Download Why do thick-tailed geckos (Underwoodisaurus milii)

Why do thick-tailed geckos
(Underwoodisaurus milii) aggregate?
Bansi Shah
School of Biological Sciences,
The University of Sydney.
Thesis submitted in partial fulfilment of the requirements for the
Degree of Bachelor of Science (Adv.) with Honours.
April, 2002.
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In the wild (rock lifted)
In captivity (shelter lifted)
Thick-tailed gecko aggregations
iv
The University of Sydney
School of Biological Sciences
Honours and Graduate Diploma Studies
Declaration
The research described in this thesis, except where acknowledged, is the original work
of the author and was a discrete project supervised by:
Prof. Rick Shine & Dr. Simon Hudson
__________________________________________________________________________________
Signature of student
__________________________________
Signature of supervisor(s) __________________________________
__________________________________
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Table of Contents
Acknowledgements .......................................................................................................... 1
Abstract............................................................................................................................ 2
Chapter 1. General Introduction ................................................................................... 3
1.1 Benefits of aggregation ............................................................................................. 4
1.1.1 Biotic factors.................................................................................................... 5
1.1.2 Abiotic factors.................................................................................................. 9
1.2 Aggregation behaviour in reptiles.............................................................................. 9
1.3 Significance of study................................................................................................12
1.4 Thesis outline...........................................................................................................12
Chapter 2. General Methods .........................................................................................14
2.1 Study species ...........................................................................................................14
2.2 Gecko collection ......................................................................................................16
2.3 Gecko husbandry .....................................................................................................18
Chapter 3. Retreat-site selection and patterns of aggregation in the field ..................20
3.1 Introduction .............................................................................................................20
3.2 Materials and Methods.............................................................................................21
3.3 Results .....................................................................................................................25
3.3.1 Gecko morphology ..........................................................................................25
3.3.2 Shelter-site use by geckos: rock dimensions.....................................................27
3.3.3 Shelter-site use by geckos: thermal regimes.....................................................31
3.3.4 Do thick-tailed geckos aggregate in the field? .................................................33
3.4 Discussion ...............................................................................................................36
3.4.1 Gecko morphology ..........................................................................................36
3.4.2 Shelter-site use by geckos: rock dimensions.....................................................38
3.4.3 Shelter-site use by geckos: thermal regimes.....................................................40
3.4.4 Do thick-tailed geckos aggregate in the field? .................................................41
3.4.5 Conclusions.....................................................................................................42
Chapter 4. Non-social influences on retreat-site selection in the laboratory ...............44
4.1 Introduction .............................................................................................................44
4.2 Materials and Methods.............................................................................................46
4.2.1 Crevice height .................................................................................................48
4.2.2 Thickness of overlying rock .............................................................................49
4.2.3 Crevice size .....................................................................................................49
4.2.4 Slope of crevice ...............................................................................................49
4.2.5 Crevice thermal regime ...................................................................................49
4.2.6 Predator scented crevice .................................................................................50
4.2.7 ‘Protected’ shelter...........................................................................................51
4.2.8 ‘New’ versus ‘old’ shelter................................................................................51
4.3 Results .....................................................................................................................52
4.4 Discussion ...............................................................................................................55
4.4.1 Crevice height .................................................................................................55
4.4.2 Crevice thermal regime ...................................................................................56
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Table of Contents
4.4.3
4.4.4
4.4.5
4.4.6
4.4.7
4.4.8
Predator scented crevice .................................................................................57
‘Protected’ shelter...........................................................................................57
Thickness of overlying rock .............................................................................58
Crevice size .....................................................................................................58
Slope of crevice ...............................................................................................59
Conclusions.....................................................................................................59
Chapter 5. Social influences on retreat-site selection in the laboratory ......................61
5.1 Introduction .............................................................................................................61
5.2 Materials and Methods.............................................................................................63
5.2.1 Do geckos aggregate when provided with ceramic tile shelters?......................64
5.2.2 Do geckos aggregate when provided with ‘new’ plastic shelters?....................67
5.2.3 Are geckos attracted to, or repelled by, restrained conspecific geckos? ...........69
5.3 Results .....................................................................................................................76
5.3.1 Do geckos aggregate when provided with ceramic tile shelters?......................76
5.3.2 Do geckos aggregate when provided with ‘new’ plastic shelters?....................79
5.3.3 Are geckos attracted to, or repelled by, restrained conspecific geckos? ...........82
5.4 Discussion ...............................................................................................................82
Chapter 6. Environmental influences on aggregative behaviour.................................88
6.1 Introduction .............................................................................................................88
6.2 Materials and Methods.............................................................................................89
6.2.1 Does predator scent induce aggregation?........................................................90
6.2.2 Does humidity influence the aggregative response?.........................................90
6.2.3 Does ambient temperature influence the aggregative response? ......................91
6.2.4 Do geckos heat and cool at different rates when with another gecko?..............91
6.2.5 Are more geckos in contact with each other when temperature changes? ........93
6.3 Results .....................................................................................................................94
6.3.1 Does predator scent induce aggregation?........................................................94
6.3.2 Does humidity influence the aggregative response?.........................................94
6.3.3 Does ambient temperature influence the aggregative response? ......................94
6.3.4 Do geckos heat and cool at different rates when with another gecko?..............96
6.3.5 Are more geckos in contact with each other when temperature changes? ........97
6.4 Discussion ...............................................................................................................97
Chapter 7. General Discussion ....................................................................................101
Chapter 8. Literature Cited.........................................................................................108
Appendix 1. Scat-piling in thick-tailed geckos............................................................118
A1.1 Introduction ........................................................................................................118
A1.2 Materials and Methods ........................................................................................119
A1.3 Results ................................................................................................................122
A1.4 Discussion...........................................................................................................127
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Acknowledgements
Many thanks go to my supervisor Rick Shine and co-supervisor Simon Hudson for their
advice and encouragement throughout the year. Rick always had time to sit, have a chat and
throw ideas around any time I walked into his office, reminding me that “she’ll be right!”
when things didn’t go quite to plan. Simon provided lots of feedback despite demanding
teaching commitments and the misfortune of being hit by a car. I am grateful to both Rick
and Simon for reading and making useful comments on the various drafts of this thesis, and
helping bring it to fruition.
This research project could not have happened without the incredible encouragement and
enthusiasm shown by Mike Kearney, who helped initiate this study and pointed me in the
direction of the ‘Magic Mountain’. Mike was always excited and ready to discuss various
possibilities and make suggestions, even while in the States. Thanks, Mike!
Thanks to those who helped collect geckos in the field; Mike and Nicole Kearney, Steve
Comber, Danny Brown and Pritesh Shah. Tim E. braved the dangers of walking through
very long grass containing all manner of highly venomous snakes (!) to help me retrieve
thermochrons from under rocks that were no longer marked.
Thanks go to everyone in the Shine lab, especially Melanie Elphick who was very helpful
and always managed to find various bits and pieces of equipment for me (and made them
work). George (Elizabeth) Barrott provided me with many quirky discussions about all
things non-herpetological… and occasionally about baby geckos and skinks. Sara
Broomhall shared (among other things) frustrating computing moments with me, and taught
me the true meaning of the word ‘chocaholic’. Sam Ruggeri patiently and cheerfully helped
me with construction of shelters, and Darren Townsend suggested methods for securing
mesh partitions in them. Mike Kearney and Glenda Wardle provided some much
appreciated statistical advice. Various people in the school kindly offered bits of
equipment; Alfie Meats lent me his video camera for a field trip and the Thompson lab lent
me thermocouples (Kylie Robert) and their refrigeration unit (Jacquie Herbert) for some allimportant experiments. Many thanks also to Malcolm Ricketts for teaching me about the
wonders of Adobe Photoshop™, and to Nish Solanki for the colour printing.
Towards the end of the study when stress levels were starting to rise, Matt Greenlees and
Carla Avolio helped feed my animals. Matt G., Carla and John Llewelyn helped with some
last minute experimental work. Many, many thanks to my proofreaders Mel, George and
Jacquie Herbert. Thanks must go also to my fellow honours sufferer, Pru Harvey, for
sharing traumatic experiences with me over coffee.
And lastly, I am grateful to my family for their support throughout my university degree,
especially to my mum who was very understanding and regularly made me delicious meals
to take home (yum!). My sister Anjli did some essential shopping for me and even
delivered hot dinners to me during thesis production… and dad kindly gave me a car during
this time.
This thesis is dedicated to my constant source of inspiration and probably the best study
animals anyone could ever ask for… the incredibly cute, cuddly and well-behaved thicktailed geckos.
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Abstract
Thick-tailed geckos (Underwoodisaurus milii) are unusual among Australian lizards (and
indeed, among lizards in general) in their tendency to aggregate in the wild. There are many
anecdotal reports of several individuals found clustered together in a single rock crevice.
My project was designed to quantify patterns of aggregation in the field, and to use
controlled trials in the laboratory to identify physical and social cues for aggregation. My
ultimate aim was to understand why the lizards display this unusual grouping behaviour.
My field data showed that thick-tailed geckos do aggregate; the lizards were found in groups
more frequently than expected under the null hypothesis of independent assortment. Retreatsites used in the field were a non-random subset in terms of several abiotic variables, and the
lizards also displayed significant selectivity for such cues in the laboratory. Lizards
aggregated in the laboratory even when suitable vacant shelter-sites were available,
indicating that aggregation in the field is not simply a response to limited availability of
appropriate shelter. The tendency to form aggregations was affected in only minor ways by
a lizard’s sex or age group, or by its familiarity with the other individuals used in the trials.
However, the aggregative response was eliminated when the opportunity for physical
interaction was removed. Lizards in physical contact with conspecifics heated and cooled
more slowly than did solitary animals, reflecting the greater effective mass (and thus,
thermal inertia) of the aggregated group. Additionally, exposure to rapidly cooling
conditions stimulated geckos to huddle together more closely.
Based on my results, I propose that aggregation behaviour in thick-tailed geckos has evolved
for biophysical advantages rather than ‘social’ interactions. More specifically aggregation
may enhance fitness by permitting the lizards to control their rates of heat exchange
behaviourally. Compared to related species, thick-tailed geckos are unusual in their large
body size, cold-climate distribution and use of thermally fluctuating rock crevices as retreatsites. Because they are exposed to profound diel and seasonal cycles in temperature, the
control of thermal exchange rates achieved via huddling may be of significant benefit in
terms of maintaining suitable body temperatures. For example, geckos may be able to
maintain relatively high temperatures late into the evening before leaving rocks to forage,
and may avoid dangerously high day-time maxima and dangerously low night-time minima
by retarding heating and cooling rates.
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CHAPTER 1
General Introduction
An aggregation is a group of two or more individuals of the same species that are gathered in
the same place and may or may not be in close physical contact with each other (amended
from Wilson 1975). Aggregations occur among ecologically and phylogenetically diverse
arrays of organisms. A great deal of variation exists in the structure and organisation of
animal societies, with the basic social unit ranging from solitary animals, to small groups of
less than 10 individuals (Breed and Gamboa 1977), to groups containing many millions of
individuals (Pereira and Gordon 2001). Aggregations may consist of either family groups or
unrelated animals (Wilson 1975). Both types of assemblages have been well documented
amongst birds and mammals (Wilson 1975).
Importantly, aggregative behaviour may incur various costs, and many animals remain
solitary throughout their lives, only coming together to mate (Wilson 1975). Some of the
costs associated with group formation include competition for resources such as food and
mates, increased transmission of parasites and diseases, and increased detectability of
multiple versus solitary animals by predators (Côté and Poulin 1995; Hass and Valenzuela
2002).
However, grouping behaviour between conspecifics may also benefit the individuals within
an aggregation, a concept known as the Allee effect (Stephens and Sutherland 1999).
Aggregations often occur whilst animals are inactive, in places such as dens, burrows, tree
hollows and under rocks (Boersma 1982; Cowan 1989; Wilkinson et al. 1998; Rasa et al.
2000; Kearney et al. 2001). Some animals also aggregate whilst carrying out specific
activities such as foraging for food, scouting for mates and migrating between habitats
(Denny et al. 2001).
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Retreat-site selection and patterns of aggregation in the field
Some aggregations may occur without direct ‘social’ cues, for instance when individuals
gather around a concentrated, high quality food source, or when individuals are attracted to
habitats of particularly high quality (Honek and Martinkova 2001; Kearney et al. 2001). For
example, ground beetles (Carabidae, Coleoptera) feed on winter rape seeds, resulting in
aggregations of beetles where the seeds occur (Honek and Martinkova 2001). Conversely,
aggregations may occur because conspecifics actively seek each other out, for example, to
mate (Wilson 1975). Amongst many territorial animals, naïve settlers prefer to settle near
conspecifics (Muller et al. 1997) irrespective of whether an alternative, equally suitable
patch of habitat is available (Stephens and Sutherland 1999). Thus, naïve settlers use the
presence of conspecifics as a cue to habitat quality and as a result, may benefit from more
effective territory defence, predator protection and attraction of prospective mates (Stamps
1988, 1991; Forbes and Kaiser 1994; Shennan et al. 1994; Muller et al. 1997).
Below, I briefly review some of the factors that may be responsible for aggregative
behaviour, with examples drawn from a variety of organisms. I then summarise published
information and ideas about aggregations in reptiles.
1.1
Benefits of aggregation
The benefits of conspecific presence may include anti-predator vigilance, predator dilution
and thermoregulation as well as a reduction of inbreeding (Courchamp et al. 1999; Stephens
and Sutherland 1999; Hass and Valenzuela 2002). In the discussion below, I have divided
some of the putative benefits of aggregative behaviour into biotic versus non-biotic factors.
Inevitably, any such classification is arbitrary; many of the benefits of aggregative behaviour
are interwoven and cannot be separated from each other.
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Retreat-site selection and patterns of aggregation in the field
1.1.1
Biotic factors
I
Group defence / predator vigilance
Group-living species may be less vulnerable to predators because of increased vigilance
(Wilson 1975). For example, adult coatis, Nasua narica (Mammalia: Procyonidae) suffer
greater rates of predation when solitary than when in a group, and rates of predation are
higher in smaller than in larger groups (Hass and Valenzuela 2002). Coatis share vigilance
duties (Burger and Gochfeld 1992) and communicate with each other using a variety of
alarm calls, and also physical contact (Russell 1982, 1983 in Hass and Valenzuela 2002).
Members of the groups even mob and attack predators such as boa constrictors (Boa
constrictor) (Janzen 1970; Russell 1981).
Herds of African elephants (Loxodonta africana) consist of several generations of females,
with a matriarch ruling over the family group (Wilson 1975). Bonds amongst the females in
the group are extremely strong and the elephants defend their own and each other’s young
from predators by forming a tight group when threatened (Wilson 1975).
II
Predator dilution
Fish in schools may be at less risk from predation than solitary fish (Pitcher and Parrish
1993). Wild guppies (Poecilia reticulata) in Trinidad alter their schooling behaviour while
inspecting predators, depending on the predation risk they incur (Magurran and Seghers
1994). When in a highly hazardous location, the guppies approached predators for
inspection in large groups, diluting the risk of predation. However, when risk from predators
was low, guppies approached predators for inspection individually (Magurran and Seghers
1994).
Many species of mammals live in herds or family groups. Herding mammals include
African wildebeest (Connochaetes taurinus) whose groups may contain thousands of
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Retreat-site selection and patterns of aggregation in the field
animals during mass migrations (Wilson 1975). At this time, the herds forage together and
function as protection for individuals and young from predation via dilution of risk (Wilson
1975). Similarly, birds often forage in flocks, which also function to reduce the predation
risk for individuals (Krebs 1974).
III
Foraging benefits and prey acquisition
Many species of birds forage in flocks and this may increase the efficiency with which food
is captured as well as enhancing intake of food per individual per unit of time (Wilson 1975;
Clark and Mangel 1986). This greater harvest efficiency may occur as a result of group
vigilance so that each animal needs to look out for predators less often (Vine 1971). In
addition, insectivorous birds are more efficient at catching flying insects when in a group
rather than on their own (Wilson 1975). Barracouta fish (Thyrsites atun) in New Zealand
form schools facultatively; those that are in schools are more efficient foragers than those
that are solitary (O’Driscoll 1998).
Lions (Panthera leo) too may benefit from group living in terms of prey acquisition.
African lions live in prides consisting of a group of related females and their young as well
as one or more males (Wilson 1975). The females hunt cooperatively, stalking prey by
fanning out around the prey animal and then rushing at it from all directions (Wilson 1975).
Without this cooperation between females, individual lions may not be able to hunt
successfully.
IV
Rearing offspring in habitats of high quality
Some groupings of animals fulfil functions related to the rearing of offspring. Birds that nest
in colonies include emperor penguins (Apentodytes forsteri) in Antarctica (Micol and
Jouventin 2001) and lesser snow geese (Chen caerulescens) in Canada (Abraham et al.
1999). Many such colonies form as a result of attraction to patches of habitat that are of
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Retreat-site selection and patterns of aggregation in the field
exceptionally high quality in terms of low risk from predation and/or high availability of
food (Wilson 1975). In addition, the availability of ice-free areas in Antarctica is limited,
although emperor penguins actually nest on the ice (Croxall 1997). Thus, aggregations
directly reflect habitat heterogeneity in these instances.
V
Division of labour
Many insects live in large colonies, often consisting of one or more queens and thousands to
millions of workers (Wilson 1975). Probably the best known examples are the highly
organised societies of Hymenopteran and Isopteran insects, which include wasps, bees and
ants, and termites respectively (Keller and Chapuisat 1999). Work is divided between
individuals depending on various internal (e.g. polymorphic, temporal and genetic
polyethism) and external (environmental) factors (Gordon 1996). In such cases, survival of
individuals is enhanced by the colony’s ability to achieve tasks that a single individual may
be unable to accomplish. These tasks include defence from predation and/or invasion
(Sakata and Katayama 2001), foraging for food (Denny et al. 2001) and reproduction (Aron
2001), as well as colonisation of new territories (Mallon et al. 2001).
VI
Increased mating opportunities
Aggregations may also arise as a result of mating activity. Leks are mating aggregations
where males display and females attend to mate (Alatalo et al. 1992; Widemo and Owens
1995). Leks thus involve intense competition for mates between individuals (Tello 2001).
Aggregation may enhance male reproductive success because a group of males is more
likely to attract females than a solitary male (Wilson 1975). Examples of lekking species
include round-tailed manakins (Pipra chloromeros) in south-east Peru, which form leks of
between two and five territorial males (Tello 2001), and buff-breasted sandpipers (Tryngites
subruficollis), which display both in leks and as solitary individuals (Lanctot et al. 1998).
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Retreat-site selection and patterns of aggregation in the field
Many insects also form leks; one of the more spectacular examples is that of male fireflies
(Lampyridae) in south-east Asia (Wilson 1975). The fireflies sit in specific trees within the
forest, flashing synchronously and rhythmically throughout the night in order to attract
females (Wilson 1975). Similarly, the fruit flies (Drosophilidae) of Hawaii possess a lek
system whereby males aggregate on the stems of tree ferns (Spieth 1968).
VII
Paternity assurance / mate-guarding
As described above, many animals congregate for mating. However, for males to be assured
of paternity, they must guard their mates. For example, Seychelles Warblers (Acrocephalus
sechellensis) have only one egg and one clutch per breeding season (Komdeur 2001). Males
can increase their fitness via extra-pair copulations, but thereby also run the risk of another
male mating with their own mate. By guarding their mates during their receptive period,
males reduce the risk of this occurrence (Komdeur 2001). The degree to which the females
are guarded depends on the paternity risk, or number of rival males present (Komdeur 2001).
By comparison, in the pipefish (Corythoichthys haematopterus), the mate-guarding role is
reversed, and females guard males (Matsumoto and Yanagisawa 2001). This probably
occurs because, unlike the conventional system where females have the greater parental
investment, males of this species care for the brood (Dawson 1977).
VIII
Increased fitness through cooperative breeding
Perhaps one of the best known cooperative breeders is the laughing kookaburra (Dacelo
novaeguineae), which lives in family groups of about six individuals, consisting of a
breeding pair and several helpers (Legge 2000; Legge and Cockburn 2000). The related
helpers, recruited from young hatched within the group, gain indirect fitness benefits by
helping the breeding pair raise more young (Legge and Cockburn 2000). Helpers at the nest
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Retreat-site selection and patterns of aggregation in the field
benefit the breeding pair in terms of increased survivorship as well as higher quality
fledglings (Legge 2000).
1.1.2
Abiotic factors
I
Moisture conservation
Desert tenebrionid beetles (Parastizopus armaticeps) form facultative aggregations in
burrows during summer droughts resulting in reduced rates of water loss (Rasa et al. 2000).
Similarly, the tropical fungus beetle Stenotarsus rotundus (Endomychidae) forms
aggregations that may increase humidity within the aggregated group, thereby reducing
water loss (Tanaka 2000). The beetles then emerge from diapause as a result of an increase
in photoperiod as well as increasing humidity (Tanaka 2000).
II
Thermoregulatory benefits
Naked mole rats (Heterocephalus glaber) form aggregations in which individuals huddle
together occur all year round. This huddling behaviour saves both energy and water but also
plays an important role in thermoregulation for the otherwise non-endothermic mammals
(Yahav and Buffenstein 1991). Similarly, Leadbeater’s possums (Gymnobelideus
leadbeateri) sleep in tree hollows in groups of up to 12 animals and huddle to keep warm,
thus reducing energy expenditure (Lindenmayer 2002).
1.2
Aggregation behaviour in reptiles
In the brief review above, I have used examples of aggregative behaviour from animals other
than reptiles. Indeed, aggregative behaviour in reptiles has attracted little scientific attention.
Sociobiologists have generally treated reptiles as simple animals with limited behavioural
and social repertoires (Brattstrom 1974, Wilson 1975). Recently, however, the phenomenon
of reptile aggregation has gained increasing attention from researchers. The most obvious
example of grouping behaviour in reptiles involves winter aggregations, as occur in many
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Retreat-site selection and patterns of aggregation in the field
species of snakes from highly seasonal habitats. Well-known examples include rattlesnakes
(Crotalus viridis) (Graves and Duvall 1987) and red-sided garter snakes (Thamnophis
sirtalis) (Joy and Crews 1987), where aggregations may contain as many as 20,000
individuals (Shine et al. 2001). Some lizard species also form winter aggregations: for
example tree lizards (Urosaurus ornatus) aggregate both in the field and under laboratory
conditions, huddling together to form tight balls (Elfström and Zucker 1999). Several
species of lizards from the scincid genus Eumeces also form winter aggregations, including
E. fasciatus in hibernacula in rotting logs and under the ground (Hamilton 1948). Broadheaded skinks (Eumeces laticeps) also aggregate under laboratory conditions even when
excess shelter-sites are available (Cooper and Garstka 1987).
In Australia, the medium-sized skink Pseudemoia spenceri has been noted to occur in large
winter aggregations of up to 50 individuals (Rawlinson 1974). Multi-species aggregations of
skinks have also been found in Australia (Rawlinson 1975), and both the brown tree snake
(Boiga irregularis) and the Oenpelli python (Morelia oenpelliensis) have each been reported
in an aggregation of three animals (Fitzgerald 2000; Peck 2000). Some of these reptilian
winter aggregations have been suggested to occur due to limited availability of suitable
hibernacula (Rawlinson 1975). However, many of the aggregations are assumed to be of
thermoregulatory significance (Hamilton 1948; Powell et al. 1977; Cooper and Garstka
1987), although there is little or no empirical support for this inference in these examples.
There is, however, some evidence that aggregations may provide thermoregulatory benefits
in large reptiles. Marine iguanas (Amblyrhynchus cristatus; 0.5 – 10 kg) are diurnal and
aggregate, or form ‘sleeping piles’ at night (Boersma 1982). Iguanas closest to the centre of
a sleeping pile are warmer than those on the periphery, and those on the periphery are
warmer than solitary iguanas (Boersma 1982). Furthermore, iguanas in the centre of the pile
remain warmer throughout the night, to just before sunrise (Boersma 1982).
