Download Evolution of allometries in the worker caste of Dorylus army ants

OIKOS 110: 231 /240, 2005
Evolution of allometries in the worker caste of Dorylus army ants
Caspar Schöning, Wanja Kinuthia and Nigel R. Franks
Schöning, C., Kinuthia, W. and Franks, N. R. 2005. Evolution of allometries in the
worker caste of Dorylus army ants. / Oikos 110: 231 /240.
The worker caste of polymorphic ant species consists of individuals of strikingly
different morphologies. Most studies of polymorphic species have focused
on intracolonial allometries and their association with division of labour and
task performance of workers of different physical subcastes. However, the factors
driving the evolution of these allometries are poorly understood. Here, we analyse
the importance of life-style (hypogaeic vs intermediate vs epigaeic) as a factor in
the evolution of allometries of functionally important body traits in Dorylus army ants
in the context of an evolutionary scenario according to which a hypogaeic life-style is
the ancestral state in this group. To this end, we conducted a detailed comparative
analysis of the allometries of nine characters for ten species belonging to all six
currently recognized subgenera and showing different life-styles. Eight of the nine traits
under consideration show a clear increase in relative size from hypogaeic to
intermediate to epigaeic life-style. These results strongly suggest that shifts in
ecological niche are linked with adaptations in these traits. The degree of overall
differentiation among species is more pronounced in larger than in smaller workers.
The pattern of division of labour in the epigaeic D. molestus indicates that two factors
that may have caused this phenomenon are new food habits and an increased need for
colony defence.
C. Schöning, Inst. für Biologie, Neurobiologie, Freie Univ. Berlin, Königin-Luise-Strasse
28/30, DE-14195 Berlin, Germany ([email protected]). / Present address:
Zoological Inst., Univ. of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen,
Denmark. / W. Kinuthia, Dept of Invertebrate Zoology, National Museums of Kenya,
P.O. Box 40658, Nairobi, Kenya. / N. R. Franks, School of Biological Sciences, Univ. of
Bristol, Woodland Road, Bristol, UK, BS8 1UG.
Polymorphism in the worker caste of ants has been
defined as non-isometric increase in size occurring over a
sufficient range of adult size variation to produce
individuals of distinctly different proportions (Wilson
1953, Oster and Wilson 1978). Workers of different
physical subcastes show marked but flexible task specialization (Wilson 1980, 1984, Moffett 1987, Franks et al.
2001, Hughes et al. 2003), and this is thought to enhance
the efficiency of the worker caste as a whole, because
workers allocate tasks to themselves for which they are
morphologically specially adapted (Oster and Wilson
1978, Bourke and Franks 1995).
Physical subcastes are defined as groups of workers
that underwent the same developmental programme
(Wheeler 1991). The development of ant larvae is
controlled by nutrition and inhibitory pheromones
(Wheeler 1991), but in one species studied so far,
different physical subcastes were partly associated with
different patrilines within a colony (Hughes et al. 2003).
The effects of environmental cues are mediated by
hormones, which act at specific thresholds and cause a
switch between alternative developmental pathways with
associated patterns of gene expression (Evans and
Wheeler 2001, Abouheif and Wray 2002). In this way,
the critical size at which the larva begins metamorphosis
and/or the growth parameters of tissues can be reprogrammed. Imaginal structures grow mostly only during
metamorphosis once food intake has stopped, so that the
Accepted 26 December 2004
Copyright # OIKOS 2005
ISSN 0030-1299
OIKOS 110:2 (2005)
231
allometries among different body parts reflect the outcome of competition between growing tissues for nutrients (Wheeler 1991, Nijhout and Wheeler 1996,
Nijhout and Emlen 1998).