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Retreat-site selection and patterns of aggregation in the field
More recently, studies on reptiles have begun to reveal a previously unsuspected complexity
in social behaviour. For example, individual recognition has been documented amongst
lizards living in social aggregations. Several species of lizards are able to discriminate
between conspecific individuals based on chemosensory cues (Cooper 1996; Main and Bull
1996; Bull et al. 2000; Bull et al. 2001), obtained via tongue-flicking (see Schwenk 1995 for
a review). Broad-headed skinks (Eumeces laticeps) discriminate between familiar and
unfamiliar individuals, with adult males tongue-flicking more often to scent cues from
unfamiliar females and males (Cooper 1996). The gregarious Australian gidgee skink
(Egernia stokesii) is able to recognise and differentiate between group and non-group
members (Bull et al. 2000). Female gidgee skinks and shingle-back lizards (Tiliqua rugosa)
are also able to recognise their own offspring (Main and Bull 1996). Similarly, juvenile tree
skinks (Egernia striolata) are able to recognise related from unrelated individuals among
unfamiliar conspecifics (Bull et al. 2001). Furthermore, tree skinks (E. striolata) form scat
(faeces) piles, which may function as individual signals and territory markers (Bull et al.
1999). This group of Australian skinks displays the most complex reptilian social systems
known to date, with genetic evidence showing that gidgee skinks (E. stokesii) live in family
groups consisting of a breeding pair and related adults, as well as young from two or more
annual cohorts (Gardner et al. 2001).
Most of the above examples of lizard aggregation, have involved species of a single lineage,
the scincid lizards (Scincidae). Research on aggregative behaviour in geckos however, has
been far more limited, although western banded geckos (Coleonyx variegatus) were noted to
occur in large aggregations in the laboratory as early as 1943 (Greenberg 1943). The
tendency of this species to aggregate was later demonstrated more rigorously via laboratory
experiments, with limited burrow availability being implicated as a reason for such
aggregation (Cooper et al. 1985). Based on chemosensory cues, male leopard geckos
(Eublepharis macularius) can discriminate between pheromonal cues of themselves versus
11
Retreat-site selection and patterns of aggregation in the field
other males, as well as between familiar and unfamiliar females (Steele and Cooper 1997).
Western banded geckos also form scat-piles away from their diurnal shelter-sites, and are
able to discriminate between their own scats and those of conspecifics (Carpenter and Duvall
1995).
1.3
Significance of study
Within Australia, the vast majority of reports of ‘social’ aggregation have been based on
members of the scincid lizard genus Egernia (see above references). However, one other
lizard species has been documented to occur frequently in relatively large aggregations
throughout the year (Wells and Wellington 1983; Kearney et al. 2001). This is the
nocturnal, saxicolous thick-tailed gecko, Underwoodisaurus milii (Wells and Wellington
1983; Kearney et al. 2001). In the only quantitative study of this taxon to date, Kearney et
al. (2001) documented non-random combinations of male, female and juvenile geckos, with
74 % of geckos found in aggregations of between two and nine individuals (Kearney et al.
2001). Thick-tailed geckos are phylogenetically very distant from other ‘social’ lizards, and
there are no reports of aggregative behaviour in any other members of the
Underwoodisaurus - Nephrurus lineage (Nephrurus species are closely related to
Underwoodisaurus species; Bauer 1990). Thus, the aggregative behaviour of this species
has presumably evolved independently of that in the other ‘social’ lizards. These geckos
therefore offer an ideal opportunity to examine questions about the significance of sociality
and aggregative behaviour.
1.4
Thesis outline
I conducted a study to determine why thick-tailed geckos aggregate. Chapter 2 describes the
study species and its collection and husbandry. Chapter 3 examines patterns of natural
aggregation as well as characteristics of shelter-sites in the field. In Chapter 4, I describe
experiments where I manipulated physical cues in the laboratory to identify retreat-site
12
Retreat-site selection and patterns of aggregation in the field
preferences; the rationale for this component of the work was that aggregations in the field
might result simply from spatial heterogeneity coupled with extreme selectivity of physical
cues by the lizards. Chapter 5 attempts to discover whether aggregation is a social
phenomenon: that is, do social cues per se drive aggregative behaviour. In Chapter 6, I look
at the consequences of environmental manipulations on grouping behaviour. Finally,
Chapter 7 proposes a novel hypothesis as to the functional significance of aggregation in
thick-tailed geckos. An additional chapter is included in the appendix, and looks at whether
thick-tailed geckos scat-pile (i.e., defecate in consistent places) because previous work has
identified scat-piling as a cue for social aggregation in other species of lizards (Bull et al.
1999).
13
Retreat-site selection and patterns of aggregation in the field
CHAPTER 2
General Methods
2.1
Study species
Thick-tailed geckos (Underwoodisaurus milii; allocated to the genus Nephrurus by Bauer
1990), also known as ‘barking geckos’, are nocturnal terrestrial lizards with relatively large
heads and prominent eyes (Fig. 2.1). They vocalise, emitting one to several barks when
engaged in aggressive interactions with conspecifics, as well as in predator defence. They
can hear relatively higher frequencies than several other species of geckos, and this may aid
in communication with conspecifics (Johnstone and Werner 2001). These geckos eat mainly
insects, spiders and scorpions but will also eat smaller lizards of other species (Cronin 2001).
Adult snout-vent length (SVL) averages approximately 80 mm (Cogger 2000), although
geckos from western New South Wales (NSW) and some offshore islands tend to be larger
than those in the rest of their range (Bauer 1990). The colouring of the lizards is highly
variable, and may range from orange to almost black dorsally, with a white to pale pink belly
(Fig. 2.1). There are numerous yellow or white spots on the body, usually forming two to
three bands on the head and four to six bands on the tail. The tail is ‘carrot-shaped’ (Swan
1990; Cogger 2000); broad and depressed, tapering towards the end. Original tails are
longer than regenerated tails, which lack the elaborate patterning.
Adult males are extremely aggressive and fight viciously with other males (pers. obs.). The
specific timing of mating in the wild is unknown, but may occur from late winter through to
early to mid summer (Greer 1989; pers. obs.). One to two eggs are laid (Henle 1991)
approximately four to six weeks later (at least in captivity), although this timing appears to
vary according to thermal conditions and food availability (pers. obs.). While there is no
data on reproduction in the field, it is likely that one to two clutches of eggs may be laid per
season (Greer 1989), whereas in captivity there may be up to four clutches. Hatchlings
14
Retreat-site selection and patterns of aggregation in the field
emerge from the egg after approximately 60 days, depending on the temperature of
incubation (see below).
a) An orangish-pink adult male, snout-vent length 79 mm
b) A purple to black adult male, snout-vent length 73 mm.
Figure 2.1: Colour variation in thick-tailed geckos (Underwoodisaurus milii).
Thick-tailed geckos occur from the east to the west coasts of southern Australia (Cogger
2000). The lizards can be found in a variety of habitats, from wet coastal heathlands and
sclerophyll forests to arid scrublands and rocky hills (Swan 1990; Cogger 2000). Within
these habitats, they are usually found under slabs of rock and beneath loose bark (Swan
1990; Johnstone and Werner 2001). These geckos may be active throughout the winter
15
Retreat-site selection and patterns of aggregation in the field
months; one male was found active on a cold (9oC) rainy winter night in mid July (pers.
obs.).
2.2
Gecko collection
All geckos were collected from Mt Korong, in Victoria, Australia (36o 45′ S, 144 o 17′ E)
from 12 – 14 July 2001 (Fig 2.2). A total of ten adult pairs (nine male-female pairs and one
male-male pair), 19 solitary adult females, 24 solitary adult males and 30 pairs of juveniles
were collected. I classified animals that were < 60 mm snout-vent length (SVL) as juveniles,
and > 60 mm SVL, as adults. These criteria were based on two previous studies. Thicktailed geckos > 60 mm SVL were classed as adults by How et al. (1990) based on dissection,
and by Kearney et al. (2001), based on the smallest individuals with observable sexual
characteristics.
In order to facilitate later experimental work, I needed pairs of lizards. This was
straightforward when two animals were found together under a rock, but required some
simple rules when groups consisted of more than two lizards. Where groups of four
juveniles were found, all geckos were taken and later split into two groups of two. For
groups of three or five juveniles, one gecko was randomly selected and replaced under the
rock where it had been found. Where a pair of juveniles was found with one adult, the
juvenile pair was taken, and the adult was taken and treated as a ‘solitary adult’. If a pair of
juveniles was found with an adult male-female pair, each pair was taken.
a) Mt Korong
16
Retreat-site selection and patterns of aggregation in the field
b) Rock outcrop at Mt Korong
Figure 2.2: Site from which geckos were collected at Mt Korong, Central Victoria.
2.3
Gecko husbandry
17
Retreat-site selection and patterns of aggregation in the field
All geckos were housed in a room in the Native Animal House in the Heydon-Laurence
Building at the University of Sydney. The room was maintained at 20oC. Cages were
housed on racks with thermal film (‘flexwatt tape’, 75 mm wide, 33 watts/m, Flexwatt
Corporation [U.S.A.]) running along the back of each rack. Thus, each cage contained a
small area with a constant heat source, with temperatures in this area ranging from 26 to
30oC. The light regime followed natural cycles. From 1 November 2001 onwards, heating
for juveniles was reduced from 24 hours a day to six hours a day (from 1000 to 1600 h), to
reduce the lizards’ growth rates. All juveniles were kept in pairs, with each individual
housed with a conspecific that had been found under the same rock (see above). In addition,
adults found as male and female pairs were housed together, and one pair of adult males was
also housed together. All other adults were caught as solitary lizards, and were housed
individually.
Juvenile lizards were housed in plastic cages (22 x 22 x 7 cm) with a sand substrate 2 cm
deep. Two shelters made of white plastic half-pipes (diameter 5 cm, 12 cm long) were
provided in each cage, with a water dish at the front of the cage. All lizards were fed
crickets twice a week up to 19 November 2001, after which time the lizards were fed once
weekly to reduce their growth rates. Vitamin and calcium supplements were dusted onto the
crickets once a week throughout the study and water was provided ad libitum.
Solitary adult lizards were housed in the same type of plastic cages (22 x 22 x 7 cm) as the
juveniles, with a 2 cm deep sand substrate. One plastic half-pipe shelter was provided in
each cage, plus a water dish. Paired adults were housed in larger plastic cages (29 x 21.5 x
6.5 cm) with a 2 cm sand substrate and two plastic half-pipe shelters. All adults were fed
crickets twice a week, with vitamin and calcium supplements dusted onto the crickets once a
week. Water was provided ad libitum.
18
Retreat-site selection and patterns of aggregation in the field
Cages containing juveniles were placed on racks in the order in which the lizards had been
caught. Adults’ cages were first numbered, and then placed randomly on racks, irrespective
of whether they were paired or solitary. As thermal and light regimes were not identical for
every position on every rack, all cages were rotated twice a week by translocating the first
cage to the end and moving all others up one position. Adult and juvenile lizards were
housed on separate racks.
Females became gravid from early to mid September. Cages of gravid females were
checked daily for eggs. Eggs laid were removed and placed in glass jars with a mixture of
60 g medium grade vermiculite: 55 ml water (= 91.7% H2O). The jars were covered with
cling wrap and placed in an incubator at 28.5oC. Hatchlings emerged 53.56 + 0.39 (SE) days
after the eggs were laid and averaged 41.10 + 0.67 (SE) mm SVL and 1.63 + 0.04 (SE) g.
19
Retreat-site selection and patterns of aggregation in the field
CHAPTER 3
Retreat-site selection and patterns of aggregation in the field
3.1 Introduction
Many animals remain inactive during various periods of the diel cycle: nocturnal animals are
usually inactive during the daytime, whereas diurnal animals are inactive at night. Such
animals need to find a suitable place in which to retreat to reduce their vulnerability to
potential hazards such as predators and environmental extremes (Lima and Dill 1990;
Eggleston and Lipcius 1992; Schlesinger and Shine 1994; Downes and Shine 1998a;
Kearney 2002). Shelter-sites may vary from the tree hollows used by sugar gliders
(Petaurus breviceps) (Traill and Lill 1997) to the rock crevices used by velvet geckos
(Oedura lesueurii) (Schlesinger and Shine 1994) and broad-headed snakes (Hoplocephalus
bungaroides) (Webb and Shine 1998).
Ectotherms rely on external sources of heat for important physiological processes such as
digestion, locomotion and growth (Huey 1982; Stevenson et al. 1985; Webb and Shine
1998). Because nocturnal ectotherms have optimal physiological performance at body
temperatures higher than those they normally experience during their active period at night
(Huey et al. 1989; Autumn and De Nardo 1995; Kearney 2002), they may be strongly
advantaged by selecting retreat-sites that offer favourable thermal regimes (Kearney 2002).
Finding an appropriate diurnal retreat-site is of particular importance to nocturnal ectotherms
because thermal conditions are highly variable during the day (Webb and Shine 1998;
Kearney 2002).
The thermal regime under a rock depends upon the rock’s size as well as its degree of
exposure to solar radiation (Webb and Shine 1998; Kearney 2002). Other attributes such as
the rock’s aspect, slope and the underlying substrate may also affect temperature and
20
Retreat-site selection and patterns of aggregation in the field
moisture conditions. Accordingly, we might expect that when selecting retreat-sites,
nocturnal saxicolous reptiles would evaluate a range of characteristics of potential crevices.
Indeed, studies on four such species have all concluded that the animals selected retreat-sites
based on an array of specific physical cues (Schlesinger and Shine 1994; Stow 1998; Webb
and Shine 1998; Kearney 2002). In addition, different age and sex classes within the same
species may select different retreat-sites (Webb and Shine 1998). Thus, in this study I aimed
to establish whether or not thick-tailed geckos, another saxicolous squamate, are also
selective with respect to the characteristics of their diurnal shelter-sites. If the geckos are
selective of traits of diurnal retreat-sites, aggregations may occur simply as a result of
limited availability of suitable shelter-sites.
To obtain data on the attributes of crevices used by free-ranging geckos compared to the
array of crevices available to the animals, I quantified these aspects during a field trip to Mt
Korong in Central Victoria, Australia. This trip also provided an opportunity to examine
patterns of aggregation of thick-tailed geckos during winter; the only previous study of this
phenomenon was based on data gathered during spring and summer (Kearney et al. 2001).
3.2 Materials and Methods
This study was designed to identify whether or not thick-tailed geckos use retreat-sites nonrandomly with respect to attributes such as the dimensions of the overlying rock, and/or the
thermal regimes under that rock. If so, we would expect to see that rocks sheltering geckos
will differ from other (unused) rocks in such characteristics. Further, I aimed to ascertain
whether age and sex classes of geckos differed in terms of retreat-site use. In addition, I
attempted to find out whether specific age and sex classes of geckos were found together
more or less often than predicted by the null hypothesis of random distribution, as well as
whether the size of rocks influenced the number of geckos sheltering beneath them.
21
Retreat-site selection and patterns of aggregation in the field
Thirty-two pairs of miniature field-portable thermal data-loggers (thermochron ibuttons,
Dallas Semiconductor Corporation, U.S.A.) were prepared by setting them to begin
recording data at 1200 h on 12 July 2001, so as to obtain data on rock thermal regimes
immediately upon placement. However, because all thermochrons were not set out at the
same time, data obtained during the first two days of recording were not used for analyses.
The thermochrons were set to record the temperature every 15 minutes for 22 days. To
ensure that the maximum numbers of thermochrons were retrieved, each thermochron was
put into a small brightly coloured balloon, which was then tied off with a long piece of
orange flagging tape.
The thermochrons were set out on 12 and 13 July 2001, at the same site from which my
study animals were obtained (Mt Korong in Victoria, Australia; 36o 45′ S, 144 o 17′ E).
Rock outcrops containing numerous loose surface rocks were located. All suitable rocks on
each outcrop were then lifted to look for thick-tailed geckos beneath them. A ‘suitable’ rock
was defined as one that could be lifted by two people (maximum rock length = 111 cm) and
excluded rocks with widths of less than 12 cm. The lower size restriction ensured that rocks
were large enough for an adult gecko to shelter beneath (an adult thick-tailed gecko’s total
length is approximately 12 cm, thus making it unlikely to be found sheltering under rocks
smaller than 12 cm in length).
When a gecko was found under a rock, the gecko’s snout-vent length (SVL) was measured
and its sex noted. Mature male thick-tailed geckos have a distinctive post-cloacal bulge
(Greer 1989), and a ‘waxy’ patch on their belly, both of which are lacking in females (pers.
obs.). The rock (‘gecko’ or used rock) was then measured for length, width and thickness.
The length of the rock was its longest dimension and the width was the greatest dimension
perpendicular to this major axis. Thickness was measured at the point at which the gecko
was found. A ‘random’ rock was then chosen by spinning a pen on the ground and walking
22
Retreat-site selection and patterns of aggregation in the field
a maximum of 10 m in the direction it pointed. The first suitable rock in that direction was
chosen as the ‘random rock’. If there was no suitable rock in that direction the pen was spun
again and the procedure repeated. The ‘random’ rock was then lifted to check for geckos,
and measured as before, with thickness being measured at the centre of the rock. No geckos
were found under these ‘random’ rocks.
A thermochron was placed under each of these two rocks, at the point at which the gecko
was found for the ‘gecko’ rock, and in the middle of the ‘random’ rock. The rocks were then
marked with flagging tape and individual numbers and replaced in their original position.
The substrate (rock or soil) and percentage shading by trees or other rocks and boulders
(0/50/100%) were also noted for each rock. This was done for 16 rocks with one or more
juvenile lizards underneath, and 16 rocks with one or more adult lizards under them. Due to
the relative shortage of adults compared to juveniles, rocks with both juveniles and adults
underneath were classed as ‘adult’ rocks.
I returned three months later (12 – 14 October 2001) to retrieve the thermochrons. While
most were recovered, six could not be found. These lost thermochrons resulted in data for
four ‘adult’ rocks and one ‘juvenile’ rock (two thermochrons lost for a single pair of ‘gecko
and ‘random’ rocks) being unavailable. Data from all thermochrons were downloaded into a
computer for analysis.
To compare rock dimensions and thermal conditions to the numbers and sizes of geckos
found under those rocks, I made further morphological measurements. In addition to SVL, I
measured head width, head length, axillar-groin length (AGL; distance between the arm-pits
of the front limb and hind limb; measured on the right-hand side for all lizards) and masses
of all the adult geckos captured. Due to the small size of the juvenile geckos, only mass and
SVL were recorded.
23
Retreat-site selection and patterns of aggregation in the field
To determine whether there was sexual dimorphism in thick-tailed geckos, I analysed SVL
using analysis of variance (ANOVA). Analysis of covariance (ANCOVA) was used to
compare the sexes in terms of traits that depend on absolute body size (mass, head length,
head width and AGL), with SVL as the covariate.
ANOVA was also used to test whether the rocks under which I found geckos of different age
and sex classes differed with respect to specific rock characteristics (i.e. thickness, length,
width and shading). ANOVA was also used as to determine whether the range of thermal
regimes differed between ‘gecko’ and ‘random’ rocks. While the mean daily temperatures
had similar variances, the ranges of temperatures had unequal variances between treatments,
and were therefore log transformed. However, variances remained unequal. Thus,
significance tests on mean daily minimum and maximum temperatures were based on
nonparametric sign tests. I performed correlation analyses to see if thermal regimes and the
dimensions of rocks were related to the body sizes and age and sex classes of geckos
sheltering under them.
To test whether the size of aggregations differed from those expected under the null
hypothesis of random occurrence, I compared observed values to those predicted from the
Poisson distribution, and used G-tests to detect any significant differences (Kearney et al.
2001). Two different analyses were used to determine whether different combinations of
adult male, adult female and juvenile geckos occurred randomly. Initially, observed
frequencies of aggregations consisting of pairs of geckos in any of the six possible
combinations were compared, using G-tests, to expected frequencies calculated using the
binomial expansion (Kearney et al. 2001). Sample sizes of aggregations with more than two
geckos in them were too small to test as above. Therefore, as in Kearney et al. (2001), I
tested whether the different age and sex classes of geckos combined randomly to form
aggregations in the following categories: (1) > 1 adult male; (2) > 1 adult female; (3) > 1
24
Retreat-site selection and patterns of aggregation in the field
juvenile; (4) > 1 adult male plus > 1 adult female; (5) > 1 adult male plus > 1 juvenile; (6) >
1 adult female plus > 1 juvenile; and (7) > 1 adult male plus > 1 adult female > 1 juvenile. A
randomisation procedure was used to estimate the expected frequencies for each of the above
categories (see Kearney et al. 2001 for explanation of randomisation procedure).
As with many studies where repeated tests are carried out on the same data, there is an issue
of artifactually ‘significant’ results arising due to type I errors. There are various techniques
available to deal with this problem, however they are based on subjective judgements in
deciding what constitutes a single set of tests (Hurlbert 1984; Kearney et al. 2001). Thus, I
have reported uncorrected values here and throughout the thesis but have also used the
sequential Bonferroni technique and have pointed out where P values move beyond
significance as a result. In this chapter, Bonferroni corrections were applied to both retreatsite use and aggregation data collected.
3.3 Results
3.3.1
Gecko morphology
Previous studies have classed thick-tailed geckos > 60 mm SVL as adults (How et al. 1987,
1990, based on dissection; Kearney et al. 2001, based on the smallest individuals with
observable sexual characteristics). I adopted the same criteria. In my data set, adult males
had an average SVL of 73.7 + 1.08 mm (SE), ranging from 64 to 85 mm. Adult females had
an average SVL of 75.0 + 1.61 mm (SE), ranging from 63 to 88 mm. There was no
significant difference in SVL, mass or head width (relative to SVL) between the adult males
and adult females (Table 3.1). There was also no significant difference in SVL or mass
between juvenile males and juvenile females (Table 3.1). ANCOVA tests showed that adult
males had significantly greater head lengths than did adult females of the same SVL. Adult
females had significantly greater axillary-groin lengths (AGL) than did adult males of the
25
2.1
45.9
Mean
1.08
0.09
0.58
+ SE
24
38
30
30
n
14.7
19.5
75.0
2.1
45.5
Mean
0.98
0.28
0.38
1.61
0.11
0.65
+ SE
19
19
19
19
19
28
28
n
0.001
5.31
3.40
192.47
0.44
0.01
0.18
F value
1, 40
1, 40
1, 40
1, 40
1, 55
1, 56
1, 56
Degrees
of
freedom
0.97
0.03
0.07
<0.0001
0.51
0.93
0.68
P value
No significant difference
Adult females have significantly
longer bodies than adult males of
the same SVL
No significant difference
Adult males have significantly
longer heads than adult females of
the same SVL
No significant difference
No significant difference
No significant difference
Result
Head width
Axillarygroin length
Females
Snout-vent
length
73.7
0.31
24
37.6
0.55
Males
Mass
19.8
0.23
24
8.5
Experiment
Snout-vent
length
15.1
0.70
24
Head length
36.2
0.44
Table 3.1: Results of ANOVA (for SVL) and ANCOVA (head length, head width, AGL and mass) testing for sexual dimorphism in morphological
traits of thick-tailed geckos. All measurements were made in millimetres, although mass was measured in grams. Significant values (P < 0.05) prior to
sequential Bonferroni corrections are in boldface font.
Gecko
age
class
Juvenile
- male
vs.
female
Adult
- male
vs.
female
8.8
Mass
Non-social influences on retreat-site selection in the laboratory
same SVL (Table 3.1). After sequential Bonferroni correction, the AGL result fell
marginally beyond the conventional level of statistical significance (P = 0.05). However,
difference in AGL between mature males and females is a widely reported phenomenon
amongst lizards (e.g. Vitt and Cooper 1985; Hews 1990; Florentino 1996) and is likely to be
a real difference between male and female thick-tailed geckos. In addition, if males have
longer heads than females of the same SVL, then they necessarily must have shorter bodies.
Thus, problems with the subjectivity of the methods used to apply Bonferroni correction
factors cast doubt on indiscriminate acceptance of its results (Cabin and Mitchell 2000). The
differences are therefore discussed below.
3.3.2
Shelter-site use by geckos: rock dimensions
Thick-tailed geckos used rocks non-randomly, and different age/sex classes of geckos were
found under different types of rocks. Every gecko found was sheltering beneath a rock that
was on rock; no geckos were found under rocks with a soil substratum (6/32 ‘random’ rocks
were on soil). Rocks that were used by geckos did not differ significantly in the percentage
shading from rocks that were not used by geckos (F1,151 = 0.34, P = 0.56), and over 75% of
rocks in both cases (used and unused) were unshaded. The thickness of rocks with geckos
under them was not significantly different from the thickness of rocks that were not used by
geckos (Table 3.2; Fig. 3.1). The range in thickness of unused rocks (2 – 13.5 cm) was
contained within the range of used rocks (0.5 – 16 cm). In contrast, length and width of
rocks used by geckos were significantly greater than rocks that were not used by geckos
(Table 3.2; Fig. 3.1). However, there was considerable overlap in the ranges of lengths and
widths of rocks that were both used (length, 26 – 120 cm; width, 18 – 89 cm) and not used
(length, 17.5 – 62 cm; width, 12.5 – 51 cm) by geckos. Over 75% of unused rocks had
lengths and widths that fell within the range of rocks that were used by the geckos, while
60% and 81.5% of the lengths and widths respectively of used rocks fell within the range of
27
Non-social influences on retreat-site selection in the laboratory
unused rocks. Sequential Bonferroni corrections were carried out combining all three
variables, but did not change significance values.
Table 3.2: Results of ANOVA testing whether or not the rocks used by geckos differed from
rocks that were not used by geckos in terms of dimensions. All measurements were made in
centimetres. Significant results prior to application of sequential Bonferroni corrections (P <
0.05) are shown in boldface font.
Dimension
Thicknes
s
Used /
unused
Mean
(cm)
Standard
Error
(cm)
Sample
size
Used
4.52
0.29
126
Unused
4.83
0.60
27
Used
59.38
1.84
126
Length
Unused
37.20
2.77
27
Used
40.88
1.24
126
Width
Unused
25.65
2.02
F value
Degrees
of
Freedom
P value
0.20
1, 151
0.65
28.00
1, 151
<0.0001
28.72
1, 151
<0.0001
27
Thickness / Length / Width (cm)
70
60
50
40
30
20
10
0
Used
Unused
Thickness
Used
Unused
Length
Used
Unused
Width
Rock dimension
Figure 3.1: Means and standard errors of dimensions of rocks (thickness, length and width)
that were used and unused by geckos. All measurements are in centimetres.
28
Non-social influences on retreat-site selection in the laboratory
Table 3.3: Results of ANOVA testing whether or not the rocks sheltering different age and
sex classes of geckos differed from each other in terms of dimensions and shading. All
measurements were made in centimetres, although shading was scored as a percentage (0, 50
or 100%). Analyses are shown separately for adult male, adult female and juvenile lizards.
Significant results (P < 0.05) prior to sequential Bonferroni corrections are shown in
boldface font.
Adult /
juvenile
Adult
male
Adult
female
Rock
characteristic
F
value
Degrees of
Freedom
P
value
Result
Thickness
1.53
1, 127
0.22
No difference in used rocks
Length
1.99
1, 127
0.16
No difference in used rocks
Width
6.57
1, 127
0.01
Rocks with adult males
significantly wider than other
used rocks
Shading
0.89
1, 127
0.35
No difference in used rocks
Thickness
0.05
1, 127
0.83
No difference in used rocks
Length
4.745
1, 127
0.03
Width
12.41
1, 127
0.0006
Shading
0.04
1, 127
0.85
No difference in used rocks
Thickness
1.16
2, 126
0.32
No difference in used rocks
Length
2.92
2, 126
0.06
No difference in used rocks
Width
8.37
2, 126
0.0004
Rocks with juveniles significantly
smaller width than other used
rocks
Shading
1.26
2, 126
0.29
No difference in used rocks
Rocks with adult females
significantly longer than other
used rocks
Rocks with adult females
significantly wider than other
used rocks
Juveniles
Of the four rock characteristics tested (thickness, length, width and shading; [excluding rock
substrate]), adult males selected rocks with greater widths compared to other rocks that were
used by geckos (Table 3.3; Fig. 3.2). Adult females selected rocks that were longer and
wider than other ‘gecko’ rocks (Table 3.3; Fig. 3.2). By contrast, juveniles selected smaller
rocks that were narrower (smaller width) than other ‘gecko’ rocks (Table 3.3; Fig. 3.2).
29
Non-social influences on retreat-site selection in the laboratory
However, there was no significant difference in mean thickness of rocks used by any of the
age/sex groups (Table 3.3; Fig. 3.2; combining data, ANOVA: F1,127 = 0.54, P = 0.46).
Sequential Bonferroni corrections were applied to rock characteristics for each sex/age class
of geckos, although shading was analysed separately. The corrections did not change
significance values for either adult males or juveniles, but moved the preference of adult
females for longer rocks beyond the conventional level of significance.
b) Rock length
6
80
5.5
70
Rock Length (cm)
Rock thickness (cm)
a) Rock thickness
5
4.5
4
3.5
60
50
40
30
3
20
No
gecko
Adult
male
Adult
female
Juvenile
Sex and age class of geckos
No gecko
Adult
male
Adult
female
Juvenile
Sex and age class of geckos
c) Rock width
60
Rck width (cm)
50
40
30
20
10
No
gecko
Adult
male
Adult
female
Juvenile
Sex and age class of geckos
Figure 3.2: Means and standard errors of dimensions of rocks (thickness, length and width)
without any geckos, with > 1 adult male, with > 1 adult female and with > 1 juvenile. All
variables are measured in centimetres. Note that vertical axes do not begin at zero.
30
Non-social influences on retreat-site selection in the laboratory
Larger rocks had more adult female and juvenile geckos under them (Table 3.4), and the
total number of geckos under a rock also increased with the rock size (Table 3.4).
Some rock dimensions were also inter-correlated; thickness was correlated with width (r =
+0.28, Z = 2.08, P = 0.04) and width was strongly correlated with length (r = +0.88, Z =
9.91, P < 0.0001). However, length was not significantly correlated with thickness (r =
+0.22, Z = 1.58, P = 0.11). As a result, it was not possible to pull out axes of the specific
sizes and shapes of rocks for comparison.
Table 3.4: Results of correlation analyses comparing rock dimensions (length and width) to
the number of geckos under the rock (n = 54). There was no correlation between rock
thickness and number of geckos under a rock. Statistically significant results prior to
Bonferroni corrections (P < 0.05) are shown in boldface font.
Rock
dimension
Length
(cm)
Width
(cm)
3.3.3
Gecko characteristic
r value
Z value
P value
Number of adult males
+0.13
0.94
0.35
Number of adult females
+0.36
2.68
0.007
Number of juveniles
+0.36
2.66
0.008
Total number of geckos
+0.45
3.48
0.0005
Number of adult males
+0.26
1.90
0.06
Number of adult females
+0.29
2.16
0.03
Number of juveniles
+0.28
2.07
0.04
Total number of geckos
+0.41
3.14
0.002
Shelter-site use by geckos: thermal regimes
Mean daily temperatures had similar variances between rocks and did not differ significantly
between used (12.0 + 0.13oC) and unused (12.2 + 0.20oC) rocks (F1,52 = 0.76, P = 0.39).
However, the range of temperatures could not be tested with ANOVA because the variance
31
Non-social influences on retreat-site selection in the laboratory
of thermal ranges for unused rocks (6.75) was significantly greater than the variance of used
rocks (1.65); (F27,27 = 0.24, P = 0.0006). Therefore, minimum and maximum temperatures
were tested for significance using nonparametric sign tests. Rocks used by the geckos had
significantly higher mean minimum temperatures and significantly lower mean maximum
temperatures compared to unused rocks, for each of the 11 days (sign tests, both significant
at P = 0.01; Fig. 3.3).
All rock dimensions; thickness (r = +0.41; Z = 3.11; P = 0.002), length (r = +0.41, Z = 3.09,
P = 0.002) and width (r = +0.49, Z = 3.79, P = 0.0002), were significantly correlated with
mean daily minimum temperatures under the 54 rocks measured. Similarly, mean daily
maximum temperatures under the same 54 rocks were significantly correlated with the three
rock dimensions measured, width (r = -0.33, Z = -2.47, P = 0.01), length (r = -0.29; Z = 2.11, P = 0.03) and thickness (r = -0.33, Z = -2.46; P = 0.01), although the correlation was
not as strong. Thus, larger rocks exhibited higher daily minimum temperatures and lower
daily maxima. However, mean daily temperatures in retreat sites were not significantly
affected by the dimensions of the rocks (thickness, r = -0.15, Z = -1.10, P = 0.27; length, r =
-0.13, Z = -0.92, P = 0.36; width, r = -0.15, Z = -1.08, P = 0.28), probably because they
simply averaged the difference between the maximum and minimum temperatures.
Mean daily minimum temperature was significantly correlated with numbers of adult males
under rocks (r = +0.29, Z = 2.13, P = 0.03) but not with numbers of adult females, juveniles
or total numbers of geckos under rocks. Mean daily minimum temperature was also
significantly correlated with the SVL of the largest (r = +0.43, Z = 2.26, P = 0.02) and
smallest (r = +0.38, Z = 1.94, P = 0.05) geckos under rocks, and also average SVL (r =
+0.44, Z = 2.29, P = 0.02) of geckos under rocks. By contrast, mean daily maximum
temperatures had no effect on either the numbers of difference age and sex classes of geckos,
32
Non-social influences on retreat-site selection in the laboratory
or the SVLs of geckos under rocks (all P > 0.05). However, application of the Bonferroni
technique moved all results beyond significance.
24
unused mean min
22
unused mean max
used mean min
20
used mean max
Temperature (oC)
18
16
14
12
10
8
6
4
14
15
16
17
18
19
20
21
22
23
24
Date in July 2001
Figure 3.3: Comparison between the means (+ standard deviation) of rocks that were either
used or not used by thick-tailed geckos at Mt Korong, Victoria.
3.3.4
Do thick-tailed geckos aggregate in the field?
Thick-tailed geckos formed aggregations that occurred more often than expected by chance
(Table 3.5). Aggregation frequencies for the different group sizes departed significantly
from the expected (Poisson) distribution (G = 62.12, 3 df, P < 0.001). Aggregations of three
or more geckos occurred significantly more than expected by chance (χ2 tests, all P < 0.05)
(Table 3.5). Bonferroni corrections did not alter the significance of results.
The sex ratio of adult geckos deviated significantly from 50:50, with more males found than
females (χ2 = 3.95, 1 df, P < 0.005). Significantly more juvenile geckos were found than
adult geckos (χ2 = 31.69, 1 df, P < 0.005; Table 3.6).
33
Non-social influences on retreat-site selection in the laboratory
Table 3.5: Observed frequencies of thick-tailed gecko aggregations at Mt Korong, Victoria,
of different size categories. The Table compares observed frequencies to expected (Poisson)
frequencies (analysed as described in Kearney et al. 2001). An asterisk (*) shows results
that are significant at P = 0.05 using χ2 tests, prior to Bonferroni corrections.
Aggregation
size
Observed
frequencies
Expected
frequencies
Deviation from
expectation
1
2
3
4
5
89
19
9
4
3
100.94
20.77
2.14
0.15
0.01
+*
+*
+*
124
124
Total
Over half the geckos found were aggregated. Adult female geckos occurred in aggregations
more often than did juveniles or males, but these differences did not attain statistical
significance (Table 3.6; χ2 = 4.31, 2 df, P = 0.12).
I also examined the composition of gecko aggregations. Observed frequencies diverged
significantly from expected (binomial) frequencies for the six possible combinations of pairs
of males, females and juveniles (χ2 = 55.65, 5 df, P < 0.001). Pairs of juveniles occurred
more often than expected by chance and adult male – juvenile and adult female – juvenile
pairs occurred less often than expected by chance (χ2 tests, all P < 0.05) (Table 3.7).
Bonferroni corrections using three categories (adult-adult, adult-juvenile and juvenilejuvenile pairs) did not alter significance. The randomisation tests showed that adult males
and juveniles occurred together less often than expected by chance (G = 6.66, 1 df, P =
0.009), and that groups including > 1 adult male and > 1 adult female occurred more often
than expected by chance (G = 7.71, 1 df, P = 0.005; Table 3.8). However, application of the
Bonferroni technique, across these tests leaves only the occurrence of groups including > 1
adult male and > 1 adult female as significantly different from the null expectation.
34
Non-social influences on retreat-site selection in the laboratory
Table 3.6: Observed frequencies of male, female and juvenile thick-tailed geckos found
aggregated under rocks at Mt Korong, Victoria. The total numbers of geckos, numbers of
solitary geckos and numbers of aggregated geckos are presented. Percentages of aggregated
geckos are also shown.
Solitary
Aggregated
Total
Percent
Aggregated
Adult Male
22
14
36
38.9
Adult Female
8
13
21
61.9
Juvenile
59
78
137
56.9
89
105
194
54.1
Sex/Age
Total
Table 3.7: Comparison between observed and expected frequencies of pairs of various
combinations of adult male, adult female and juvenile thick-tailed geckos from Mt Korong
in Victoria. As in Kearney et al. (2001), expected frequencies are calculated by binomial
expansion for comparison with all possible combinations of pairs of geckos. An asterisk (*)
shows results that are significant at P = 0.05 prior to Bonferroni corrections using χ2 tests.
Combination
Adult male – adult male
Adult female – adult female
Juvenile – juvenile
Adult male – adult female
Adult male – juvenile
Adult female – juvenile
Observed
frequencies
1
1
18
5
0
0
Expected
frequencies
1.3
3.1
4.1
4.0
4.8
7.4
Deviation from
expectation
+*
+
-*
-*
I also examined the composition of gecko aggregations. Observed frequencies diverged
significantly from expected (binomial) frequencies for the six possible combinations of pairs
of males, females and juveniles (χ2 = 55.65, 5 df, P < 0.001). Pairs of juveniles occurred
more often than expected by chance and adult male – juvenile and adult female – juvenile
pairs occurred less often than expected by chance (χ2 tests, all P < 0.05; Table 3.7), and
Bonferroni corrections using three categories (adult-adult, adult-juvenile and juvenilejuvenile pairs) did not alter significance. The randomisation tests showed that adult males
and juveniles occurred together less often than expected by chance (G = 6.66, 1 df, P =
0.009), and that groups including > adult male and > one adult female occurred more often
35
Non-social influences on retreat-site selection in the laboratory
than expected by chance (G = 7.71, 1 df, P = 0.005; Table 3.8). Application of the
Bonferroni technique did not change significance levels.
Table 3.8: Analyses comparing observed and expected frequencies of different
combinations of thick-tailed geckos at Mt Korong, Victoria. Expected frequencies are
calculated by a randomisation procedure (see Kearney et al. 2001) and compared to all
aggregations of two or more geckos. An asterisk (*) shows results that are significant at P =
0.05 using G tests, prior to Bonferroni corrections.
Observed
frequencies
Expected
frequencies
Deviation from
expectation
> 1 adult male
1
2.7
-
> 1 adult female
1
1.0
=
> 1 juvenile
31
24.6
+
> 1 adult male and > 1 adult female
9
3.1
+*
> 1 adult male and > 1 juvenile
6
13.9
-*
> 1 adult female and > 1 juvenile
6
8.5
-
> 1 adult male and > 1 adult female
and > 1 juvenile
3
2.0
+
Combination
3.4 Discussion
3.4.1
Gecko morphology
Sexual size dimorphism occurs in many animals (Andersson 1994), and frequently in lizards
(Doughty and Shine 1995; Sugg et al. 1995; Herrel et al. 1999). Thick-tailed geckos are
sexually dimorphic for head length and axillar-groin length (AGL) relative to SVL as adults.
Adult males have significantly longer heads than adult females at the same SVL, and adult
females have longer bodies (AGLs) than adult males at the same SVL. It is not possible to
determine the selective factors resulting in these differences; however, some of the possible
benefits derived are discussed briefly below. Additionally, my data do not reveal whether
there is an ontogenetic shift in sexual size dimorphism because juveniles were not measured
for characteristics other than SVL and mass, which will change as juveniles attain maturity.
36
Non-social influences on retreat-site selection in the laboratory
Many lizards are sexually dimorphic for head length, with males having longer heads than
females (Hews 1990; Andersson 1994). Increased head size may play an important role in a
number of aspects, including intrasexual interactions such as male – male combat and
territorial contests (Trivers 1976; Anderson and Vitt 1990; Bull and Pamula 1996), as well as
intersexual interactions, such as copulatory bites (Hews 1990; Herrel et al. 1996).
Dimorphism in head size may also aid in partitioning resources to reduce intersexual
competition (Stamps 1977). Male thick-tailed geckos are extremely aggressive and inflict
injuries upon each other, as well as on adult females, using their jaws as their major
offensive weapon (pers. obs.). In addition, males grasp females behind the head by biting
them during copulation, as in many other geckos (Greer 1989; pers. obs.). My data do not
allow me to evaluate whether or not these aspects of sexual behaviour are involved as
selective forces for the observed patterns of sexual dimorphism.
Adult female thick-tailed geckos tended to have longer bodies (i.e. AGL) than adult males at
the same SVL. This dimorphism is similar to that found in some lacertid lizards (Florentino
1996) and the girdled lizard Cordylus macropholis (Mouton et al. 2000). The Australian
southern leaf-tailed gecko (Phyllurus platurus) also has a similar sexual dimorphism
(Doughty and Shine 1995). This longer body size may provide a ‘fecundity advantage’
(Fitch 1978) in allowing the females to produce larger offspring. Unlike many other lizards,
most Australian geckos, including thick-tailed geckos, produce only two eggs per clutch
(Greer 1989) and therefore increased AGL cannot confer advantages in terms of greater
clutch size (Doughty and Shine 1995). The eggs laid by thick-tailed geckos are extremely
large, taking up a significant portion of the abdominal cavity (Greer 1989) so that a greater
AGL probably provides more space for eggs (Shine and Doughty 1995).
37
Non-social influences on retreat-site selection in the laboratory
3.4.2
Shelter-site use by geckos: rock dimensions
Rock-use by thick-tailed geckos was highly non-random. All of the geckos that I found
were sheltered in rock-on-rock crevices. Similarly, broad-headed snakes (Hoplocephalus
bungaroides) avoided rocks on soil (Webb and Shine 1998). In contrast, degree of shading
of rocks had no significant effect on shelter-use by thick-tailed geckos. This pattern differs
from that found for marbled geckos (Christinus marmoratus) and broad-headed snakes.
Marbled geckos selected unshaded rocks as retreat-sites in spring and shaded rocks in
summer (Kearney 2002). Similarly, broad-headed snakes actively selected thin exposed
rocks during spring, but avoided these during summer because they were too hot (> 40oC:
Webb and Shine 1998). In my study, 75% of both used and unused rocks were unshaded,
possibly reflecting a shortage of suitable shelter-sites that were shaded. Alternatively, it is
possible that during winter (the time of my own sampling), shading is of less consequence as
radiation from the sun is much reduced, and heating and cooling rates of rocks are less
dependent on the degree of shade.
Rock-use by thick-tailed geckos differed among age classes. Adult geckos generally were
found under larger rocks than were juvenile geckos. The causal basis for such an
ontogenetic shift is unclear. It is possible that the larger rocks are preferable to all geckos
irrespective of age, but that adults aggressively displace juveniles. Alternatively, this
ontogenetic difference in rock sizes used may be driven by other factors, such as thermal
consequences of rock size or the increased availability of larger and wider crevices under
larger rocks (Kearney 2002). A similar pattern of rock-use has been found with broadheaded snakes, in which juveniles used rocks that were significantly smaller than those used
by adults (Webb and Shine 1998).
Rocks that were used by thick-tailed geckos were larger (length and width) than rocks that
were unused. In addition, larger rocks generally had more thick-tailed geckos sheltering
beneath them. Aggregations of marbled geckos were also significantly more frequent under
38
Non-social influences on retreat-site selection in the laboratory
larger rocks (Kearney 2002). This pattern may be due to an active preference for larger
rocks, combined with a limited availability of such large rocks. Alternatively, the larger
crevices under these rocks may simply allow more individuals to fit into them. In addition,
large rocks may be more attractive to geckos because they are much more thermally stable,
with lower maxima and higher minima (Huey et al. 1989).
A very different organism, the marine spiny lobster (Panulirus argus) also displays this
pattern of shelter-use, with smaller lobsters using smaller shelters and larger ones using
larger shelters (Eggleston and Lipicus 1992). This pattern is interesting for comparison with
that seen in thick-tailed geckos because lobsters also changed their shelter-site preferences
according to the presence or absence of predators. In addition to this, lobsters selected larger
shelters in the presence of conspecifics, and particularly so when predators were around
(Eggleston and Lipicus 1992). Thus, lobsters selected shelter-sites not only according to
availability but also according to the numbers of conspecifics present and the presence or
absence of predators (Eggleston and Lipicus 1992). By analogy, thick-tailed geckos might
also select larger rocks when they are able to shelter with conspecifics, and smaller rocks
when solitary.
There was no difference in thickness between ‘used’ and ‘unused’ rocks, and the range of
thickness of used rocks encompassed that of unused rocks. Marbled geckos at the same site
were also found to use rocks irrespective of thickness (Kearney 2002). However, this null
result may partly reflect methodological problems. Only rocks that could be lifted by two
people were examined, perhaps generating a bias in the results. In addition, rocks were not
of uniform thickness along their length and breadth, and my measurements of thickness
(made using arbitrary rules for both used and unused rocks) may have failed to capture
subtle effects of this variation on thermal regime.
39
Non-social influences on retreat-site selection in the laboratory
3.4.3
Shelter-site use by geckos: thermal regimes
The thermal environment under a rock was related to its physical characteristics. As
expected, rock size influenced daily minimum and maximum temperatures, with larger rocks
having lower maxima and higher minima. Thick-tailed geckos, particularly larger lizards,
occurred under rocks that had higher minima and lower maxima than did ‘random’ rocks
(Fig. 3.3). Thus, the rocks used by thick-tailed geckos were more thermally stable, such that
geckos were not exposed to very high or very low temperatures. Living in such a cold
climate, with ambient temperatures falling below 4oC during winter nights (Anon. 1993 in
Kearney 2002), selecting rocks that cool down at a slower rate may enable the lizards to
maintain body temperatures high enough for the activity period during the early evening.
Furthermore, maintaining a higher body temperature for a longer period of time may
facilitate other physiological processes such as digestion and reproductive processes,
including vitellogenesis and spermatogenesis (Huey 1982).
Sheltering under rocks that have lower mean daily maximum temperatures may also be
important in ensuring survival. Rocks at Mt Korong attain maximum temperatures above
30oC occasionally during spring and regularly during summer, with maxima up to 60oC
(Kearney 2002). While there are no data available on the critical thermal maximum
(CTMax) of thick-tailed geckos, the CTMaxs of six other Australian gecko species range
from approximately 40 to 44oC (Greer 1989). In addition, thick-tailed geckos survived for
three hours at temperatures of 37.5oC but suffered high mortality in the following 24-hour
period (Licht et al. 1966). At temperatures of 40.3oC, geckos survived for approximately 25
minutes (Licht et al. 1966). Preferred body temperature (PBT) of thick-tailed geckos (in the
laboratory) during the day ranged from approximately 23 to 28oC (Greer 1989).
40
Non-social influences on retreat-site selection in the laboratory
3.4.4
Do thick-tailed geckos aggregate in the field?
Large aggregations of thick-tailed geckos occurred more often than expected by chance, with
more than half of all geckos found being in aggregations. Similarly, a previous study of
thick-tailed geckos at the same locality showed that the animals formed large aggregations
more often than expected by chance during autumn and spring, with groups of up to nine
lizards (Kearney et al. 2001). The presence of aggregations at times other than winter
therefore rules out the possibility that the geckos form groups only in winter, as occurs in
many other squamate reptiles in Australia (Powell et al. 1977).
There are many possible reasons why animals aggregate, including attraction to patches of
habitat that are of exceptionally high quality with respect to food, predator avoidance and/or
thermoregulatory opportunities. Another possibility is that individuals are actually attracted
to conspecifics. Intuition argues against the latter phenomenon because many species of
lizards may be territorial, resulting in aggressive interactions between individuals (Marcellini
1977; Henkel and Schmidt 1995), and this includes male thick-tailed geckos (pers. obs.).
However, aggregations under rocks may provide increased mating opportunities, group
defence, increased vigilance against predators, and also protection from the elements
(Stamps 1988; Elfström and Zucker 1999; Kearney et al. 2001). Conversely, a large
aggregation under a rock may result in a stronger scent trail attracting potential predators and
also local depletion of resources such as food.
Information on the age and sex composition of aggregations can also provide insights into
the reasons why groups occur. My data show that the composition of aggregations was nonrandom. Pairs of juveniles occurred more often than expected by chance, but groups of three
or more juveniles were not particularly common. The presence of large numbers of pairs of
juvenile geckos suggests the possibility that they may be siblings; clutch size is two in this
species (Greer 1989). Juvenile geckos were found less often than expected by chance with
41
Non-social influences on retreat-site selection in the laboratory
either an adult male or an adult female, or in groups that included both an adult male and an
adult female. The composition of groups during spring and autumn was also non-random,
with juvenile geckos and adult females found together less often than expected by chance
(Kearney et al 2001). However, that was the only significantly non-random pattern found by
Kearney et al. (2001). I found no evidence of avoidance between juveniles and adult
females. My data suggest that juveniles may, in fact, avoid adult males (or vice versa), as
they were rarely found together, either in pairs or as parts of larger groups. This discrepancy
in results between the two studies may be due to seasonal shifts in the tendency to aggregate.