Physical subcastes of polymorphic species are usually
identified based on allometries and/or the size-frequency
distribution of workers (Hollingsworth 1960, Raignier
et al. 1974, Franks 1985, Cagniant 1991, Tschinkel
1998), but this approach has been criticised (Feener et
al. 1988). As the size-frequency distribution of the
standing worker population represents the sum of
many separate (or continuous, if the queen(s) lay(s)
eggs continuously) brood sets modified by size-dependent mortality rates (Calabi and Porter 1989, Wheeeler
1991), the use of size-frequency distributions as an
indicators of developmental mechanisms is problematical. Moreover multimodality is hard to demonstrate
statistically. It has been shown that complex allometries
can be the product of reprogramming of growth parameters (Nijhout and Wheeler 1996), but it is nevertheless
hard to detect physical subcastes from smoothly curvilinear graphs. Finally, as switches between alternative
developmental pathways can occur in different tissues at
different times or not at all in others, interpretations
concerning subcastes will depend on the choice of
morphometric characters.
The allometries among body parts within ant colonies
represent a form of static allometry and can often be well
described by Huxley’s growth function y /b/xa
(Huxley 1932). Artificial selection experiments and a
great number of comparative studies have shown that
allometries in holometabolous insects can evolve quite
rapidly (Emlen and Nijhout 2000). As selection in social
insects not only operates at the individual level but also
at the colony level, the allometries found in ants and
bumblebees differ from the allometries of solitary
holometabolous insects in that they are an adaptive
colony trait. Previous studies have shown that these vary
between populations (Owen and Harder 1995,
McArthur et al. 1996), between different families within
colonies of a facultatively polygnous formicine species
(Fraser et al. 2000), between young pleometrotic and
haplometrotic colonies in facultatively polygnynous
myrmicine species (Kenne et al. 2000), with colony size
(Kenne et al. 2000, Nowbahari et al. 2000) and perhaps
with season (Fig. 3 in Rissing 1987).
While numerous studies have examined allometries
within species, comparative analyses that may reveal
which factors drive the evolution of these allometries
and allow insights into the functional morphology of
workers with different body proportions are generally
sparse (but see Wehner 1983, 1987, Feener et al. 1988,
Zollikofer 1994).
The results of recent studies inspired by the size-grain
hypothesis (Kaspari and Weiser 1999) indicate that the
characteristics of different spatial niches in a habitat act
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as selection pressures for the morphology of the ant
species using these different niches (Espadaler and
Gómez 2001, Parr et al. 2003, Farji-Brener et al. 2004).
Farji-Brener et al. 2004 also suggested that colonies of
polymorphic ant species could enhance their foraging
success by sending out workers of different sizes to
explore and forage in environments with different levels
of rugosity. However, at least in some species such as
polymorphic army ants, which by definition employ
obligate collective foraging, workers of different sizes
work together in the same microniche. If different spatial
niches in a habitat do indeed favour specific ant worker
morphologies, one would expect workers of all sizes in
these species to show the same respective niche specific
adaptations.
The army ant genus Dorylus represents a well-defined
monophyletic group (Bolton 1990, Brady 2003) with
six currently recognized subgenera (Bolton 1995).
The worker caste is highly polymorphic in all species
studied so far (Emery 1901, Hollingsworth 1960,
Raignier et al. 1974, Berghoff et al. 2002b). Species of
the subgenera Dichtadia , Rhogmus, Typhlopone, Alaopone and Dorylus s.str. hunt and nest in the soil.
Only under exceptional circumstances do workers of
these species appear on the surface: when foraging
workers have to cross hard-packed soil (C. Schöning
pers. obs.) or encounter unusually rich food sources
(Berghoff et al. 2002a), during interspecific battles
(Leroux 1979, Gotwald 1995, p. 159) and when
the males emerge from the nest (Wheeler 1922). The
use of ecological niches by species of the subgenus
Anomma is more complex. While the typical ‘‘driver
ants’’ (e.g. D. wilverthi , D. molestus ) conspicuously
forage on the surface and climb vegetation in search
of prey, several species hunt in the leaf litter stratum (e.g.
D. kohli , Wasmann 1904, Raignier and van Boven 1955;
D. gerstaeckeri , Gotwald 1974). Gotwald (1978) proposed that the extant Dorylus species derive from an
ancestor with a completely hypogaeic life-style and that
a few species adopted an epigaeic foraging style secondarily. He based this inference on the observation that
workers are eyeless and have a reduced number of
maxillary and labial segments in all species regardless
of life-style.