It is possible that aggression towards juveniles changes with season and mating periods, as
occurs in at least one species of Australian skink, Egernia saxatilis (D. O’Connor, pers.
comm.).
Despite these significantly non-random patterns, it is also important to note that the
aggregations of thick-tailed geckos were very variable in composition and generally,
frequencies of each combination were close to those expected under the null hypothesis of
random co-occurrence with respect to age and sex (Table 3.8). This diversity makes it
highly unlikely that aggregations of thick-tailed geckos are ‘family groups’ of an adult male,
an adult female and their offspring.
3.4.5
Conclusions
Clearly, thick-tailed geckos used rocks in a non-random pattern. Adults occurred under
larger rocks than juveniles, although both groups used larger rocks than available overall,
and larger rocks tended to act as shelters for greater numbers of geckos. The geckos used
rocks that were more thermally stable than were most other rocks sampled on the outcrops,
with higher mean daily minima and lower mean daily maxima. These patterns of
significantly non-random rock-use suggest that limited shelter-site availability may be
responsible for, or contribute to, the aggregative behaviour seen; used rocks were longer and
42
Non-social influences on retreat-site selection in the laboratory
wider than unused rocks (Fig. 3.1). However, it is important to note that the lengths and
widths of over 75% of unused rocks fell within the range of used rocks, suggesting instead,
that while geckos used rocks non-randomly, there were still plenty of suitable shelter-sites
available. However, biases may exist within rock data arising from the methodology;
‘random’ rocks were located relative to ‘gecko’ rocks (within 10 m of each other). Sampling
unused rocks without regard to used rocks, perhaps using quadrats, may provide a more
accurate representation of the rocks available to the geckos.
Thick-tailed geckos also formed large aggregations (of up to five individuals) under rocks,
with juveniles pairing more often than expected by chance and rarely being found with adult
males.
These patterns are interesting, but cannot reveal causation. For example, it is not possible to
separate the effect of rock size from that of the temperature profile under that rock: geckos
may select large rocks for their temperature profiles rather than their size per se.
Nonetheless, these data provide the framework for further laboratory work, which is required
to clarify specific causal influences on shelter-site selection and aggregation. I will examine
these questions in later chapters.
43
Non-social influences on retreat-site selection in the laboratory
CHAPTER 4
Non-social influences on retreat-site selection in the laboratory
4.1
Introduction
Habitat requirements differ substantially among animal species and may constrain the
geographic limits of species as well as abundance within their range (Schlesinger and Shine
1994; Jones 2001; Thery 2001). Such habitat preferences are sometimes very specific. For
example, echidnas (Tachyglossus aculeatus) in south-east Queensland prefer hollow logs
and depressions under fallen tree roots as their daily shelter-sites (Wilkinson et al. 1998).
Kelp gulls (Larus dominicanus) in southern Africa nest on the ground and prefer horizontal
areas, probably to avoid eggs and chicks rolling out of their nests, with suitable shelter such
as 25 to 50% vegetation cover or protruding rocks (Burger and Gochfeld 1981). Similarly,
common brushtail possums (Trichosurus vulpecula) in New Zealand have specific habitat
requirements, preferring to den above ground in trees (Cowan 1989). The gekkonid lizard
Gehyra variegata spends a large part of its life in arboreal bark crevices less than one metre
above the ground (Bustard 1970a). Thus, while a wide range of potential shelter-sites are
typically available in any given habitat, any given animal species may only utilise specific
components of it.
The adaptive significance of specific habitat-selection ‘choices’ is sometimes clear. For
example, predator avoidance is of the utmost importance in seeking out shelter-sites
(Downes and Shine 1998a). In addition, animals require thermally suitable shelters to avoid
becoming either too hot or too cold (Huey et al. 1989; Downes and Shine 1998a). This
aspect is of particular importance for ectothermic animals, which utilise heat from their
environment for their behavioural and physiological processes (Huey 1982). Some
nocturnal ectotherms rely greatly on thermally suitable diurnal retreat-sites to achieve
44
Non-social influences on retreat-site selection in the laboratory
optimal body temperatures (Schlesinger and Shine 1994; Webb and Shine 1998; Kearney
2002).
45
Non-social influences on retreat-site selection in the laboratory
Selection of specific habitat attributes by various squamate reptiles, including lizards, has
been demonstrated experimentally in several studies (Schlesinger and Shine 1994; Downes
and Shine 1998b; Stow 1998; Webb and Shine 1998). Although diurnal species have been
the primary subjects of research, nocturnal, saxicolous taxa (including geckos) have also
attracted study. For example, southern leaf-tailed geckos (Phyllurus platurus) showed
consistent preferences with respect to crevice width, shape and height above substrate (Stow
1998). Similarly, velvet geckos (Oedura lesueurii) exhibited strong preferences with respect
to crevice width, shape, temperature, height above substrate and size of overlying rock
(Schlesinger and Shine 1994).
Subsequent studies on the same species showed that social factors may also influence habitat
selection (Downes and Shine 1998b). For example, dominant animals may drive
subordinate animals out of high-quality retreat-sites (Downes and Shine 1998b). Before
investigating social influences on shelter-site selection, we need to have a context for such
studies.
Natural habitats are highly complex and it is difficult to separate variables from one another
to identify the specific cues influencing shelter-site selection. Thus, I conducted a series of
experiments in the laboratory to determine the habitat preferences of thick-tailed geckos with
respect to crevice height, size, slope and temperature, thickness of overlying rock and the
degree of protection provided by the shelter. These preferences may restrict the habitat
available to the geckos, resulting in increased social interactions and possibly aggregations.
That is, thick-tailed geckos might frequently occur in groups not because they are ‘social’,
but because they are highly selective with respect to (relatively scarce) physical attributes of
retreat-sites. Conversely, habitat preferences or restrictions may not drive either social
interactions or aggregations. I also used these initial studies to identify the type of shelters
45
Non-social influences on retreat-site selection in the laboratory
preferred by the geckos. Knowledge of these preferences enabled me to design subsequent
experiments to investigate social behaviour in the geckos.
4.2
Materials and Methods
Each of 25 plastic nally bins (57 x 36 x 19.5 cm) was divided into two using pieces of
Styrofoam covered with plastic and sealed with silicon. Thus, each bin provided two
separate experimental units (36 x 28 x 19.5 cm; Fig. 4.1). Five nally bins (10 experimental
units) were placed on each of five shelves in a room maintained at 20oC. Opaque covers
with nine ventilation holes (5 mm in diameter) were placed over each bin to ensure that
geckos could not escape, and to eliminate extraneous visual cues.
The aim of the experiment was to clarify shelter-site preferences by providing two
alternative cues and allowing each gecko to make a choice between them. The null
hypothesis for each experiment was that adult and juvenile thick-tailed geckos would show
no preference for either of the two alternative cues provided. Thus, I used two shelters per
experimental unit, differing with respect to a single cue. Ceramic tiles (11 cm x 11 cm; 11
mm thick) were used as substrates for the two shelters within each experimental unit. The
tiles were placed at opposite ends of the unit for each experiment. Washed beach sand was
added and built up to the same height as the tile (11 mm) (Fig. 4.1). Tiles were also used to
form the upper part of each shelter, with their height above the lower tile determined by the
size of plastic spacers forming three ‘legs’ to hold them up. Shelter height was 12 mm for
all experiments, unless stated otherwise. Thus, each experimental unit contained two
potential shelter-sites, both consisting of the gaps between two ceramic tiles.
For each experiment, I used 50 geckos: 26 juveniles, 12 adult males and 12 adult females
(unless stated otherwise). Only one gecko was placed in each experimental unit at a time.
These animals were chosen randomly from the available 60 juveniles, 24 adult males and 19
46
Non-social influences on retreat-site selection in the laboratory
Figure 4.1: Experimental set-up used to investigate shelter-site selection by thick-tailed
geckos. The plastic nally bin (57 x 36 x 19.5 cm) was divided by a foam partition to provide
two experimental units (36 x 28 x 19.5 cm). Each unit had washed beach sand as a substrate,
and contained two alternative shelter-sites (tiles separated by plastic spacers). Only the
upper tiles are visible in this photograph.
adult females. Where two different shelter types were used, the positions (left or right) of
these were also randomised, as was the order in which the animals were put into the
experimental units.
Experiments were conducted between mid August and late November 2001. Each
experiment was set-up in the afternoon and animals were placed in the experimental units
between 1600 and 1930 h in the evening. They were left in the experimental units overnight,
and removed between 0830 and 1100 h the following morning. In the morning, the shelter
under which each gecko was resting was noted. If a gecko had not selected a shelter at this
time, it was excluded from the results. Animals were given at least one nights’ rest between
successive experiments. Each gecko was only used once per experiment. Following each
experiment, all tiles were soaked in hot soapy water for a minimum of 30 minutes. They
47
Non-social influences on retreat-site selection in the laboratory
were then vigorously scrubbed, soaked for another 30 minutes in hot water, rinsed and airdried.
Data on shelter-site use by adults and juveniles were analysed separately using Pearson’s
Chi-square tests. Adults and juveniles were considered separately as they may have different
preferences, resulting from factors such as body size and sexual maturity. I used
contingency table analyses to test if there was a significant difference between responses of
adults versus juveniles. If no difference was found, the data were pooled and also tested
against the null hypothesis of no overall shelter-site selection, using Chi-square tests.
Sequential Bonferroni corrections were applied, using adult, juvenile and combined data as
three separate data sets. However, due to their subjective nature as discussed in Chapter 3,
uncorrected values are reported but any cases where a change in significance occurs are
pointed out.
I conducted several separate experiments to test the role of various shelter characteristics in
retreat-site selection. These attributes were: crevice height, thickness of overlying rock,
crevice size, crevice slope, predator scent, temperature, and degree of closure of crevice.
4.2.1
Crevice height
This experiment was designed to test whether the lizards would select a crevice barely large
enough for them to squeeze into versus one that provided additional height. Thus, the first
step was to select crevices of appropriate heights relative to gecko body sizes. For juveniles,
the narrow crevice was 8 mm high and the wide crevice was 12 mm high. The narrow
crevice for adults was 12 mm high and the wide crevice was 18 mm high. In each case, all
animals could squeeze into the narrow crevice and easily fitted into the wide crevice. The
experiment was conducted over two days, with juveniles being tested on the first day and
adults on the second day.
48
Non-social influences on retreat-site selection in the laboratory
4.2.2 Thickness of overlying rock
The aim was to test whether geckos chose to shelter in a crevice overlaid by a thicker or
thinner upper rock (or in this case, tile). To simulate this situation, I set up shelters with
either three tiles above the crevice (i.e. 33 mm thickness of overlying rock) or one tile above
the crevice (11 mm thickness of overlying rock). The experiment was conducted over two
days, with 25 geckos (both adults and juveniles) being tested on the first day, and 25 on the
following day.
4.2.3 Crevice size
I provided a choice of shelter-sites differing in the overall size (length x width) of the
overlying rock. To create this difference, the large shelter was constructed of two substrate
plus two shelter tiles placed side by side (22 x 11cm) while the small shelter remained the
same as in previous experiments (11 x 11 cm). This experiment was also conducted over
two days, with 20 geckos being tested on the first day and 30 geckos on the following day.
4.2.4
Slope of crevice
The aim was to compare the lizards’ use of crevices that were either horizontal or steeply
sloping (as frequently occurs in crevices under rocks in the field). Wedges made of
Styrofoam were put under one substrate tile in each experimental unit, with sand built up
around it, producing a slope of between 35o and 40o. The trials were conducted over two
days, with the first 25 geckos (both adults and juveniles) being tested on the first day, and
the remaining 25 the next day.
4.2.5
Crevice thermal regime
The aim of this experiment was to determine if thick-tailed geckos use thermal cues in
shelter-site selection. Thermal film (140 mm wide, 65 watts/m, Thermofilm Australia Pty.
Ltd.) was laid along one edge of each of five wooden boards. Each board was connected to a
49
Non-social influences on retreat-site selection in the laboratory
dimmer switch, allowing fine temperature control. One board was then placed on each of
the five shelves on which experimental units were placed. The side on which the thermal
film was placed was alternated to ensure that ‘hot’ shelters were closest to the wall in half
the units and furthest from the wall in the other half. Nally bins were then placed sideways
(longer side against the wall) on the shelves so that one shelter in each experimental unit was
placed on top of the thermal film.
The temperature of the room was reduced to 16 oC. The substrate tile, sand and shelters were
then placed in each experimental unit as above. The temperature of the thermal film was
adjusted until all substrate tiles on the thermal film were between 25 and 30oC (as measured
by a Raytek infrared thermometer). All substrate tiles not on the thermal film were between
16 and 19 oC, generating a choice of ‘hot’ and ‘cold’ shelter-sites for each gecko.
Experiments were run over two days, with 30 animals (both adults and juveniles) being
tested on the first day, and the remaining 20 on the following day.
4.2.6 Predator scented crevice
To examine whether geckos actively avoid shelter-sites that contain the scent of a predatory
snake, I used scent from a captive red-bellied black snake (Pseudechis porphyriacus,
Elapidae). This species is broadly sympatric with thick-tailed geckos in southern Australia
(Cogger 2000) and consumes a wide variety of lizards, including geckos (Shine 1977, 1999).
Substrate tiles were exposed to snake scent by placing them inside the cage of the snake.
However, the tiles used as the tops of the shelters were unscented, to mimic conditions likely
to be encountered by lizards in the field. Because only 20 tiles could fit into the enclosure of
the snake at any one time, the experiment was conducted on three different days. For each
set of trials, the tiles were placed in the home cage of the snake for a week in order to pick
up its scent. I then removed the tiles, wearing a pair of disposable gloves to prevent transfer
50
Non-social influences on retreat-site selection in the laboratory
of human scent to the tile, and placed them in the experimental units. The next set of tiles
was then placed in the snake’s cage for a week.
4.2.7
‘Protected’ crevice
I designed this experiment to clarify whether or not geckos would select shelter-sites that
offered a higher degree of concealment (i.e. a small rather than a large degree of visual
exposure of a lizard within a crevice). To achieve this effect, the gap beneath one shelter in
each experimental unit was covered on three-and-a-half sides with strips of manila folder
attached with Blu-Tack (Bostik, Australia), leaving an opening of 5.5 cm at the front of the
shelter. The other shelter was left with all sides uncovered, as in previous experiments.
Experiments were run over a period of two days, with 26 animals (both juveniles and adults)
being tested on the first day and the remaining animals on the next day.
4.2.8
‘New’ vs. ‘old’ shelter
Based on results from the above trials (see below), it became apparent that changes in shelter
design might substantially enhance my ability to detect other influences – especially social
influences – on retreat-site selection. The rationale for this decision was that I needed to
simulate the conditions within natural shelter-sites as closely as possible in order to evaluate
social influences on retreat-site selection.
Thus, the aim of this experiment was to improve the design of the experimental units and to
incorporate the preferences shown by the geckos in previous experiments. Square, plastic
pot plant trays (13 cm x 13 cm x 2.2 cm; Fig. 4.2) were used as the ‘new’ shelters. Each tray
had a 4 cm-wide ‘door’ cut out 1 cm from a corner. ‘Old’ shelters were ceramic tiles (as
used in the trials described above) with a crevice height of 18 mm. This experiment was
also conducted over a period of two days with 26 animals (both juveniles and adults) being
tested on one day and the remainder the following day.
51
Non-social influences on retreat-site selection in the laboratory
Figure 4.2: ‘New’ plastic shelter (13 x 13 x 2.2 cm) adapted from square plastic pot plant
trays with a 4 cm wide ‘door’ cut in one side.
4.3
Results
The thick-tailed geckos were highly selective for six out of the eight shelter item traits
examined (Table 4.1; Fig. 4.3):
1) Both adult and juvenile geckos showed a highly significant preference for a narrow
crevice rather than a wide crevice.
2) Both adult and juvenile geckos showed a significant preference for a horizontal shelter
rather than one that sloped, with a gradient of 35 to 40o.
3) Both age classes of lizards also showed a highly significant preference for higher (26 to
30oC) rather than lower temperatures (16 to 19oC).
4) Adult geckos showed significant avoidance of predator scent, whereas the juveniles
showed a similar but not statistically significant avoidance. When data for all geckos
were combined, contingency table analysis revealed no significant difference (χ2 = 1.21,
1 df, P = 0.27) between adults and juveniles, and a strong overall avoidance of snake
scent (χ2 = 9.0, 1 df, P < 0.005).
5) Both adults and juveniles showed highly significant preferences for ‘protected’ rather
than ‘unprotected’ shelters.
6) Both age classes also showed a highly significant preference for the ‘new’ protected
plastic shelters rather than the ‘old’ unprotected ceramic tile shelters (Table 4.1; Fig.
4.3).
52
Non-social influences on retreat-site selection in the laboratory
Table 4.1: Results of shelter item trait preferences of adult and juvenile thick-tailed geckos.
(*χ2 crit (P = 0.05, df = 1) = 3.841, **χ2 crit (P = 0.005, df = 1) = 7.879, NS = not significant). All
significant values (P < 0.05) prior to Bonferroni corrections are shown in boldface font.
Note that in some cases the total sample size is < 50 because one or more geckos did not
select a shelter, and were therefore excluded from the results.
Shelter item
trait
Crevice height
Alternate
shelters
available
Preferred shelter
- Narrow
crevice
- Wide
crevice
Narrow crevice
Result
χ2 Values and
significance
23/26 juveniles
20/23 adults
43/49 total
15.39**
9.78**
26.47**
13/26 juveniles
15/24 adults
28/50 total
0.00 NS
1.50 NS
0.72 NS
9/26 juveniles
14/24 adults
25/50 total
1.96 NS
0.67 NS
0.00 NS
- Thick ‘rock’
- Thin ‘rock’
No preference
shown
Crevice size
- Large
crevice
- Small
crevice
No preference
shown
Slope of crevice
- Sloping
crevice
- Horizontal
crevice
Horizontal crevice
20/24 juveniles
19/24 adults
39/48 total
10.67**
8.17**
18.75**
Crevice thermal
regime
- High
temperature
- Low
temperature
Crevice at high
temperature
23/26 juveniles
20/24 adults
43/50 total
15.39**
10.67**
25.92**
Predator scented
crevice
- Scented
crevice
- Unscented
crevice
Unscented crevice
17/26 juveniles
18/23 adults
35/49 total
2.46 NS
7.35*
9.00**
‘Protected’
versus
‘unprotected’
crevice
- Protected
crevice
- Unprotected
crevice
Protected crevice
25/26 juveniles
19/23 adults
44/49 total
22.15**
9.78**
31.04**
‘New’ versus
‘old’ shelter
- New crevice
- Old crevice
New crevice
26/26 juveniles
21/23 adults
47/49 total
26.00**
15.08**
41.37**
Thickness of
overlying ‘rock’
(proportion of geckos in
thick shelter shown)
(proportion of geckos in
large shelter shown)
53
110
100
90
80
70
60
50
40
30
20
10
0
*
* *
Height
Height
(narrow)
Thickness
Thickness
(none)
(no preference)
*
* *
*
* *
Slope
Thermal
sSize
Slope (horizontal)
Thermal regime
(no preference)
(horizontol)
(warm)
Shelter item trait
*
*
Predator scent
scent
Predator
(unscented)
(unscented)
*
*
*
Protected
Protected vs
vs
unprotected
unprotected
(protected)
(protected)
*
*
*
'New'
vs 'old'
'old'
New' vs
(new)
('new')
juveniles
adults
total
Figure 4.3: Percentage of adult and juvenile thick-tailed geckos that selected shelter items based on various traits (preferences shown in brackets: narrow,
horizontal, high temperature, unscented, protected and ‘new’ shelters). The dotted line indicates the percentage of geckos expected per shelter under a null
hypothesis of no retreat-site selection. An asterisk (*) shows cases that display a bias strong enough for statistical rejection of the null hypothesis prior to
the application of Bonferroni corrections.
Percentage of geckos showing preference
Social influences on retreat-site selection in the laboratory
However, no preference was evident for the other two aspects of shelter-sites that I
manipulated (Table 4.1; Fig. 4.3):
1) Neither adults nor juveniles showed any significant preference for thick shelters over
thin shelters.
2) Neither adults nor juveniles showed any preference for large shelters over small shelters.
Contingency table analyses revealed no significant differences between responses of adult
and juvenile geckos (P > 0.05 for all tests). In addition, sequential Bonferroni corrections
did not change any significance values.
4.4
Discussion
Previous studies on two gecko species concluded that these lizards are highly selective with
respect to the attributes of their diurnal retreat-sites (Schlesinger and Shine 1994; Downes
and Shine 1998b; Stow 1998). My study further supports the notion that saxicolous
nocturnal gekkonid lizards are able to evaluate a range of physical characteristics of potential
retreat-sites, including crevice height, presence of predator scent, gradient of crevice,
temperature within shelter and degree of cover or protection around the shelter. The retreatsites selected by thick-tailed geckos probably function mostly to avoid predators and to
allow for thermoregulation (Schlesinger and Shine 1994). Below, I will first discuss
attributes important in retreat-site selection, and then consider aspects of the crevice that
appear to be unimportant for the lizards.
4.4.1 Crevice height
Thick-tailed geckos showed a strong preference for narrow shelters (just large enough for
them to fit into) rather than wider crevices in which there was some space between their
bodies and the overlying shelter. Other saxicolous gecko species (southern leaf-tailed
geckos, Phyllurus platurus, and velvet geckos, Oedura lesueurii) also select narrow, rather
55
Social influences on retreat-site selection in the laboratory
than wide crevices (Schlesinger and Shine 1994; Doughty and Shine 1995; Stow 1998). The
narrower the crevice, the less likely it is that a predator will be able to gain access to the
lizard. Protection against predators may well have been an important selective force for the
geckos, resulting in a strong preference for these structural features. However, narrow
crevices may also allow for better thermoregulatory opportunities, as the gecko is able to
gain heat directly from the overlying rock by conduction (Bustard 1967; Kearney and
Predavec 2000).
4.4.2
Crevice thermal regime
My results suggest that thick-tailed geckos actively thermoregulate by selecting warm rather
than cold retreat-sites. This behaviour allows the geckos to maintain higher body
temperatures, thus enhancing their ability to avoid predators and allow physiological
processes such as digestion to continue (Downes and Shine 1998b). However, the thermal
regimes presented in my experiments may mimic natural conditions rather poorly. In the
wild, rocks that heat up during the day would cool down throughout the night,
approximating ambient temperature by dawn. In the field, small or thin rocks may heat up
more quickly and attain higher temperatures, but would also lose heat faster throughout the
night due to a larger surface area to volume ratio (Huey et al. 1989; Rock et al. 2002).
Although large rocks would cool more slowly at night, they would also heat up more slowly
during the day and thus, attain lower temperatures (Werner 1990). Therefore, by dawn few,
if any, of the rocks would have temperatures as high as provided in my experiments (26 30oC; see Chapter 4 for data on rock temperatures in the field).
Nonetheless, thermal cues may still be available in natural habitats. Many geckos have peak
activity within the first three hours after dusk, after which they select their retreat-site for the
following day. This pattern has been observed in nocturnal house geckos (Gehyra
variegata), stone geckos (Diplodactylus vittatus) (Bustard 1967, 1968) and velvet geckos
(Oedura lesueurii) (Schlesinger and Shine 1994). During this time, rocks may retain a
56
Social influences on retreat-site selection in the laboratory
significant amount of the heat gained during the day. Thus, significant thermal differentials
may persist for long enough to influence rock selection.
4.4.3 Predator scented crevice
Velvet geckos (Oedura lesueurii) and western banded geckos (Coleonyx variegatus) are both
able to detect predators using chemical cues (Dial et al. 1989; Downes and Shine 1998a).
Avoidance of predators was found to be a higher priority than thermoregulation or social
cues for retreat-site selection by velvet geckos (Downes and Shine 1998a, b). Similarly, the
presence of predator scent under a shelter resulted in avoidance of that shelter by adult thicktailed geckos in my own study. While juvenile geckos did not show statistically significant
avoidance of predator scent, contingency table analyses revealed no significant difference
between the shelter-site selection of adults versus juveniles. When the data were combined
for juveniles and adults, they revealed a highly significant overall avoidance of predator
scent. Avoiding areas used by predators confers obvious survival advantages to the geckos.
4.4.4 ‘Protected’ shelter
Thick-tailed geckos showed a strong preference for retreat-sites where the lizards were more
effectively concealed, either by strips of Manila paper around a ceramic tile, or by the sides
of a plastic pot plant tray with a small opening cut in one side. Forty-four and 47 out of 49
individuals respectively chose these protected shelters over the unprotected ones. Such a
choice may have survival advantages to the lizard. If the opening to the retreat-site is small,
then a predator may be less likely to encounter it, or to be able to enter. Additionally, less
light penetrates such a shelter and the lizard may therefore be less likely to be detected
visually by diurnal predators. In addition, such a shelter may provide protection from the
elements. For example, wind and rain (water) are less likely to penetrate a shelter with a
small opening that a shelter with a large opening.
57
Social influences on retreat-site selection in the laboratory
4.4.5 Thickness of overlying rock
Neither the adults nor the juveniles showed any preference for a thick over a thin shelter.
These results are similar to those in the field, where there was no difference in thickness of
rocks that were used and unused by thick-tailed geckos (Chapter 3). In the field during
winter, marbled geckos (Christinus marmoratus) are often found under relatively thin rocks
(Kearney 2002), perhaps because such rocks warm up at a faster rate and thus allow the
animals to attain high body temperatures. Thick-tailed geckos are also found under
relatively thin rocks during winter (4.52 + 0.29 [SE] cm, Chapter 3). Larger rocks do not
warm up as much at this time, and thus may not be as suitable for diurnal shelter-sites.
During summer, however, thin rocks heat up very quickly, potentially resulting in lethally
high temperatures beneath them (Kearney 2002). Under such conditions, the geckos shelter
under thicker rocks with less extreme temperature swings (Kearney 2002). Because of this
seasonal variation in the relationship between rock thickness and crevice temperature, lizards
may benefit by selecting retreat-sites based on temperature per se rather than on the less
reliable cue of rock thickness.
4.4.6 Crevice size
Neither adult nor juvenile geckos showed any preference for large over small shelters. This
result is contrary to the results of shelter-site usage by these geckos in the field where they
selected the larger (width and length) of the rocks available (Chapter 3). Similarly, a
previous study documented a significant preference for larger shelters in velvet geckos
(Oedura lesueurii) (Schlesinger and Shine 1994). However, considerably larger shelters
were offered than in my experiment, with geckos selecting large rocks that were 363 cm2
over smaller ones that were 181 and 121 cm2 (large rocks in my experiment were 242 cm2
and small rocks were 121 cm2). Interestingly, once rocks reached a certain size, velvet
geckos showed no preference for those that were larger still (Schlesinger and Shine 1994).
These results suggest that there is a certain threshold of rock size that is preferred by geckos.
58
Social influences on retreat-site selection in the laboratory
A similar study on marbled geckos (Christinus marmoratus) also showed a significant
preference for larger shelters (M. Kearney, unpubl. data). Again, the large shelters used (1
m x 1 m) were five times the size of the small shelters (20 cm x 20 cm) (M. Kearney, pers.
comm.). However, when both the large and small shelters had their sides covered with only
small openings to allow access, the marbled geckos showed no preference for large rocks
over small rocks (M. Kearney, pers. comm.). Thus, the reduced light levels under a large
shelter item may have been the factor that induced a preference for these shelters in
Kearney’s lizards, as well as the greater chance of a lizard encountering the large crevice
simply due to its size.
4.4.7 Slope of crevice
Both adult and juvenile thick-tailed geckos showed a preference for horizontal shelters rather
than shelters with a 35 – 40o slope. In the field, thick-tailed geckos are frequently found
sheltering under rocks in areas that are sloping, often steeply. This slope may potentially
provide benefits such as better drainage or better thermal properties due to increased or
decreased exposure to direct sunlight, as well as convective heat exchange. However, these
results suggest that the occurrence of the geckos under such rocks may be a secondary factor
arising from the greater incidence of suitable rocks in sloping areas. By contrast, velvet
geckos (Oedura lesueurii) are found in both horizontal and vertical crevices in the field and
showed no preference for either in the laboratory (Schlesinger and Shine 1994).
4.4.8 Conclusions
My results show that thick-tailed geckos are highly selective with respect to shelter-site
selection. They showed a strong preference for narrow, horizontal, protected shelters that
had high temperatures (26 - 30oC). The lizards also avoided shelters that contained the scent
of a predator (red-bellied black snake, Pseudechis porphyriacus). However, the lizards did
not select shelters on the basis of size or thickness of overlying rocks. Instead, lizards
59
Social influences on retreat-site selection in the laboratory
appear to select shelter-sites based on physical conditions inside crevices. Thus, two
important criteria that thick-tailed geckos used in selecting a diurnal retreat-site in my
experiments were the degree of protection and the thermoregulatory opportunities provided
by the shelter.
This selectivity offers a potential explanation for the aggregations of thick-tailed geckos in
the field. That is, optimal retreat-sites may be rare, concentrating most of the geckos within
an area into a few sites.
However, in the field, over 80% of used rocks had widths and
lengths that fell within the range of unused rocks, suggesting that shelter-sites were not
limited. Conversely, due to the large number of variables inter-correlated in the field, it is
not possible to determine if, in fact, this is the case. To determine whether or not limited
shelter-site availability is a sufficient explanation for aggregation in Underwoodisaurus
milii, we need to not only examine field data (Chapter 3), but also to conduct laboratory
experiments on the role of social cues in shelter-site selection (Chapter 5).
60
Social influences on retreat-site selection in the laboratory
CHAPTER 5
Social influences on retreat-site selection in the laboratory
5.1 Introduction
Social behaviour is well documented for many animals. However, the scientific literature on
this topic reveals strong taxonomic biases. For example, mammals and birds have received
considerable study in this respect. By contrast, squamate reptiles (lizards and snakes) have
generally been considered as socially ‘primitive’ animals with a limited behavioural
repertoire (Brattstrom 1974). Recent studies have forced a re-evaluation of the concept that
squamate reptiles have ‘simple’ social systems. For example, aggregations of some lizard
species occur throughout the year and may provide social advantages such as easier access to
mates, protection against predation through increased vigilance, group defence, greater
resource accessibility as well as protection from the elements (Stamps 1988). The South
African armadillo lizard (Cordylus cataphractus), for example, occurs in aggregations all
year round, with 85% of all lizards collected being in groups of two or more (Mouton et al.
1999). Long-term monogamy has been reported in shingleback lizards (Scincidae: Tiliqua
rugosa) (Bull 1988; Bull et al. 1998). Many lizards are known to discriminate between
conspecifics and recognise members of the opposite sex (Cooper and Trauth 1992) as well as
individuals (Steele and Cooper 1997; Bull et al. 1999; Bull et al. 2000). However, probably
the most remarkable social behaviour yet reported in reptiles is that found in the Australian
scincid genus Egernia. Genetic evidence has shown that the gidgee skink (Egernia stokesii)
lives in family groups consisting of a breeding pair, offspring from two or more cohorts, and
also related adults (Gardner et al. 2001).
Studies on gecko social behaviour have been more limited. In 1943, Greenberg recorded
aggregation behaviour during summer (breeding season) in western banded geckos
(Coleonyx variegatus) under laboratory conditions. Australian eyed geckos (Oedura
61
Social influences on retreat-site selection in the laboratory
occelata) (now O. monilis) (Bustard 1971) and Bynoe’s geckos (Heteronotia binoei)
(Bustard 1970b) were found to occur frequently as adult male – female pairs in the wild, and
pairs of a single adult with a juvenile were also common in Bynoe’s geckos (Bustard 1970b).
Western banded geckos aggregated under controlled laboratory conditions, perhaps because
of limited burrow availability (Cooper et al. 1985).
More recently, marbled geckos (Christinus marmoratus) and thick-tailed geckos were shown
to aggregate in the wild more often than would be expected under a model of random
assortment (Kearney et al. 2001). Aggregations of as many as nine thick-tailed geckos and
10 marbled geckos were found (Kearney et al. 2001), although there are anecdotal reports of
as many as 20 thick-tailed geckos within an aggregation (M. Kearney, pers. comm.). Nonrandom combinations of male, female and juvenile thick-tailed geckos were found, with
adult females rarely occurring with juveniles and juvenile pairs being found frequently
(Kearney et al. 2001). While possible reasons for these aggregations were proposed, the
determination of causal relationships was beyond the scope of Kearney et al.’s (2001) study.
Thus, while several studies have shown that lizards aggregate, these reports have generally
been based on only a small number of lizard taxa. In particular, large viviparous Australian
skinks (genera: Egernia and Tiliqua) have attracted most study. Apart from over-winter
aggregations of inactive animals (Rawlinson 1974, 1975), the only other Australian lizard
taxon recorded quantitatively to aggregate frequently in large numbers is the thick-tailed
gecko (Kearney et al. 2001).
Few studies on reptile aggregation have determined whether or not this behaviour occurs as
a result of limited shelter-sites, or identified whether individuals show specific attraction or
avoidance behaviour to particular age and sex classes of conspecifics. The social behaviour
of juvenile lizards has been largely ignored. Consequently, I conducted a series of
62
Social influences on retreat-site selection in the laboratory
laboratory experiments based on shelter-site selection to determine whether aggregation still
occurs when enough shelter-sites are provided for individual retreats, and to investigate
attraction and/or avoidance behaviour between different age and sex classes of thick-tailed
geckos. The presence of such behaviours between geckos may determine the size and
composition of lizard aggregations in the wild, and may help explain their incidence.
5.2
Materials and Methods
The aims of these experiments were to determine:
1) whether or not thick-tailed geckos aggregate even when surplus shelter-sites are
available;
2) whether the tendency to form such aggregations differs among sex and age groups;
3) whether aggregation within this species is driven by specific attraction or avoidance
behaviours between particular age and sex classes of geckos (juveniles, adult males
and adult females); and
4) what kinds of cues stimulate aggregative behaviour.
Experiments were set up as in Chapter 4, with 25 nally bins (57 x 36 x 19.5 cm) each divided
into two to provide 50 (small) experimental units (36 x 28 x 19.5 cm). Several of the
experiments using adult geckos were conducted in full nally bins (large experimental units)
to provide more distance between shelters; my aim with this modification was to ensure that
the geckos perceived them as separate shelters. Most experiments were conducted using
square, plastic pot plant trays (13 cm x 13 cm x 2.2 cm) as shelters. Each tray had a 4 cm
wide ‘door’ cut out 1 cm from a corner (see Fig. 4.2, Chapter 4). These plastic trays were
used in preference to the ceramic tile shelters (see Chapter 4) employed previously because
they better approximate natural conditions encountered by the lizards, and were actively
selected by lizards offered a choice between the ‘old’ and ‘new’ types of shelter (see Chapter
63
Social influences on retreat-site selection in the laboratory
4). To ensure comparability with previous experiments, I also conducted a few initial
experiments using the ‘old’ ceramic tile shelters.
Experiments were conducted between October 2001 and February 2002. As in Chapter 4,
each experiment was set up in the afternoon and animals were left in experimental units
overnight (from 1600 – 1930 h in the evening to 0830 – 1100 h in the morning). In the
morning, I recorded the shelter under which each experimental gecko was resting. If a gecko
had not selected a shelter at the time of checking, that lizard was excluded from the results.
Lizards were given at least one nights’ rest between successive experiments and each gecko
combination was only used once per experiment. The maximum possible sample sizes were
used for each experiment. However, in some cases, the number of geckos used was limited
by availability; any sick or injured animals were not used for experiments until completely
recovered. Following each experiment, all shelters were soaked in hot soapy water for a
minimum of 30 minutes. They were then vigorously scrubbed, soaked for another 30
minutes in hot water, rinsed and air-dried.
Data collected for each experiment were tested against null hypotheses using Pearson’s Chisquare tests. I also conducted contingency table analyses to determine whether there was
any difference in response between adult and juvenile geckos when subjected to the same
experimental conditions. As mentioned in Chapters 3 and 4, Bonferroni corrections were
applied to the three sets of data collected. Again, due to the arbitrary and subjective
judgements required, uncorrected values are reported, although cases where the technique
results in a shift in significance are pointed out.
5.2.1
Do geckos aggregate when provided with ceramic tile shelters?
The following experiments were conducted using small experimental units (unless stated
otherwise) with ceramic tile shelters (11 x 11 cm; tiles separated by plastic spacers) and
64
Social influences on retreat-site selection in the laboratory
washed beach sand as a substrate. The null hypothesis for each experiment was that the
geckos would neither aggregate nor avoid each other; Table 5.1 provides a brief summary of
the experimental combinations and set-ups used.
Table 5.1: Summary of experimental set-up and combinations for trials using ceramic tiles
as shelters. Substrate and overlying tiles were separated by plastic spacers. Washed beach
sand was used as a substrate.
Experimental
geckos
Juveniles
Adults
Geckos
tested
Size of
experimental
unit
Shelter
size
(cm)
Shelter
height
(mm)
Other
alterations
Sample
size
Familiar housed
together
Small
11 x 11
12
None
27
Unfamiliar housed apart
Small
11 x 11
12
None
28
Small
11 x 11
12
None
10
Small
22 x 11
18
None
10
Large
22 x 11
18
Front of shelter
covered with
cardboard
10
18
3.5 sides covered
with strips of
manila folder,
staggered ‘doors’
9
Familiar housed
together
(9 male –
female pairs
and 1 male –
male pair)
Small
5.2.1.1
22 x 11
Juveniles
I
Do juveniles aggregate with familiar conspecific
juveniles?
The aim of this experiment was to ascertain whether juvenile geckos that were captured
under the same rock and subsequently housed together (‘familiar’), would aggregate when
offered a choice of identical shelter-sites. Small experimental units were used. Shelter
height was 12 mm because all geckos could fit under these and my previous experiments
showed that geckos preferred narrow crevices (Chapter 4). Twenty-seven pairs of juvenile
65
Social influences on retreat-site selection in the laboratory
geckos were tested. Each pair of lizards was placed at the centre of each experimental unit
simultaneously in the evening, and checked the following morning.
II
Do juveniles aggregate with unfamiliar conspecific
juveniles?
This experiment aimed to determine whether juvenile geckos that had been caught under
different rocks and subsequently housed separately (‘unfamiliar’), would aggregate when
offered a choice between identical shelter-sites. The experiment was set up as above, using
28 randomly assigned pairs of juvenile geckos. Each pair of geckos was placed in the
experimental bins at the same time in the evening, and removed the following morning.
5.2.1.2
Adults
I
Do adults aggregate with familiar conspecific adults?
The aim of this experiment was to determine whether pairs of adults that had been caught
and subsequently housed together (‘familiar’) would aggregate when offered a choice
between identical shelter-sites. Ten pairs of adults were used, nine of which were male –
female pairs, the other being a male – male pair (except for the fourth trial where one male –
female pair was excluded due to illness). These were the combinations in which pairs of
adults were caught, and the experiment aimed to clarify whether familiarity of geckos
resulted in aggregation. I repeated the experiment using several different experimental setups for this part of the study to ensure that the geckos perceived each shelter as being
separate from the other (field data showed that adults preferred larger rocks, which are
probably also further away from each other).
The first set-up was as for juveniles, in a small experimental unit with two separate ceramic
tile (11 x 11 cm) shelters, with a height of 12 mm. The second was also in a small unit, but
using two tiles side by side to create two large shelters (22 x 11 cm) and with a shelter height
of 18 mm to ensure all gravid females could also fit comfortably. The third experimental
66
Social influences on retreat-site selection in the laboratory
set-up involved using a large unit with two large shelters (two tiles side by side, 22 x 11 cm;
18 mm high) on either end. In addition, strips of cardboard were cut out (22 x 2 cm) and
attached to the front of the shelter so that geckos could not see each other from opposite
shelters. In the last set-up, I used small units with two large shelters (22 x 11 cm). Strips of
manila cardboard (57.5 x 2 cm) were used to cover the sides of the shelter, leaving an 8.5 cm
opening on either the left or the right side of the front of each shelter. This staggering of
openings ensured that geckos in separate shelters were not able to see each other.
5.2.2
Do geckos aggregate when provided with ‘new’ plastic shelters?
These experiments were conducted in a similar fashion to those in section 5.2.1.1, with a
ceramic tile as the substrate within the shelter and washed beach sand as a substrate in the
rest of the experimental unit. However, ‘new’ square plastic pot plant trays (13 cm x 13 cm
x 2.2 cm) were used as shelters (see Fig. 4.2, Chapter 4). As stated above, these shelters
were used because they more closely approximate natural conditions encountered by the
thick-tailed geckos in the field, and were preferred by geckos in the laboratory (see Chapter
4). I used small experimental units for experiments with juvenile geckos, and large
experimental units for all experiments with adult geckos and also for experiments with both
adults and juveniles. Once again, the null hypothesis for each experiment was that the
geckos would neither aggregate nor avoid each other when offered a choice between
identical shelter-sites. Where I assigned pairs of geckos randomly, it was regardless of
whether the lizard was housed in a pair or individually. See Table 5.2 for a brief summary
of experimental combinations and set-ups used.
5.2.2.1
Juveniles
I
Do juveniles aggregate with familiar conspecific juveniles?
This experiment was used to determine whether familiar geckos aggregated with the more
secure, ‘new’ shelters. Twenty-eight pairs of geckos were tested.
67
Social influences on retreat-site selection in the laboratory
Table 5.2: Summary of experimental set-ups used with ‘new’ plastic shelters (13 cm x 13
cm x 2.2 cm). Sand was used as a substrate in each experimental unit and a ceramic tile was
used as a substrate within each shelter.
Size of
experimental
unit
Sample
size
Small
28
Small
28
Large
10
Unfamiliar
Male – female
Large
10
Unfamiliar
Female – female
Large
8
Unfamiliar
Male – male
Large
12
Unfamiliar
Adult female – juvenile
Large
16
Unfamiliar
Adult male – juvenile
Large
23
Experimental
Combination of geckos tested
geckos
Juveniles
Familiar (housed together)
Unfamiliar (housed apart)
Adults
Adult –
juvenile
II
Familiar
(housed together; 9 male – female
pairs and 1 male – male pair)
Do juveniles aggregate with unfamiliar conspecific
juveniles?
I used this experiment to determine whether unfamiliar geckos aggregated when the ‘new’
shelter was provided. I tested 28 pairs of randomly assigned geckos.
5.2.2.2
Adults
I
Do adults aggregate with familiar conspecific adults?
This experiment aimed to ascertain whether familiar adults aggregated when provided with
the ‘new’ shelter. As in previous experiments, ten pairs of adults were tested; nine male –
female pairs and one male – male pair.
68
Social influences on retreat-site selection in the laboratory
II
Do unfamiliar male – female adult pairs aggregate?
I designed this experiment to establish whether randomly assigned pairs of adult males and
adult females would aggregate. Sixteen pairs of geckos were tested.
III
Do unfamiliar female – female adult pairs aggregate?
I used this experiment to ascertain whether or not pairs of females would aggregate. No
adult female – female pairs were caught together, and all pairs were assigned randomly.
Eight pairs of geckos were tested in this experiment.
IV
Do unfamiliar male – male adult pairs aggregate?
This experiment aimed to find out whether or not pairs of males would aggregate. One adult
male – male pair was caught together, but all pairs used in the experiment were assigned
randomly. Twelve pairs of male geckos were tested.
5.2.2.3
Adults with juveniles
I
Do unfamiliar adult female – juvenile pairs aggregate?
This experiment was designed to determine whether randomly assigned adult female –
juvenile pairs would aggregate. Sixteen pairs of geckos were tested.
II
Do unfamiliar adult male – juvenile pairs aggregate?
Using this experiment, I aimed to ascertain whether or not randomly assigned pairs of adult
male – juvenile geckos would aggregate. Twenty-three pairs of geckos were tested.
5.2.3
Are geckos attracted to, or repelled by, restrained conspecific geckos?
The preceding trials all involved two geckos, both of which were free to select their own
retreat-site. Thus, any patterns of attraction or avoidance might be caused by the behaviour
of either participant. To identify which gecko was responsible for such a result, I conducted
69
Social influences on retreat-site selection in the laboratory
trials in which one lizard was restrained and only the other individual was free to select
between the available retreat-sites.
The following set of experiments was conducted to establish whether or not specific age and
sex groups of geckos are attracted to or repelled by individuals of other specific age and sex
groups (i.e. adult males – adult females, adult females – adult males, adult males – adult
males, etc.). To conduct these trials, I needed to restrain a gecko inside one shelter to give
the experimental gecko a choice between that gecko and the second shelter containing a
‘control’ object (a wooden clothes peg) similar in size to a gecko. A pilot study was
conducted initially to find a simple and effective method to restrain a gecko. Previous
studies have used string to tether geckos inside a shelter (Cooper et al. 1985). However, an
animal restrained in such a manner is likely to become highly stressed or aggressive, offering
an unnatural stimulus. Therefore, I first used small mesh bags made out of fine curtain
material, with holes approximately 2 mm2. Geckos were placed inside bags, which were
then stapled closed and kept in place within the shelter by tying the bag to the bottom tile
(ceramic tile shelters were used with a height of 18 mm). However, this method also
stressed the animals, with geckos trying to escape from their bags.
A more complex system was then developed using the plastic pot plant tray shelters. Plastic
mesh (holes 4 x 5 mm) dividers were inserted diagonally into each shelter and fastened using
fishing wire tied to three sets of drilled holes (Fig. 5.1). Thus, each shelter was divided
diagonally into two, with one half bounded by the two sides of the shelter and the mesh, and
the other half the same but with an opening providing access into the shelter for the
experimental gecko. Large experimental units were used for experiments unless stated
otherwise. Each evening, the geckos to be restrained were placed behind the divider of one
shelter and a wooden peg was placed behind the divider of the other shelter. A wooden peg
was used (rather than plastic objects) as an object that was of an approximately similar size
70
Social influences on retreat-site selection in the laboratory
Figure 5.1: ‘New’ plastic shelter (13 x 13 x 2.2 cm) with a mesh partition (4 x 5 mm holes)
used to determine whether specific age/sex classes of geckos were attracted to or repelled by
each other.
and shape (long) as a gecko because it was easily available. Each experimental gecko was
placed in the centre of an experimental unit, with the other gecko remaining behind its
divider throughout the trial. Using this system, there was no evidence of stress in the
restrained geckos; after some initial digging in an attempt to escape, the lizards settled down
and were resting when checked the following morning. The large mesh size allowed geckos
to obtain scent cues via tongue-flicking. A ceramic tile was placed on top of each plastic
shelter to weigh it down and ensure that geckos behind the divider were not able to escape.
If a gecko did escape, the data from that trial were excluded from the analysis. Thirteen
different experiments were conducted; see Table 5.3 for a brief summary of experimental
combinations and set-ups used.
5.2.3.1 Juveniles
I
How do juveniles respond to restrained conspecific juveniles?
This experiment was designed to determine whether an experimental juvenile gecko was
attracted to (i.e. selected a shelter-site containing) a restrained juvenile gecko. I assigned
pairs of juveniles randomly, regardless of which geckos were housed together. Twenty-eight
pairs of geckos were tested, with each gecko in each pair being both a restrained lizard and
71
Social influences on retreat-site selection in the laboratory
an experimental lizard in successive experiments, resulting in a total sample size of 56.
Small experimental units were used.
Table 5.3: Summary of experimental combinations and set-ups for experiments using mesh
partitions inside each shelter. The shelter containing a gecko was chosen randomly, and a
peg was placed in the other shelter. Sand was used as a substrate.
Experimental
gecko
Experimental stimulus
Size of unit
Sample size
1 juvenile
Small
28
3 juveniles
Small
28
10 familiar scats
Small
28
Small
30
Adult male
Large
20
Adult female
Large
32
1 adult female and 2
juveniles
Large
16
Adult female
Large
16
Adult male
Large
12
Juvenile
Large
20
Adult female
Large
8
Adult male
Large
16
Juvenile
Large
16
Juvenile
(from own cage)
Shed skin
(equivalent to one gecko’s skin)
Adult male
Adult female
72
Social influences on retreat-site selection in the laboratory
II
How do juveniles respond to a group of three restrained juveniles?
In this experiment, I aimed to find out whether individual juvenile geckos were attracted to a
group of three juvenile geckos. Fourteen randomly assigned groups of geckos were tested
initially, with another 14 groups assigned and tested later, to increase the sample size and
therefore the power of the experiment. These data were analysed both separately for each
day, and combined. Three geckos were placed behind the divider of one shelter and a single
peg was placed behind the divider of the other shelter (this provided consistency for the
purposes of comparing data with the previous experiment). I used small experimental units
for this experiment.
III
How do juveniles respond to the presence of familiar scats?
Previous experiments (Appendix 1) showed that thick-tailed geckos scat-pile. This result
suggests that scat-piles may act as social signals to the lizards and provide information about
conspecifics, as they do in at least one species of scincid (Bull et al. 1999) and one gekkonid
lizard (Carpenter and Duvall 1995). Therefore, I designed this experiment to test whether
thick-tailed geckos would either preferentially select a shelter-site containing scats from their
own cage, or avoid it. I placed 10 scats of varying age, from freshly deposited up to oneand-a-half months old, in the centre of one shelter. One gecko was randomly chosen from
each cage (total 28 geckos) and these animals were used as the experimental lizards. Each
gecko was placed overnight inside the (small) experimental unit.
IV
How do juveniles respond to the presence of shed skins of other juveniles?
Several species of lizards are able to distinguish males from females and also familiar from
unfamiliar individuals based on chemosensory cues, including scats (see Schwenk 1995 for a
review; Steele and Cooper 1997; Bull et al. 1999). Plausibly then, the lizards may also use
shed skins as a social signpost to detect the presence and/or sex or identity of other geckos.
Thick-tailed geckos frequently tongue-flick (pers. obs.), presumably to obtain chemosensory
73
Social influences on retreat-site selection in the laboratory
information from their surroundings. They shed entire skins and do not eat them (pers.
obs.). These skins are frequently found lying under rocks in the field (pers. obs.).
Therefore, I designed this experiment to determine whether juvenile thick-tailed geckos
would select a shelter-site containing a shed skin from another (randomly selected) juvenile
gecko. Skins were collected from home cages during routine husbandry and placed in a ziplock bag inside a fridge. Shed skins were removed from the fridge two hours before the
experiment to bring them to room temperature. One random shed skin was placed inside one
of the shelters in each experimental unit. Thirty juvenile geckos were chosen randomly and
one was placed in each (small) experimental unit overnight.
V
How do juveniles respond to restrained adult males?
In this experiment, I aimed to find out whether juvenile geckos were attracted to adult male
geckos. Thus, I assigned and tested 20 adult male – juvenile pairs of geckos. The adult male
was placed behind the divider of one shelter while the juvenile gecko was the experimental
animal.
VI
How do juveniles respond to restrained adult females?
This experiment aimed to ascertain whether juvenile geckos were attracted to adult female
geckos. Sixteen pairs of juvenile plus adult female geckos were designated and tested. The
adult female was placed behind the divider of one shelter while the juvenile was the
experimental gecko. This experiment was repeated a week later with a new set of 16 random
pairs. The purpose of this repetition was to increase sample sizes, and thus the power of the
tests. These data were analysed both separately for each day, and combined.
74
Social influences on retreat-site selection in the laboratory
VII
How do juveniles respond to a restrained group of three geckos (one adult
female, two juveniles)?
In this experiment, the aim was to determine whether a juvenile gecko would be attracted to
a group of geckos consisting of one adult female and two juveniles. Sixteen groups of one
adult female plus three juvenile geckos were randomly assigned. The female gecko and two
juvenile geckos were placed behind the divider of one shelter while the remaining juvenile
served as the experimental lizard. One peg was used in the second shelter rather than three
to provide consistency with previous experiments, allowing direct comparison of data.
5.2.3.2
Adult males
I
How do adult males respond to restrained adult females?
The aim of this experiment was to find out whether adult male thick-tailed geckos were
attracted to adult female thick-tailed geckos. Sixteen male – female pairs of geckos were
assigned randomly and tested, regardless of whether they had been housed in pairs or
individually. Females were placed behind the divider of one shelter, with a peg in the other
shelter. Experimental males were then placed in the centre of each experimental unit.
II
How do adult males respond to restrained conspecific adult males?
The aim of this experiment was to determine whether male geckos were attracted to other
male geckos. I randomly assigned and tested 12 pairs of adult male lizards. One male was
placed behind the divider of one shelter and the second male was the experimental gecko.
III
How do adult males respond to restrained juveniles?
The aim of this experiment was to determine whether adult male geckos were attracted to
juvenile geckos. I assigned 20 adult male plus juvenile pairs of geckos randomly and tested
them. A juvenile gecko was placed behind the divider of one shelter, with a peg behind the
75
Social influences on retreat-site selection in the laboratory
divider of the second shelter. The adult male was the experimental lizard and was free to
roam around within the experimental unit.
5.2.3.3
Adult females
I
How do adult females respond to restrained conspecific adult females?
This experiment was designed to ascertain whether female geckos were attracted to other
female geckos. Eight pairs of females were assigned randomly and tested. One adult female
was placed behind the mesh partition while the second adult female was the experimental
lizard.
II
How do adult females respond to restrained adult males?
This experiment aimed to determine whether adult female geckos were attracted to adult
male geckos placed behind the divider. Again, 16 pairs of geckos were assigned randomly
and tested. This experiment was repeated a second time approximately a week later, with a
new set of 16 random pairs, none of which had been tested with each other in the previous
trial. The purpose of repeating the experiment was to increase sample size and thus, the
power of the test. Data from each day were analysed separately and also combined.
III
How do adult females respond to restrained juveniles?
I designed this experiment to determine whether adult female geckos were attracted to
juveniles. Sixteen pairs of adult female plus juvenile geckos were randomly assigned and
tested. The juvenile lizard was placed behind the divider of one shelter and the female was
the experimental lizard.
5.3
Results
5.3.1
Do geckos aggregate when provided with ceramic tile shelters?
I first consider trials using crevices created with a single tile and a crevice height of 12 mm.
Juvenile thick-tailed geckos showed a significant level of aggregation, with 81.5% of those
76
Social influences on retreat-site selection in the laboratory
collected and housed together aggregating and 92.9% of those collected and housed apart
aggregating (Table 5.4; Fig. 5.2).
Table 5.4: Results of aggregative responses by juvenile and adult thick-tailed geckos with
tiled shelters (*χ2 crit (P = 0.05, df = 1) = 3.841, **χ2 crit (P = 0.005, df = 1) = 7.879, NS = not
significant). P values reported prior to sequential Bonferroni corrections. Note that in some
cases the sample sizes are different from that stated in the methods, where one or more
geckos within an experimental unit had not selected a shelter at the time of scoring.
Experimental
gecko
Juvenile
Adult
Experiment
Aggregation /
Avoidance
Result
χ2 Values and
significance
Familiar (housed together),
small unit, small shelter, 12
mm high
Aggregation
22/27 pairs
together
8.33**
Familiar (housed apart), small
unit, small shelter, 12 mm
high
Aggregation
26/28 pairs
together
20.57**
Familiar (housed together),
small unit, small shelter, 12
mm high
Avoidance
1/10 pairs
together
6.40*
Familiar (housed together),
small unit, large shelter, 18
mm high
None
6/10 pairs
together
0.40NS
Familiar (housed together),
large unit & shelter, 18 mm
high, cardboard strip on front
None
8/10 pairs
together
3.60 NS
Familiar (housed together),
small unit, large shelter, 3.5
sides covered
Aggregation
8/9 pairs
together
5.44*
For adults, the first three trials listed in Table 5.4 revealed no significant aggregation in
familiar geckos. In fact, in the first trial, the geckos showed a significant level of avoidance.
However, in the last trial, there was a significant level of attraction between familiar adult
geckos (see Table 5.4; Fig. 5.2). It is important to note that the proportion of familiar
juvenile geckos that aggregated (81.5%) was similar to the proportion of familiar adults that
aggregated in the third trial (80%), yet one is significant and the other is not. Thus, small
sample sizes may have resulted in the rejection of the null hypothesis in that case.
Sequential Bonferroni corrections moved the last (adult) trial to non-significance also,
although this too is likely to be due to the necessarily small sample size.
77
Social influences on retreat-site selection in the laboratory
78
110
100
90
80
70
60
50
40
30
20
10
0
*
Familiar,
small unit &
shelter,
12 mm high
Juveniles
*
Unfamiliar,
small unit &
shelter,
12 mm high
*
Familiar,
small unit &
shelter,
12 mm high
Experimental treatment
Familiar
small unit,
large shelter,
18 mm high,
3.5 sides
covered
*
Non-social influences on retreat-site selection in the laboratory
Familiar,
small unit,
large shelter,
18 mm high
Familiar,
large unit &
shelter,
18 mm high,
front covered
Adults
Figure 5.2: Percentage of adult and juvenile thick-tailed geckos that aggregated in trials using ceramic tile shelters. The percentages of gecko pairs
showing a bias towards aggregation are shown. The dotted line indicates the percentage of pairs of geckos expected under a null hypothesis of equal
distribution. An asterisk (*) shows cases that display a bias strong enough for statistical rejection of the null hypothesis, prior to the application of the
sequential Bonferroni technique.
Percent aggregated
Social influences on retreat-site selection in the laboratory
These data suggest an ontogenetic difference in aggregative behaviour. The strongest
comparison between juveniles and adults involves data on pairs of lizards that had been
captured and housed together. Indeed, contingency table analysis revealed a significant
difference in aggregation patterns (χ2 = 13.03, 1 df, P = 0.0003) between adult and juvenile
gecko pairs in these trials using narrow (12 mm) crevices.
5.3.2
Do geckos aggregate when provided with ‘new’ plastic shelters?
Juvenile thick-tailed geckos again showed a significant level of aggregation, both with
familiar (caught and housed together) and unfamiliar (caught and housed separately)
conspecifics (Table 5.5; Fig. 5.3). Adult geckos that were captured and housed together did
not show statistically significant aggregation, although the proportion of aggregating
individuals was actually higher than that in juveniles housed together. This bias did not gain
statistical significance due to the unavoidably small sample size of eight pairs (Table 5.5;
Fig. 5.3).
Unfamiliar adult male - adult female gecko pairs showed significant aggregation as did the
adult female pairs (Table 5.5; Fig. 5.3). Adult males showed neither attraction nor
avoidance of each other (Table 5.5; Fig. 5.3), and in three (out of 12) of the male-male trials,
one of the geckos had not selected a shelter-site when the lizards were checked in the
morning. This behaviour (seen infrequently in other trials within my study) may be related
to another unusual aspect of these trials: all males had been fighting, evidenced by the
presence of new bite marks on their bodies and heads, as well as blood in several instances.
Adult female and juvenile geckos showed significant aggregation, however adult male and
juvenile geckos did not (Table 5.5; Fig. 5.3). There was no significant difference in the
proportion of juvenile males versus females that selected shelters with (7/12 males, 6/11
females, χ2 tests, both P > 0.05) and without adult males inside.
79
Social influences on retreat-site selection in the laboratory
Table 5.5: Results of aggregation trials using juvenile and adult thick-tailed geckos with
‘new’ protected plastic shelters (*χ2 crit (P = 0.05, df = 1) = 3.841, **χ2 crit (P = 0.005, df = 1) = 7.879,
NS
= not significant). Significant results (P < 0.05) obtained before the application of the
sequential Bonferroni technique are in boldface font.
Experimental
geckos
Experiment
Aggregation /
Avoidance
Result
χ2 Values and
significance
Juvenile
Familiar (housed together),
small unit
Aggregation
20/28 pairs
together
5.14*
Familiar (housed
separately), small unit
Aggregation
19/27 pairs
together
4.48*
Familiar (housed together),
large unit
None
7/8 pairs
together
3.57
Unfamiliar
Random male - female, large
unit
Aggregation
14/16 pairs
together
9.00**
Unfamiliar
Random female – female,
large unit
Aggregation
8/8 pairs
together
8.00**
Unfamiliar
Random male – male, large
unit
None
4/9 pairs
together
0.11 NS
Unfamiliar
Adult female – juvenile,
large unit
Aggregation
12/16 pairs
together
4.00*
Unfamiliar
Adult male – juvenile, large
unit
None
13/23 pairs
together
0.52 NS
Adult
Adult juvenile
Contingency table analysis showed that there was no significant difference in aggregation
patterns between adult and juvenile gecko pairs that were either familiar (χ2 = 0.21, 1 df, P =
0.64) or unfamiliar (χ2 = 0.85, 1 df, P = 0.36) with each other.
In summary, juvenile geckos aggregated strongly, and adults less strongly, in these
laboratory tests. The sequential Bonferroni technique was applied to the three sets of
experiments separately, and did not result in changes in significance for juveniles or adults.
However, the technique moved the aggregative response of adult females with juveniles
beyond the conventional level of significance.
80
120
110
100
90
80
70
60
50
40
30
20
10
0
Housed
together,
small unit
*
Housed
apart,
small unit
*
Juveniles
Housed
together,
large unit
*
Random
male female,
large unit
*
Random
Random
female male female, male, large
large unit
unit
Adults
Experimental treatment
*
Adult
Adult male
female - - juvenile,
juvenile, large unit
large unit
Adults with juveniles
Figure 5.3: Percentage of adult and juvenile thick-tailed geckos that aggregated in trials using the ‘new’ plastic shelters. The percentages of gecko pairs
showing a bias towards aggregation are shown. The dotted line indicates the percentage of pairs of geckos expected under a null hypothesis of equal
distribution. An asterisk (*) shows cases that display a bias strong enough for statistical rejection of the null hypothesis, prior to the application of the
Bonferroni technique.
Percent aggregated
Social influences on retreat-site selection in the laboratory
5.3.3
Are geckos attracted to, or repelled by, restrained conspecific geckos?
Juvenile geckos showed neither attraction to nor avoidance of individual conspecific geckos,
other than adult females, that were constrained inside one of the shelters (Table 5.6; Fig.
5.4). However, this result too was no longer significant after application of the sequential
Bonferroni technique (juvenile, adult male and adult female data sets considered separately).
Similarly, the lizards showed neither significant attraction to, nor avoidance of groups of
three restrained geckos, scat-piles, or shed skins (Table 5.6; Fig. 5.4).
All combinations of adult geckos (male – female, female – male, female – female, male –
male) displayed neither significant attraction, nor avoidance towards the restrained gecko
(Table 5.6; Fig. 5.4). Similarly, adult male and adult female geckos showed neither
attraction to, nor avoidance of, juvenile geckos (Table 5.6; Fig. 5.4). Contingency table
analysis revealed no significant differences in the proportions of aggregative responses
between juvenile – juvenile, adult male – male and adult female – female pairs (χ2 = 0.84, 2
df, P = 0.66).
5.4
Discussion
Clearly, thick-tailed geckos aggregated in the laboratory as well as in the field (Chapter 3).
Grouping occurred in the laboratory despite the provision of two identical shelter-sites
within experimental units, suggesting a genuine attraction between conspecifics. Despite the
selectivity displayed by the geckos with respect to the physical characteristics of their
retreat-sites (Chapter 4), results of these trials indicate that aggregation in this species is not
simply a result of the rarity of suitable shelter-sites in the field. Thus, it would appear that
these geckos are indeed social animals.
82
Social influences on retreat-site selection in the laboratory
Table 5.6: Results of trials testing aggregative responses by juvenile and adult thick-tailed
geckos when one to three geckos were restrained (*χ2 crit (P = 0.05, df = 1) = 3.841, **χ2 crit (P =
NS
= not significant). Significant results (P < 0.05), obtained prior to
0.005, df = 1) = 7.879,
application of the Bonferroni technique, are shown in boldface font.
Experimental
gecko
χ2 Values and
significance
Experimental
stimulus
Attraction /
Avoidance
1 juvenile
None
Scat-pile (10 scats)
None
Shed skin of
another juvenile
None
Adult male
None
Adult female
Avoidance
9/32 pairs
together
6.13*
3 juveniles
None
9/26 pairs
together
2.46NS
1 adult female and
2 juveniles
None
9/15 pairs
together
0.60 NS
Adult female
None
6/16 pairs
together
0.09 NS
Adult male
None
6/11 pairs
together
1.00 NS
Juvenile
None
8/20 pairs
together
0.80 NS
Adult female
None
3/8 pairs
together
0.50 NS
Adult male
None
20/32 pairs
together
2.00 NS
Juvenile
None
10/16 pairs
together
1.0 NS
Juvenile
Adult male
Adult female
Result
29/53 pairs
together
17/28
individuals with
scats
18/30
individuals with
shed skins
7/20 pairs
together
0.47 NS
2.43 NS
1.20 NS
1.80 NS
83
70
60
50
40
30
20
10
0
Juvenile Scat-pile Shed skin
(10
of
scats)
another
juvenile
Adult
male
Juveniles
*
Adult
female
3
1 adult
juveniles female
&2
juveniles
Adult
male
Experimental design
Adult
female
Juvenile
Adult males
Adult
male
Adult
female
Juvenile
Adult females
Figure 5.4: Percentage of juvenile and adult thick-tailed geckos that aggregated when the plastic shelters with mesh partitions were used. Percentages of
geckos showing a bias towards attraction to conspecifics are shown. The dotted line indicates the percentage of attraction to conspecifics expected under a
null hypothesis of neither attraction nor avoidance. An asterisk (*) shows cases that display a bias strong enough for statistical rejection of the null
hypothesis, prior to the application of the sequential Bonferroni technique.
Percent attracted
Juvenile geckos aggregated when provided with two unprotected (‘old’) shelters, and
continued to aggregate when two protected (‘new’) shelters were provided. Interestingly, the
preference of marbled geckos (Christinus marmoratus) for a large over a small shelter was
eradicated when only a small opening was left to allow entry (M. Kearney, pers. comm.). A
plausible explanation for this response is that marbled geckos are primarily attracted to
shelters with reduced amounts of light penetration under the shelter, and the level of security
offered as a result of being further away from an opening. This further suggests that
aggregation in thick-tailed geckos does not occur simply to reduce the amount of light
penetrating under the shelter, or to give the gecko an opportunity to wedge itself into a tight,
secure position.
The aggregative response of adults when provided with unprotected shelters is somewhat
more complicated. However, the avoidance behaviour seen when small shelters with a low
crevice height (12 mm) are used may help explain why adults are found significantly more
often under large rocks than under small rocks in the wild (Chapter 3). Large rocks permit
the lizards to aggregate due to their size, allowing more animals to physically fit under the
rocks. In addition, the results suggest that distance between shelters, or rocks, per se does
not define a separate ‘rock’ (geckos aggregated when shelters were close together rather than
further apart), and is less important in promoting aggregation than the inability of geckos to
see each other from ‘separate’ rocks or shelters. While the Bonferroni technique moved this
result beyond significance, it is likely to be due to the unavoidably small sample size rather
than due to a lack of aggregation.
Interestingly, thick-tailed geckos did not group preferentially with familiar rather than
unfamiliar conspecifics. By contrast, in some species of lizards, juveniles and sub-adults in
a group show no aggression towards each other but are aggressive towards unfamiliar
intruders, as in the skink Egernia cunninghami (Banks 1986). Others, such as Egernia
85
Scat-piling by thick-tailed geckos
stokesii actually live in family groups (Bull 1988; Bull et al. 1998). In addition, size and sex
was not important in determining aggregations in my trials. Most combinations of geckos
aggregated significantly when free to move. The exceptions were pairs of adult males, and
adult males with juveniles. Both of these groups showed neither aggregation nor avoidance.
The lack of avoidance between adult male geckos is intriguing because they are extremely
aggressive and fight with one another, sometimes to the death (pers.obs.). In addition, bite
marks were evident on the bodies and heads of all males at the end of the experiment.
However, it is possible that in this experimental set-up, one male continually chased another
male throughout the night from one shelter to the other, perhaps stopping only when the
lights came on in the morning, and thus ending up in the same shelter despite active
avoidance of one animal by the other. Alternatively, the males may have fought during their
activity period at night and chosen their diurnal retreat-sites without regard for each other.
In any case, this finding is similar to that found in the field, where adult males neither
aggregate with, nor avoid each other (Chapter 4). Male – male combat and territoriality
occurs in many species of geckos, such as Bynoe’s geckos (Heternotia binoei) (Bustard
1970b) and velvet geckos (Oedura leseuerii) (Downes and Shine 1998b). I observed combat
between male thick-tailed geckos on numerous occasions, and territoriality probably also
occurs. However, further observational studies are required to clarify the specific processes
involved in interactions between adult male thick-tailed geckos.
Juveniles did not aggregate with adult male geckos in trials where both lizards were free to
select retreat-sites, although the reasons for this lack of aggregation are unclear. The sex of a
juvenile had no effect on whether it selected a shelter with or without an adult male in it.
Casual observation of interactions between adult males and juveniles revealed no overt
aggression. Nonetheless, this lack of grouping may be related to the generally aggressive
nature of adult males.
86
Scat-piling by thick-tailed geckos
When mesh partitions were used to physically separate geckos, the aggregation response was
eliminated. The only other pattern emerging was an avoidance of adult females by juveniles.
However, the significance was eliminated by the application of sequential Bonferroni
corrections and may have occurred spuriously due to a large number of tests being
conducted. Nonetheless, it is interesting to note that the only major pattern found by
Kearney et al. (2001) in the field was a lack of adult female – juvenile aggregations.
Thus, the question remains, what attributes attract geckos to each other? The presence of
shed skins and familiar scats did not elicit attraction, and placing geckos behind a divider
eradicated the aggregation response. Thick-tailed geckos aggregated only when they were
able to maintain close physical contact with one another. This tendency suggests that there
may be some specific advantage gained by being in physical contact with conspecifics.
Aggregation between animals may change rates of heating and cooling (Boersma 1982;
Yahav and Buffenstein 1991), and evaporative water loss (Tanaka 2000), as well as
providing benefits in terms of predator evasion (Hoogland 1983). One way to identify which
(if any) of such factors is important, is to examine facultative shifts in the degree of
aggregative behaviour in response to environmental cues. This approach is taken in Chapter
6.
87
Scat-piling by thick-tailed geckos
CHAPTER 6
Environmental influences on aggregative behaviour
6.1 Introduction
As discussed above, aggregative behaviour has been recorded for many animals, from
mammals to invertebrates. Some of these aggregations occur for social reasons (e.g. Legge
and Cockburn 2000; East and Hoffer 2001), but this is not always the case. Other plausible
causes for such behaviour may be various environmental factors, which may promote or
inhibit social or aggregative behaviour. Such factors may include changes in photoperiod,
humidity, temperature, and presence of a predator (Hoogland 1983; Elfström and Zucker
1999; Tanaka 2000).
Aggregation may reduce an individual’s risk of predation through increased vigilance and
predator dilution (Wilson 1975; Hoogland 1983; Pitcher and Parrish 1993). For example,
gidgee skinks (Egernia stokesii) in groups react more quickly to threats than do solitary
skinks (Lanham 2001). Another cause for aggregation may be humidity; the tropical fungus
beetle, Stenotarsus rotundus (Endomychidae), aggregates to reduce water loss when
environmental humidity is low (Tanaka 2000). Changes in environmental temperature, such
as the onset of winter, may also induce aggregative behaviour as in several species of snakes
and lizards, discussed previously (Hamilton 1948; Graves and Duvall 1987; Elfström and
Zucker 1999; Shine et al. 2001).
Group living may also provide thermoregulatory benefits. Marine iguanas (Amblyrhynchus
cristatus) aggregate, or form ‘sleeping piles’ at night, such that iguanas closest to the centre
of a pile are warmer than those on the periphery, which in turn are warmer than solitary
iguanas (Boersma 1982). Furthermore, iguanas in the centre of the pile remain warmer
throughout the night to just before sunrise (Boersma 1982). Naked mole rats
88
Scat-piling by thick-tailed geckos
(Heterocephalus glaber) aggregate all year round and huddling behaviour plays an important
role in thermoregulation for the otherwise non-endothermic mammals (Yahav and
Buffenstein 1991). Similarly, gidgee skinks (Egernia stokesii) that are in contact with other
lizards remain warmer at night than those that are solitary (Lanham 2001).
The phenomenon of aggregation in thick-tailed geckos occurs throughout the year, including
winter (Kearney et al.2001; Chapter 3). My laboratory experiments confirmed that this
phenomenon is due to the lizards’ tendency to select retreat-sites containing conspecifics
(Chapter 5), as well as their active selection of multiple physical attributes of retreat-sites
(Chapter 4).
While thick-tailed gecko aggregations may occur for social reasons, it is not clear whether
other environmental factors may affect this grouping behaviour. One potential approach to
clarify the functional significance of aggregative behaviour is to manipulate cues and record
whether the frequency of aggregation changes in response. This chapter presents the results
of three experiments of this type, in which I changed variables (predator scent, humidity and
temperature) that might plausibly be related to the advantages of aggregation. I also
conducted additional experiments to see (1) whether aggregation modified the rate at which
geckos heated and/or cooled, and (2) if geckos tended to cluster together as temperature
changed.
6.2 Materials and Methods
The aim of the following set of experiments was to investigate what kinds of cues may either
stimulate or suppress aggregation in juvenile thick-tailed geckos. Each experiment was set
up differently and the procedures are described below. Note that as in previous experiments,
if geckos had either not made a selection or had escaped at the time of scoring, they were
excluded from the data set. Data were analysed using Pearson’s Chi-square tests, unless
89
Scat-piling by thick-tailed geckos
stated otherwise. The sequential Bonferroni technique was not applied in this section as
each set of experiments tested for different factors.
6.2.1
Does predator scent induce aggregation?
The purpose of this experiment was to determine whether the presence of predator scent
modifies the degree to which juvenile geckos are attracted to a group of three juvenile
geckos, with which no physical contact is possible. In previous experimental work, I
examined such responses in the absence of predator scent, and found no attraction (see
Chapter 5). Two plastic shelters with mesh partitions were used (see Chapter 5, Fig. 5.1).
Fourteen groups of geckos were randomly assigned and tested. Pieces of cotton rope were
placed inside the cage of a captive red-bellied black snake (Pseudechis porphyriacus) for
several weeks to pick up its scent. These pieces of rope were then cut into smaller pieces 15
cm and 10 cm long. Each experimental unit (large, rather than small as in Chapter 5 to allow
for sufficient area for placement of scented rope) contained three pieces of rope: one 15 cm
long piece in the centre between the two shelters and two more 10 cm long pieces
perpendicular to the central piece, forming an ‘I’ pattern. A group of three juvenile geckos
was placed behind the divider of one shelter while the other shelter contained a single peg
(this allowed a direct comparison of results with previous experiments). Fourteen such
groups were tested (although 28 groups were tested in the previous trial). The experimental
lizard was then placed in the unit and allowed to roam freely overnight. In the morning, I
scored its retreat-site selection. Note that my previous experiment on avoidance of predator
scent (Chapter 4) involved scent inside one shelter but not the other. In contrast, the present
set-up had predator scent generally through the enclosure but not in either shelter.
6.2.2
Does humidity influence the aggregative response?
This experiment aimed to find out whether humidity affected the degree of aggregation in
juvenile thick-tailed geckos. If low humidity causes aggregation, then high humidity should
90
Scat-piling by thick-tailed geckos
reduce or eliminate the grouping response. I added two litres of water to the sand that was
used as a substrate in the experimental units. The sand was mixed thoroughly with the water
and then placed in the (small) experimental units as before, with two alternate plastic shelters
available (see Chapter 4, Fig. 4.2) to the 29 randomly allocated pairs of geckos. One extra
experimental unit was set up (without lizards inside it) using the damp sand. A relative
humidity logger (Hobo RH, Onset Computer Corporation) was placed inside one shelter to
measure humidity overnight in this extra unit. Similarly, to measure relative humidity (RH)
for the ‘low humidity’ treatment, a data logger was placed inside one shelter in a unit
containing dry sand during the previous experiment.
6.2.3
Does ambient temperature influence the aggregative response?
Thick-tailed geckos aggregated significantly at 20oC, the temperature at which almost all my
previous trials had been conducted. If aggregation is driven by ambient temperature, then
warming or cooling may modify or eliminate the grouping behaviour. Thus, the aim of these
experiments was to determine if the geckos continued to aggregate at high (28oC) and low
(16oC) temperatures. The room in which the experiments were conducted was first set to
28oC. Twenty-seven pairs of familiar geckos (housed together) were tested. Each pair was
placed in one (small) experimental unit, containing two plastic shelters, and allowed to roam
freely. For the second experiment, I set the temperature in the experimental room to 16oC. I
then placed each of the same 27 pairs of geckos into experimental units overnight. The
resulting data could then be compared to those obtained in previous trials with identical setups, except that room temperature was kept at 20oC.
6.2.4
Do geckos heat and cool at different rates when with another gecko?
I designed this experiment to determine whether a juvenile thick-tailed gecko cools and
heats up more slowly when huddled with an adult lizard compared to when it is on its own.
Adult geckos were used both because these groupings occurred frequently in the field, and
also due to their larger body mass. Two randomly assigned experimental groups were
used: an adult female with a juvenile, and a juvenile on its own. Fifteen replicates were
tested for each experimental group and for each of the experiments below.
91
Scat-piling by thick-tailed geckos
6.2.4.1
Does presence of another gecko change the rate at which a lizard heats up?
This experiment was used to measure the heating rate of a juvenile gecko when alone, versus
when huddled with another gecko. Three incubators were set to 10oC. The geckos were
placed in plastic half-pipe shelters (5 cm diameter, 12 cm long), which were then sealed on
either side with paper to prevent the geckos from escaping. The bottom of each shelter was
not sealed and rested on the clear bottom of the experimental container (22 cm x 13 cm x
7cm) for each pair or individual gecko. The containers were then covered with a plastic lid
with wire mesh over one side (approximately 9 cm x 9 cm). Thirty randomly distributed
containers were placed in each of the three incubators (ten per incubator). The geckos were
then left for two hours to allow them to equilibrate at approximately 10oC.
After two hours, I checked the core body temperature of one solitary juvenile gecko from
each incubator by inserting a thermocouple wire (with Parafilm [American Can Company,
U.S.A] wrapped around the naked wires to create a softer probe) approximately 5 to 8 mm
into its cloaca. This temperature was taken to ensure that geckos had equilibrated thermally.
I checked whether or not the paired geckos were in contact by looking through the clear
bottom of the experimental container. I then turned each incubator up to 28oC. At the end
of an hour, I again checked whether or not the geckos were in contact and then took core
(cloacal) body temperatures of all juvenile geckos.
6.2.4.2 Does presence of another gecko change the rate at which a lizard cools down?
This experiment measured cooling rates of juvenile thick-tailed geckos when alone, versus
when huddled with another gecko. The experiment was initially carried out as described
above except that the (three) incubators were set to 28oC at the beginning of the trial and
turned down to 10oC after two hours. However, the thermal stratification between the top
and bottom shelves of the incubators while cooling was too great (approximately 10oC) to
discern any differences. Thus, this experiment was repeated as above using (two) different
92
Scat-piling by thick-tailed geckos
incubators with little thermal stratification within them. Core body temperature readings of
one solitary juvenile gecko per incubator were taken before the temperatures in the
incubators were turned down. An hour later, core body temperature readings were taken for
all juvenile geckos, as described above.
6.2.5
Are more geckos in contact with each other when temperature changes?
One of the above experiments (section 6.2.3) examined aggregative responses at three
different ambient temperatures. Plausibly, however, aggregation might be a behavioural
response to changing temperature rather than any particular mean temperature. In this
experiment, I changed the temperatures the lizards were exposed to, and looked for any
effect on clustering (i.e., whether or not the geckos were in contact with one another). A
thermal gradient (208 x 98 cm) with a cooling system at one end and a heating system at the
other end was used to create three sections with cool (10oC), moderate (18oC) and high
(29oC) temperatures. Ambient temperature in the room was 18oC.
For this experiment, I used pairs of juveniles in their home cages (22 x 22 x 7 cm; see
Appendix 1, Fig. A1.1c for illustration). For normal husbandry, cages were placed on racks
that had a heated section (temperatures ranging from approximately 28 to 30oC) at the back
where the hides (plastic half-pipes; 5 cm diameter, 12 cm long) were placed (see Chapter 2).
Each cage contained a sand substrate approximately 2 cm deep, some of which was removed
for this experiment, to create a uniform depth of 1 cm for all cages. The hides normally used
in the gecko cages were removed, and one plastic shelter (13 x 13 x 2.2 cm; see Chapter 4,
Fig. 4.1) was placed inside each cage. Eight cages were then randomly placed on each
section of the thermal gradient representing the three temperature treatments (cold, moderate
and hot). The cages were left on the gradient for one hour. I then returned and scored
whether or not geckos were in contact with each other.
93
Scat-piling by thick-tailed geckos
6.3 Results
6.3.1
Does predator scent induce aggregation?
In the presence of predator scent, juvenile thick-tailed geckos showed neither significant
attraction to, nor avoidance of, individual conspecific geckos that were constrained inside
one of the shelters (Table 6.1; Fig. 6.1). The proportion of aggregating individuals was
similar to that seen in earlier trials using the same set-up except for the presence of predator
scent (χ2 = 0.0, 1 df, P > 0.99). Contingency table analysis showed that there was no
difference in aggregative response between treatments (χ2 = 0.00, 1 df, P > 0.99).
6.3.2
Does humidity influence the aggregative response?
The geckos showed significant aggregation at high humidity (99.5% RH, measured by data
logger), with 72% of them being together (Table 6.1; Fig. 6.1). This result is very similar to
that recorded at lower humidity (77.0% RH) (Chapter 5, 70.3% together; χ2 = 4.48, 1 df, P <
0.05), and contingency table analysis revealed no significant difference (χ2 = 0.00, 1 df, P >
0.99) between treatments. Note that while 77% relative humidity is not low per se, it
provides a useful comparison for the purposes of this experiment, which aimed to determine
whether high humidity reduced the aggregative response.
6.3.3
Does ambient temperature influence the aggregative response?
Juvenile geckos did not aggregate significantly at ambient temperatures of 16oC or at 28oC
(Table 6.1; Fig. 6.1). This result is in contrast to the aggregation displayed in previous
experiments where the room temperature was 20oC (see Chapter 5). However, contingency
table analysis revealed that there was no significant difference (χ2 = 1.27, 2 df, P = 0.53) in
the aggregative response among these three thermal treatments.
Table 6.1: Results of aggregation trials (between juvenile geckos) when environmental
conditions are changed (*χ2 crit (P = 0.05, df = 1) = 3.841, **χ2 crit (P = 0.005, df = 1) = 7.879, NS = not
94
Scat-piling by thick-tailed geckos
significant). Significant results are shown in boldface font. Note that data for some
treatments (no predator scent; low humidity; medium temperature) are taken from trials
described in Chapter 5.
Environmental
factor
Predator scent
Humidity
Experimental
Aggregative
conditions
Treatment
response
and choices
Large unit,
divided
shelter; one
with 3
juveniles, one
with a peg
Small unit,
un-divided
shelter; 2
juveniles free
to move
Present
None
5/14
groups
together
1.14 NS
Absent
None
9/26
groups
together
2.46 NS
High
(99.5%)
Aggregate
21/29
pairs
together
5.83*
Low
(77.0%)
Aggregate
20/28
pairs
together
5.14*
Low
(16oC)
None
17/27
pairs
together
1.815 NS
Medium
(20oC)
Aggregate
20/28
pairs
together
5.14*
High
(28oC)
None
15/27
pairs
together
0.33 NS
Temperature
Small unit,
un-divided
shelter; 2
juveniles free
to move
80
Percent aggregated
70
Result
χ2 values
and
significance
*
*
*
60
50
40
30
20
10
95
Scat-piling by thick-tailed geckos
Figure 6.1: Percentage of thick-tailed geckos that aggregated with changing environmental
cues (predator scent, humidity and temperature). The percentages of pairs of geckos
showing a bias towards aggregation are shown. The dotted line indicates the percentage of
pairs of geckos expected under a null hypothesis of equal distribution. An asterisk (*) shows
cases that display a bias strong enough for statistical rejection of the null hypothesis.
6.3.4
Do geckos heat and cool at different rates when with another gecko?
After one hour’s heating in the incubator from a temperature of 10oC the core body
temperatures of juvenile geckos that were in contact with an adult gecko (23.6 + 0.61oC
[+SE]) were significantly lower than those of juvenile geckos that were not in contact with
adult geckos (25.2 + 0.27oC [+SE]) (F1,28 = 7.43, P = 0.01; Fig. 6.2a). Similarly, when
cooled down in the incubator from a temperature of 28oC to 10oC, juvenile geckos that were
huddled up with an adult gecko had significantly higher core body temperatures (16.44 +
0.14oC [+SE]) than did juveniles that were not in contact with an adult gecko (15.37 + 0.21
o
C [+SE]) (F1,7 = 19.30, P = 0.0001; Fig. 6.2b).
b) Body temperature after one hour of
cooling
26
17
25
16
24
23
Temperature (oC)
Temperature (oC)
a) Body temperature after one hour of
heating
15
96
14
Scat-piling by thick-tailed geckos
Figure 6.2: Body temperatures (+ SE) of juvenile thick-tailed geckos after one hour in the
incubator when being (a) heated and (b) cooled. Differences between geckos in contact and
not in contact were statistically significant for both treatments. Note that vertical axes do not
begin at zero.
6.3.5
Are more geckos in contact with each other when temperature changes?
Significantly more geckos in the cool temperature (10oC; 6/7 pairs) treatment were in contact
with each other after one hour than geckos that were in either the moderate (18oC; 2/7 pairs)
or high (29oC; 2/7 pairs) temperature treatments (χ2 = 6.11, 2 df, P = 0.047). Note that in
each treatment, one cage had a gecko that was not inside the shelter, and was therefore
excluded.
6.4 Discussion
Facultative changes in the aggregative response potentially offer valuable insights into the
factors inducing this behaviour. However, none of the first three factors that I manipulated
(predator scent, humidity and mean temperature) significantly affected the aggregative
response of my geckos. Unfortunately, these null results are difficult to interpret, because it
is not clear whether or not I mimicked the biology of the geckos accurately in these cases.
For example, in the experiment where predator scent was used, a lack of lizard tracks in the
sand suggested that the geckos did not move around much. Indeed, the test animal may have
run immediately to one shelter and remained there to avoid predation risk. The scent of the
predator may have been too strong and it would be worth repeating the experiment with less
intense scent so as not to inhibit movement.
97
Scat-piling by thick-tailed geckos
Thick-tailed geckos continued to aggregate when the humidity was high (99.5%), indicating
that aggregation is not a direct response to low humidity. However, although aggregation
can occur even at high humidity, there may nonetheless be situations in which grouping
reduces the rate of water loss through a reduction in the surface area to volume ratio. Fungus
beetles, Stenotarsus rotundus (Tanaka 2000), and naked mole rats, Heterocephalus glaber
(Yahav and Buffenstein 1991), both benefit from a reduced rate of evaporative water loss as
a result of aggregation. It would be interesting to measure rates of evaporative water loss of
aggregated and non-aggregated lizards under high and low humidity.
While significant aggregation did not occur at either high or low ambient temperatures in the
above trials, but did occur at 20oC in a previous trial (Chapter 5), contingency table analysis
showed that there was no significant difference between the results. Thus, the geckos did
not facultatively change their grouping behaviour in response to changes in temperature. By
comparison, gidgee skinks (Egernia stokesii) aggregated and formed large groups more at
low temperatures (16oC) than at higher temperatures (22 and 28oC) (Lanham 2001).
Although temperature apparently did not influence aggregation in these trials, the incubator
trials showed the reverse. That is, aggregation can affect a lizard’s temperature. Being in
contact with another gecko reduced the rate at which the geckos heated and cooled. This
reduction in heating and cooling rates may have important implications for thick-tailed
geckos because they occur in parts of Australia which experience highly variable thermal
regimes, with cold winters and hot summers. Mean daily minimum temperatures of 3.5oC
can occur under surface rocks at Mt Korong during winter, with occasional frosts and nights
where ambient temperature is below 0oC. Mean daily maximum temperatures average
28.9oC during summer, while some days may exceed 35oC (Anon. in Kearney 2002).
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Similarly, grouped gidgee skinks (Egernia stokesii) also cooled at a slower rate than solitary
skinks (Lanham 2001).
When checked an hour after the experiment commenced, geckos that were exposed to
decreasing temperatures (from approximately 28oC down to 10oC) were in contact with one
another significantly more often than those that were kept warmer. This result suggests that
huddling occurs as temperature falls below a certain threshold. Thus, huddling appears to be
initiated by a change in temperature rather than a constant high or low temperature. Thicktailed geckos encounter temperatures well below 10oC regularly in the wild (see Chapter 3),
so this experiment used a biologically realistic range of thermal regimes. However,
temperatures under natural rocks may rarely fall as rapidly as was the case in this laboratory
trial.
Physiological processes and locomotor performance of ectotherms often vary with
temperature, and even a small difference of 1 or 2oC may have a significant impact on
organismal performance (Lanham 2001). A modest reduction in cooling rate might therefore
influence the geckos’ ability to forage in the evening, or to evade predators. Additionally,
on occasions when the geckos are not warm enough to forage, they may still maintain higher
body temperatures for longer; again enabling them to more effectively evade predators,
digest food and increase growth rates. On severely cool nights, cooling more slowly may
even reduce the likelihood of death through freezing. In summer, rocks exposed to full sun
can heat up to very high levels (42.0 – 59.0 oC) (Kearney 2002). Thick-tailed geckos
exposed to 37.5oC showed high mortality during the following 24-hour period, and at
approximately 40oC the geckos survived less than 25 minutes (Licht et al. 1966). Thus,
heating up more slowly may have substantial biological significance.
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The relatively small (< 5 g) tree lizard (Urosaurus ornatus) aggregates in groups of two to
seven lizards during winter, forming tight balls, and choosing south-facing rather than northfacing crevices (Elfström and Zucker 1999). South-facing crevices were, on average, 0.13oC
warmer than north-facing crevices (Elfström and Zucker 1999). These results suggest that
even a small temperature difference may be important in allowing occasional activity and
increasing rates of physiological processes beyond some important threshold. Thus, even in
these small lizards, aggregation and huddling may result in slightly higher body temperatures
allowing some activity to occur and perhaps aiding in winter survival. The juvenile thicktailed geckos tested in the heating and cooling, and huddling experiments above, were
approximately the same size and weight as adult Urosaurus ornatus. Thus, heating and
cooling rates, and huddling of thick-tailed geckos in aggregations may play an important role
in the phenomenon of grouping behaviour. This possibility will be explored further in
Chapter 7.
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CHAPTER 7
General Discussion
My study was stimulated by the observation that thick-tailed geckos aggregate in the wild: a
rare phenomenon among lizards. Besides thick-tailed geckos, only marbled geckos
(Christinus marmoratus) have been quantitatively shown to group in the wild, although less
than 28% of marbled geckos found were in aggregations (Kearney et al. 2001).
Experimentally, a single species, the western banded gecko (Coleonyx variegatus), has been
demonstrated to aggregate (Cooper et al. 1985). The only previous study with empirical
data on this topic for Underwoodisaurus milii is that by Kearney et al. (2001), who reported
that 74% of thick-tailed geckos found during spring and autumn at Mt Korong in Central
Victoria were in aggregations ranging from two to nine individuals. In this study, I gathered
an additional data set on field groupings at this site during winter. I also conducted
controlled experiments in the laboratory to clarify the cues responsible for aggregation, with
the ultimate aim of understanding reasons for the occurrence of this puzzling behaviour.
Below, I review major results from my study, and then suggest a hypothesis for the
functional significance of aggregative behaviour in thick-tailed geckos.
My field data clearly showed that thick-tailed geckos do aggregate, and occur more
frequently in large groups than expected under the null hypothesis of random occurrence.
This phenomenon occurs during winter as well as in autumn and spring. The retreat-sites
used in the field are a non-random subset of available retreat-sites in terms of several abiotic
variables. In addition, the lizards displayed selectivity for such cues, and also exhibited
aggregation behaviour, in the laboratory. In general, when a pair of geckos is presented with
two identical shelters, most combinations of adult male, adult female and juvenile geckos are
found together in the same shelter the next morning. This result indicates that aggregation in
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the field is not simply a response to limited availability of suitable shelter-sites. Aggregation
occurs regardless of whether conspecifics are familiar or unfamiliar.
Adult male thick-tailed geckos are extremely aggressive and fight ferociously with other
mature males. However, pairs of adult males did not occur less often than expected by
chance in the field, and males did not actively avoid each other in the laboratory.
Additionally, juveniles and adult males neither aggregated with, nor avoided each other.
The reasons for this result remain unclear: while adult males are aggressive towards each
other, casual observation and pilot studies suggest that they do not show hostility towards
juveniles. However, it is possible that seasonal shifts occur, determining the levels of
aggression displayed by adult males towards each other, and perhaps also to juveniles.
A mesh partition that prevented physical interaction between geckos eradicated the
aggregation response. The only other pattern evident in these trials was an avoidance of
adult females by juveniles, although it was eliminated when the Bonferroni technique was
applied. Lizards from these two groups aggregated when both geckos were free to move,
and adult females did not show any aggression towards juveniles. Therefore, it is possible
that this result occurred spuriously due to the large number of tests conducted. It is
interesting to note, however, that the only previous study of field aggregations (also at Mt
Korong) in thick-tailed geckos found that juveniles and adult females occurred together less
often than expected by chance (Kearney et al. 2001). This behaviour remains a puzzle.
The presence of familiar scats or shed skins did not attract or repel juvenile geckos. Studies
on several other species of lizards have demonstrated an ability of the lizards to discriminate
between conspecifics using scats (Carpenter and Duvall 1995; Bull et al. 1999), as well as
bodily scent secretions of a conspecific (Cooper 1996; Bull et al. 2000). In this case, it is
possible that scats and shed skins may play a role in providing information about a
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conspecific that has been in the area, but are not themselves a cue for shelter-site selection.
This lack of attraction to scats and skins fits with the notion that aggregation only occurs
when geckos can maintain physical contact with one another, and thus the mere presence of
a conspecific nearby is irrelevant.
Experiments in which I manipulated the presence of predator scent, humidity or mean
ambient temperature did not change aggregative behaviour in thick-tailed geckos. Various
studies on both reptilian and non-reptilian organisms have shown that aggregations occur to
dilute the risk of predation (Eggleston and Lipicus 1992), to reduce water loss when
humidity is low (Rasa et al. 2000; Tanaka 2000) or when ambient temperature is low, as in
winter aggregations (Elfström and Zucker 1999). However, my study suggests that grouping
in thick-tailed geckos is not a facultative response to dry conditions, predator cues or high or
low mean temperature.
Juveniles in contact with another gecko heated and cooled more slowly than did juvenile
geckos on their own. Similarly, gidgee skinks (Egernia stokesii) in contact with other skinks
cooled more slowly than did solitary skinks (Lanham 2001). Marine iguanas
(Amblyrhynchus cristatus) form ‘sleeping piles’ overnight, and aggregated iguanas cool
more slowly than solitary ones, and those at the centre of piles cool more slowly than those
on the outside (Boersma 1982).
When the temperature inside a retreat-site fell rapidly, significantly more geckos were in
contact with each other (huddling), than when temperature changed more slowly or remained
constant. Thus, it appears that a rapid change in temperature rather than a high or low
temperature per se causes an increase in the number of individuals in contact with one
another.
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In combination, these results suggest a novel hypothesis about the functional significance of
aggregative behaviour in thick-tailed geckos. The hypothesis is as follows: that grouping
behaviour allows the geckos to adpress closely to other individuals, thus increasing effective
total mass and decreasing the surface area to volume ratio, and hence reducing rates of
heating and cooling. The animals thus obtain significant behavioural control over rates of
heat exchange, simply by minor postural adjustments that influence degree of contact with
conspecifics. The major lines of evidence in support of this hypothesis are as follows:
1. Importantly, the lizards aggregated only when they were able to have physical contact
with each other. When this opportunity was eliminated, they no longer grouped. The
same pattern was evident in my trials of crevice height selection; in a narrow (12 mm)
crevice, the only geckos to aggregate were those small enough to push closely against
each other (see Chapter 5).
2. The geckos were extremely selective with respect to their diurnal shelter-sites; in the
field, during winter they used rocks that were more thermally stable than ‘random’
rocks, with higher daily minima and lower daily maxima.
3. These lizards are unusual in their cold-climate distribution, large body size and use of
thermally variable rocks. All of their congeners (Underwoodisaurus sphyrurus and
Nephrurus species) occur in warmer climates, while sympatric geckos (Christinus
marmoratus, Diplodactylus vittatus, D. intermedius and Heternotia binoei) are much
smaller (Cogger 2000). Thus, they are the only large Australian geckos occurring in
such cold climates and sheltering in rock crevices (which show strong diel fluctuations
in temperature), rather than soil burrows (which are thermally more stable; Williams et
al. 1999). Thus, they occur in situations where control over rates of heating and
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cooling may enhance fitness, and are also large enough for aggregation to affect rates
of heat exchange.
4. In an environment where mean daily minimum temperatures average 3.5oC during
winter, with extremes sometimes below 0oC (Anon. 1993 in Kearney 2002), it may be
particularly important for the geckos to cool down slowly in the evening to enable
locomotion for foraging and to evade predation, and also to maintain physiological
processes (digestion, growth, reproduction; Huey 1982; Stevenson et al. 1985). At
extremely low temperatures, the lizards may need to maintain body temperatures
higher than ambient simply to survive, and even small differences may be important.
Maintenance of body temperature several degrees above ambient may be possible as a
result of huddling with a group of geckos, in addition to sheltering under thermally
stable rocks.
5. During summer, mean daily maximum temperatures average 29.8oC, with temperatures
greater than 30oC occurring regularly (Anon. 1993 in Kearney 2002). At this time,
temperatures under rocks frequently exceed 40oC (Kearney 2002); high enough to kill
thick-tailed geckos (Licht et al. 1966). While there is a seasonal shift in rock-use from
thin rocks in winter to large boulders and deep crevices in summer (Kearney 2002),
geckos may nonetheless experience occasional days with excessively high
temperatures at any time throughout the year. Therefore, huddling behaviour may
enable thick-tailed geckos to remain a few degrees cooler during the hottest parts of the
day when temperatures reach lethal levels. Once again, even small differences may be
important in such cases.
6. In addition to this, thick-tailed geckos that are aggregated under a rock have the
opportunity to modify their rate of heat exchange by changing the amount of physical
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contact with other geckos under the rock. Huddling more closely will slow down the
rate of heat exchange, while remaining isolated will increase the rate of heat exchange.
7. In my study, I used just two geckos to determine whether heating and cooling rates
were affected by the presence of a conspecific. However, larger aggregations occurred
often in the field, and geckos within such a group are likely to lose and gain heat at a
rate that is still slower. That is, the hypothesis predicts that the geckos may benefit not
only from sheltering with a conspecific, but even more by sheltering with several
conspecifics.
How can we test this hypothesis? Further studies are required to investigate the composition
of thick-tailed gecko aggregations in the field and how they change through time. This may
tell us whether there is a change in the degree of aggregation during the year, and also
whether certain patterns of aggregation occur at certain times of year. Additional
experiments and mathematical modelling are needed to examine the thermal advantages that
may be gained through aggregation, as well as their effect on various physiological
processes, activity times, predator evasion and survival rates of the geckos. It would also be
interesting to conduct additional experiments using larger sample sizes to quantify the
degree of intimate contact between geckos within an aggregation at still lower temperatures
(approximately 5oC), and particularly the thermal consequences of differences in this aspect.
Gidgee skinks are large (~200 g) and socially complex lizards (Lanham 2001) that live on
thermally variable rocky outcrops and shelter in crevices (Cogger 2000). The presence of
huddling behaviour, and its thermal consequences in this lizard further suggests that the
aggregative behaviour seen may have arisen initially for thermal advantages. Aggregation
may have forced individuals to interact with one another due to their close proximity,
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providing an opportunity for the subsequent evolution of complex sociality in the Egernia
lineage.
Aggregation is a widespread phenomenon and is seen in animals from invertebrates such as
beetles, to vertebrates including birds, mammals and reptiles (Yahav and Buffenstein 1991;
Elfström and Zucker 1999; Legg and Cockburn 2000; Tanaka 2000; East and Hoffer 2001).
The specific causes for aggregation may differ from species to species, although the most
obvious reasons involve social interactions between conspecifics (Legge and Cockburn
2000; East and Hoffer 2001). However, other factors may also confer benefits to
aggregation, ranging from predator evasion (Connell 2000) to physical aspects such as
reduction in rates of heat (especially important for ectotherms; Huey 1982) and water loss
(Yahav and Buffenstein 1991; Tanaka 2000). This diversity in phylogenetic and ecological
(functional) aspects of aggregative behaviour means that there is immense potential for
comparative studies. Research on non-traditional ‘model’ organisms, such as lizards, may
provide a valuable perspective to elucidate the generality of conclusions from studies on
aggregation in more ‘popular’ organisms such as endotherms, in which much aggregative
behaviour is assumed to play a primarily social function (Wilson 1975).
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CHAPTER 8
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Scat-piling by thick-tailed geckos
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Scat-piling by thick-tailed geckos
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117
Scat-piling by thick-tailed geckos
APPENDIX 1
Scat-piling by thick-tailed geckos
A1.1 Introduction
Scat-piling is the deposition of faeces by individuals in a specific place so that they
accumulate into piles (Bull et al. 1999). This behaviour occurs in a range of taxa. For
example, many species of mammals have long been known to deposit faeces in specific
positions (e.g. Sneddon 1991; Roper et al. 1993; Vila et al. 1993). European rabbits
(Oryctolagus cuniculus) have communal latrines, or dunghills, which act as a method of
information exchange between rabbits. For example, young males visit latrines frequently,
possibly to investigate odours of dominant males (Sneddon 1991). Iberian wolves (Canis
lupus) deposit faeces at conspicuous sites, such as near trails and at junctions, and on ashes,
carrion and decaying bark, probably to display territory ownership (Vila et al. 1993).
Similarly, badgers (Meles meles) deposit faeces at latrines at the boundaries of their territory
and also close to the centre, probably as a form of mate-guarding by males and possibly also
to defend the burrow system (Roper et al. 1993). The subterranean naked mole rat
(Heterocephalus glaber) lives in a series of tunnels with communal nest and toilet chambers
(Jarvis 1985).
Scat-piling in lizards has been investigated only recently. Currently, few lizards have been
demonstrated to scat-pile, and these lizards are mainly from a single lineage: the Australian
scincid genus Egernia (Bull et al. 1999). Six out of the 26 species of Egernia, including E.
stokesii (White 1976; Swan 1990) and E. striolata (Bustard 1970c), scat-pile close to their
basking sites (Greer 1989), while Egernia hosmeri has communal scat-piles (Stammer
1976). Bull et al. (1999) demonstrated that E. striolata could use chemosensory cues to
discriminate between their own scats and those of conspecifics, suggesting that scat-piles
may function to signal residency.
118
Scat-piling by thick-tailed geckos
One species of gecko, the western banded gecko (Coleonyx variegatus), has also been shown
to scat-pile (Carpenter and Duvall 1995). Discrete scat-piles were observed for both males
and females when housed individually, but communal defecation was not demonstrated
experimentally. It was suggested that scat-piles might act as social signposts from which
conspecifics can derive information about each other (Carpenter and Duvall 1995).
Few other lizards have been formally investigated with respect to scat-piling, although I
have also observed captive eastern spiny-tailed geckos (Diplodactylus intermedius) and
southern leaf-tailed geckos (Phyllurus platurus) depositing scats in specific locations.
Similarly, early in my study I noticed that thick-tailed geckos appeared to be scat-piling in
their home cages. Because studies on Egernia stokesii have suggested that scat-piling offers
a significant cue for social aggregation (Bull et al. 1999), I investigated the phenomenon in
thick-tailed geckos also.
A1.2 Materials and Methods
The aim of this experiment was to determine whether the lizards were scat-piling within
their home cages (see Chapter 2 for details of set-up). All cages were cleaned out and all
sand was removed. Fresh, washed ‘Sydney sand’ was added to each cage, to a depth of
between two and three centimetres. The lizards were fed twice a week as usual and the scats
were left in the cages where they were deposited. All paired juvenile geckos’ cages were
checked after two weeks, while the cages containing paired adult geckos and solitary adult
geckos were checked after five weeks, to allow enough time for scats to be deposited (adult
geckos produce fewer and larger scats than do juvenile geckos). I divided each cage into
several quadrats, using a template made out of bamboo sticks, and then counted the number
of scats per quadrat in each cage.
119
Scat-piling by thick-tailed geckos
The cages containing adult pairs, solitary adults and juvenile pairs were set-up differently.
Adult pairs were housed in larger plastic cages (29 x 21.5 x 6.5 cm) than solitary adults and
juvenile pairs (22 x 22 x 7 cm). As a result, paired adults’ cages were divided into 12
quadrats (7.25 x 7.15 cm each) (Fig A1.1a) and solitary adult and paired juveniles’ cages
were divided into nine quadrats (7.3 x 7.3 cm each) (Figs A1.1b and A1.1c).
All cages containing two geckos had two plastic half-pipe shelters, whereas solitary geckos
had one. Shelters in each cage overlapped two to four quadrats and water bowls overlapped
one to two quadrats. However, this overlap was the same for all three groups of geckos and
cages being compared to each other.
Data collected for scat-piling by adults (paired and solitary) and juveniles (paired) were
analysed for each cage against a null hypothesis of equal frequencies of scat deposition in
each quadrat, using a G-test. To compare data sets obtained for solitary adults and paired
juveniles (housed in same sized cages), I conducted a one-factor repeated-measures
ANOVA.
a) Division of paired adult gecko cages into quadrats.
29 cm
1
2
3
4
5
6
7
8
21.5 cm
9
10
11
12
120
Scat-piling by thick-tailed geckos
b) Division of solitary adult gecko cages into quadrats.
22 cm
1
2
3
4
5
6
7
8
22 cm
9
c) Division of paired juvenile gecko cages into quadrats.
22 cm
1
2
3
4
5
6
7
8
9
22 cm
Figure A1.1: Diagrammatic representation of home cages of adult (paired and solitary) and
juvenile (paired) thick-tailed geckos. In all cases, rectangles represent plastic shelters and
circles show water bowls.
121
Scat-piling by thick-tailed geckos
I conducted a second experiment to test whether juvenile thick-tailed geckos used the
presence of a scat-pile containing some of their own scats as a cue to deposit more scats on
the pile. Two plastic shelters (13 cm x 13 cm x 2.2 cm; see Chapter 4; Fig. 4.2) were placed
in each of 28 experimental units (36 x 28 x 19.5 cm) with a sand substrate. Twenty-eight
juvenile geckos were chosen randomly, and 10 scats of varying age (up to 1½ months old)
from each gecko’s home cage were placed randomly inside one of the two shelters in each
unit. Geckos were placed in experimental units at approximately 1700 h and removed at
0930 h the following morning. Of the 28 geckos, 16 had deposited one or more scats. The
units were divided into two and the freshly deposited scats were scored as either in the same
half of the cage as the scat-pile, or in the ‘empty’ half. Data collected for new scat
deposition were analysed using a Pearson’s Chi-square test.
A1.3 Results
Every pair of juvenile geckos showed highly significant scat-piling after two weeks (Table
A1.1, Fig. A1.2), against the null hypothesis of equal scat deposition per quadrat. Scatpiling by juvenile geckos tended to be concentrated in the corners of the cage, with 71% of
the scats deposited in this area (Table A1.1). In the second experiment, juvenile thick-tailed
geckos showed no significant tendency to deposit fresh scats next to or on scat-piles
containing their own scats (χ2 = 0.25, 1 d.f., P > 0.05).
All ten pairs of adult geckos also showed significant scat-piling when scored after five
weeks (Table A1.2). Paired adult geckos deposited fewer scats (19%) at the back of the cage
near the shelters and in the middle (29%), than at the front near the water bowl (52%).
However, statistical analysis was not carried out due to the unavoidably small sample size.
Twenty-two out of 23 solitary adult geckos showed significant levels of scat-piling after five
weeks (Table A1.3). Like the paired adults, solitary adult geckos deposited scats primarily
122
Scat-piling by thick-tailed geckos
at the front of the cage (55%) near the water bowl rather than at the back (23%) or in the
middle (22%) of the cage (Fig. A1.3).
Table A1.1: Scat-piling by paired juvenile thick-tailed geckos within their home cages after
a period of two weeks (*χ crit(P = 0.05, d.f. = 8) = 15.51, **χ crit(P = 0.005, d.f. = 11) = 21.96).
Cage
no.
1
Back
2
3
4
Middle
5
6
7
Front
8
9
Sum
G-test
Statistic
J1
J2
0
0
3
0
9
0
0
0
0
0
3
0
0
2
0
3
2
20
17
25
33.88**
78.11**
J3
0
0
0
0
0
0
13
5
9
27
63.01**
J4
0
0
31
0
0
10
0
0
1
42
129.56**
J5
0
0
15
0
1
2
7
7
4
36
49.78**
J6
1
2
2
0
1
2
7
7
4
26
23.79**
J7
0
0
4
0
0
0
0
11
9
24
56.31**
J8
0
5
0
0
0
0
12
7
0
24
55.90**
J9
54
0
0
0
0
0
0
0
0
54
237.30**
J10
0
0
0
0
0
3
0
4
16
23
63.24**
J11
0
0
0
0
0
0
12
2
1
15
47.09**
J12
0
0
0
1
0
0
18
0
0
19
75.66**
J13
1
0
0
5
0
0
11
2
0
19
43.23**
J14
1
0
1
0
0
5
10
5
1
23
35.08**
J15
1
0
0
1
0
0
14
0
0
16
55.48**
J16
0
5
1
0
1
1
2
6
1
17
18.74*
J17
1
17
0
1
0
0
0
0
0
19
67.94**
J18
3
0
0
0
10
0
2
3
1
19
33.61**
J19
0
0
10
0
0
7
0
0
5
22
50.06**
J20
0
0
0
0
0
2
0
5
55
62
220.37**
J21
19
0
0
0
0
0
0
0
0
19
83.50**
J22
5
0
17
6
1
0
0
0
0
29
66.06**
J23
2
0
1
5
1
1
17
3
1
31
45.11**
J24
0
0
19
0
0
2
0
0
1
22
75.33**
J25
10
0
0
3
5
1
2
1
0
22
32.18**
J26
10
0
0
0
0
0
1
1
0
12
39.15**
J27
0
0
0
2
0
0
25
7
2
36
93.92**
J28
0
0
0
0
0
11
0
2
14
27
70.10**
J29
15
8
0
1
0
0
1
2
0
27
57.96**
123
Scat-piling by thick-tailed geckos
J30
0
0
12
0
1
1
0
2
6
22
44.58**
Total
123
40
122
25
21
51
156
85
153
776
293.92**
a) A juvenile gecko pair from cage J4 scat-piled in one shelter (shelters removed).
b) A juvenile gecko pair from cage J5 scat-piled along the side of one shelter
(shelters removed).
124
Scat-piling by thick-tailed geckos
J5
125
Scat-piling by thick-tailed geckos
c) A juvenile gecko pair from cage J17 scat-piled between the two shelters (shelters removed).
Figure A1.2: Scat-piling by juvenile thick-tailed geckos in their home cages (22 x 22 x 7 cm)
after a period of two weeks.
Table A1.2: Scat-piling by paired adult thick-tailed geckos within their home cages after a
period of five weeks (*χ crit(P = 0.05, d.f. = 11) = 19.68, **χ crit(P = 0.005, d.f. = 11) = 26.76).
Cage
no. &
sex
A1
(m + f)
A2
(m + f)
A3
(m + f)
A4
(m + m)
A5
(m + f)
A6
(m + f)
A7
(m + f)
A8
(m + f)
A9
(m + f)
A10
(m + f)
Total
Back
Middle
Front
Sum
G-test
statistic
1
2
3
4
5
6
7
8
9
10
11
12
5
7
1
3
2
4
5
5
7
2
13
1
55
25.22**
0
1
1
0
1
0
0
4
2
1
16
17
43
89.15**
8
3
0
0
28
1
1
0
26
1
0
0
68
159.98**
5
1
2
0
2
10
2
6
10
8
11
5
62
36.50**
6
11
5
2
0
6
23
0
18
8
2
0
81
89.61**
2
0
0
0
1
3
1
0
3
10
13
25
58
106.88**
3
2
0
2
5
7
8
1
18
20
1
0
67
82.18**
11
0
1
0
3
0
4
0
15
12
1
1
48
78.23**
1
5
2
15
5
0
0
15
0
3
12
2
60
73.31**
3
1
0
5
2
1
6
10
9
5
8
8
58
32.44**
44
31
12
27
49
32
50
41
108
70
77
59
600
144.25**
125
Scat-piling by thick-tailed geckos
Table A1.3: Scat-piling by solitary adult thick-tailed geckos within their home cages after a
period of five weeks (*χ2 crit(P = 0.05, d.f. = 8) = 15.51, **χ2 crit(P = 0.005, d.f. = 11) = 21.96, NS = Not
significant).
Cage
no. &
sex
1
2
3
4
5
6
7
8
9
A11
0
0
1
1
0
0
5
12
0
0
8
1
0
0
5
1
3
0
5
0
0
0
1
0
0
1
0
0
2
0
0
0
10
1
0
0
(f)
A12
(f)
A13
(m)
A14
(m)
A15
(m)
A16
(m)
A17
(m)
A18
(m)
A19
(m)
A20
(f)
A21
(m)
A22
(f)
A23
(m)
A24
(m)
A25
(m)
A26
(f)
A27
(f)
A28
(f)
A29
(f)
A30
(f)
A31
(m)
A32
(f)
A33
(m)
Total
Back
Middle
Front
Sum
G=test
Statistic
12
31
58.69**
8
2
24
38.33**
3
5
3
20
20.02*
5
1
0
3
11
18.27*
1
0
8
11
7
29
48.18**
0
0
4
2
4
4
24
35.02**
1
6
3
3
4
1
1
20
13.84NS
1
0
0
0
8
2
1
7
19
34.89**
0
0
12
0
1
7
0
0
3
23
50.31**
1
0
0
2
7
0
2
13
3
28
42.51**
0
1
1
0
2
1
0
1
18
24
59.75**
0
0
1
3
8
0
5
3
3
23
25.98**
10
0
0
5
0
0
5
3
0
23
41.67**
3
2
0
3
3
0
14
1
0
26
41.28**
0
0
4
1
0
3
3
1
5
17
18.74*
9
9
0
0
1
0
2
12
0
33
55.76**
0
1
0
2
8
0
6
6
0
23
35.89**
0
1
2
0
1
0
6
2
9
21
31.01**
4
1
2
3
3
2
11
0
0
26
27.41**
3
0
1
0
0
2
8
5
1
20
26.79**
18
1
1
8
2
0
1
5
0
36
56.38**
1
1
0
1
0
0
4
13
7
27
45.70**
4
0
3
0
0
2
0
3
6
18
23.59**
55
22
49
41
41
37
97
110
94
546
127.09**
126
Scat-piling by thick-tailed geckos
10
Solitary adults
9
Paired juveniles
Mean number of scats
8
7
6
5
4
3
2
1
0
1
1
2
2
3
3
4
back
4
5
5
6
middle
6
7
8
7
9
8
10
9
11
front
Quadrat
Figure A1.3: Mean number of scats deposited (+ S.E.) per quadrat by solitary adult and
paired juvenile geckos.
Because paired juvenile and solitary adults were housed in the same sized cages with the
same number of segments, I could directly compare these two data sets in terms of the
locations of scats. For this purpose, I conducted a one-factor repeated-measures ANOVA
with age class as the factor and segment as the repeated measure. The analysis confirmed
that some segments received significantly more scats than others (see Fig A1.3; F8, 208 =
2.08, P < 0.006), but that adults and juveniles did not differ overall in the number of scats
produced (F1, 26 = 0.87, P = 0.36), nor in the distribution of scats among segments
(interaction F8, 208 = 0.89, P = 0.52). Both age groups deposited scats primarily in the cooler
(front) parts of their cages (Fig A1.3).
A1.4 Discussion
Both adult and juvenile thick-tailed geckos scat-piled significantly within their cages.
Juvenile geckos scat-piled when housed in pairs; adult geckos scat-piled both when housed
individually and when housed in pairs. Scat-piling was non-random, with juvenile pairs
127
Scat-piling by thick-tailed geckos
depositing 71 % of their scats in the corners of the cages, many of them being in one of the
two shelters. Adult geckos, whether paired or solitary, scat-piled more at the front of the
cage (near the water bowl) than at the back of the cage near their shelters. There was a
general tendency to pile scats closer to the edges than to the centre of the cages.
Amongst geckos, only the western banded gecko (Coleonyx variegatus) and Texas banded
geckos (Coleonyx brevis) have been reported to scat-pile in the field (Carpenter and Duvall
1995). Western banded geckos, when housed individually, formed scat-piles. They also
scat-piled when introduced into an arena that had been previously marked via scat-piling by
a member of the opposite sex (Carpenter and Duvall 1995). While communal defecation
was not demonstrated via experimental evidence presented in their study, Carpenter and
Duvall (1995) observed communal defecation when several geckos were housed together in
a common terrarium.
Western banded geckos (Coleonyx variegatus) scat-piled in shelters for 61% of the trials
conducted, and these shelters were most often the ones not used as diurnal retreat-sites by
the geckos (Carpenter and Duvall 1995). Similarly, I observed qualitatively that thick-tailed
geckos that scat-piled in shelters tended to do so in only one of the two available shelters.
Furthermore, they did not use the shelter containing scats as their diurnal retreat-site. This
behaviour may serve as an anti-predator mechanism if predators detect the presence of
geckos by scent (Ford and Burghardt 1993), offering potential survival advantages to the
lizard (Carpenter and Duvall 1995).
The tree skink (Egernia striolata) (Bull et al 1999) and the western banded gecko (Coleonyx
variegatus) (Carpenter and Duvall 1995) have been shown to differentiate between their own
scats and those of other conspecifics, as well as from control substances. Western banded
geckos (C. variegatus) deposited more scats in the end of the arena where an extract of their
128
Scat-piling by thick-tailed geckos
own faecal scent (in a 1:1 chloroform: methanol mixture) was applied than the end where the
chloroform/ methanol mixture alone was applied (Carpenter and Duvall 1995).
Thick-tailed geckos in my study, however, did not preferentially deposit scats on piles
containing their own scats, perhaps because some of the scats were up to one and a half
months old. Two-week-old scats elicited significantly fewer tongue-flicks by tree skinks (E.
striolata) than did one-week-old scats (Bull et al. 1999). Thus, the presence of old scats may
have weakened the signal to the geckos resulting in deposition of scats randomly relative to
the piles that I placed in the experimental units. Alternatively, thick-tailed geckos may
deposit scats in specific preferred locations, irrespective of the presence of scat-piles
elsewhere.
The chemical cues within scats that serve as signals to animals are likely to be chemicals of
relatively low, transient volatility (Carpenter and Duvall 1995; Bull et al. 1999). Thus, if
scats serve as social signals or indicators, they must be replenished regularly to retain the
strength of the signal, resulting in scat-piles (Bull et al. 1999). The specific signal is not
known, although it may be pheromonal. In several lizard species, pheromones are produced
by the urodeal glands situated close to the cloaca (Cooper et al. 1986; Cooper and Trauth
1992; Bull et al. 1999). Such glands have been identified in the broad-headed skink
(Eumeces laticeps) (Cooper et al. 1986) and in the cordylid lizard Gerrhosaurus
nigrolineatus (Cooper and Trauth 1992).
A lizard’s ability to discriminate its own scats from those of a conspecific (as shown by Bull
et al. 1999) suggests a social function for scat-piling, rather than a simple coincidence
resulting from the lizards spending most of their time in a specific area. Such a
discriminatory ability may play an important role in marking territories, or searching for
mates. For example, red-backed salamanders (Plethodon cinereus) use scats as territorial
advertisements (Jaeger 1986; Jaeger et al. 1986). The salamanders are also able to
129
Scat-piling by thick-tailed geckos
differentiate between scats of a familiar neighbour of an adjoining territory from those of a
stranger, facilitating ‘dear enemy’ recognition (Jaeger 1981; Jaeger et al. 1986; Horne and
Jaeger 1988).
Nonetheless, the observation that animals defecate in consistent places does not necessarily
mean that scat-piles play a significant role in social interactions. It is possible that geckos
scat-piling in shelters do so because they only attain optimal body temperatures for digestion
during the day while sequestered under rocks (Carpenter and Duvall 1995). Many Egernia
species have also been noted to deposit scats close to their basking site where they attain
optimal temperatures (Bull et al. 1999). These scats may then be co-opted by natural
selection to act as social signals.
However, this optimal temperature explanation is unlikely because most adult thick-tailed
geckos deposited scats in the open rather than under shelters, while many juvenile geckos
deposited scats along the outside of shelters. In both cases, these locations were the cooler
parts of the cage, with the shelters being warmer.
In summary, my data show that thick-tailed geckos form scat-piles under laboratory
conditions. Studies on other lizards, including the Western banded gecko (Coleonyx
variegatus; Carpenter and Duvall 1995), and several species of Egernia skinks (Bull et al.
2001) suggest that scat-piling can, and does occur in the field. Studies on both geckos and
skinks have shown that several species of lizards are able to discriminate between their own
scats and those of conspecifics, and hence such scat-piles may play a significant social role.
Whether or not scats provide such cues for thick-tailed geckos is in interesting question and
could be the focus of future studies.
130