The aim of the present study is to examine
the importance of life-style (foraging stratum use
and nesting habits) as a selection factor in the evolution
of allometries in the Dorylus worker caste. We have
chosen to analyse the allometries of such characters
whose functional value can be demonstrated or at
least be inferred with reasonable confidence. They
should all be subject to diverse selection pressures in
the different ecological niches and therefore likely reflect
possible adaptations associated with ecological niche
shifts.
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Division of labour
In order to understand why morphological differentiation is more pronounced in larger workers (results) we
examined additionally the pattern of division of labour
in the epigaeicly hunting Dorylus molestus. Workers
engaged in 14 different tasks (sample sizes 113 /1667
workers per task) were collected, using forceps or
aspirator, from the same colony mentioned above over
a period of 2 weeks to test for size-related division
of labour and to check which tasks large workers
specialize on. The tasks are listed and defined in the
legend of Fig. 6.
Material and methods
Samples
Specimens used for this study were collected during field
work in Kenya and Uganda between March 2001 and
September 2002 and stored in 70% ethanol. Additionally, samples from south east Asian representatives of the
genus were provided by Stefanie Berghoff. All six
currently recognized subgenera were thus covered. In
four species, samples were collected from the migration
trail or directly from the nest and therefore likely
represent the entire size range of workers in that colony.
However, for the remaining six species nests or migrations could not be found so that these samples include
only foragers and thus most probably consist only of
subsets of the worker caste size range. In all 10 species,
workers were taken from a single colony. Intraspecific
variability is low compared to interspecific variability
(C. Schöning unpubl.). For morphometric analysis about
100 uninjured workers each were chosen from the
samples so as to represent the entire size range evenly.
Callow workers were excluded. Details on origin and
type of sample are summarized in Table 1. Representative specimens will be deposited in the Department of
Invertebrate Zoology, National Museum of Kenya,
Nairobi and the Natural History Museum, Berlin.
Based on own observations or information given in
the literature, species were categorized as exhibiting one
of the following three life-styles:
epigaeic (E) / hunting on the ground surface and in the
vegetation; conspicuous swarm raids attracting ant
birds; large, conspicuous earth nests.
intermediate (I) / hunting in the leaf litter but not in the
vegetation; swarm raids can be so conspicuous as to
attract ant birds; nests have never been found (presumably as in hypogaeic species, below).
hypogaeic (H) / usually hunting in the soil; normally not
seen in the leaf litter or on the ground except when
passing through areas with for hard packed soil; nests
almost undetectable (Weissflog et al. 2000, Berghoff et
al. 2002a, b, 2003 for information on D. laevigatus and
D. cf. vishnui ).
Morphometrics
All measurements were taken using a MS 5 Leica
stereomicroscope fitted with an ocular micrometer. The
maximum possible magnification was used whilst keeping a structure within the range of the ocular micrometer
(6.3 /64). We used methods recommended by Seifert
(2002) to minimise measurement errors. Measurement
error (ME) was assessed by repeated measurements and
was quantified as % ME following Bailey and Byrnes
(1990) for a set of 10 D. molestus individuals encompassing the entire size range. These were scored for all
characters five times. A one-way analysis of variance was
then performed to partition variance into within- and
between-individual components. The definitions, abbreviations, percent measurement errors, and presumed
functional significance of the characters under consideration are as follows:
Maximum head width (HWmax, 0.27%) / maximum
measurable head width. Wider heads can accommodate
larger mandibular muscles and may therefore allow
larger biting force (Paul 2001).
Anterior head width (HWa, 0.62%) / distance between points where mandibles touch head capsule when
fully expanded with forceps. A wider anterior head width
allows the mandibles to be opened wider when biting
predators or prey.
Length of the funiculus of the antenna (FL, 1.74%) /
length of the left funiculus, i.e. length of the distal
jointed, sensory part, of the antenna. Funiculus length
Table 1. Dorylus samples used for morphometric analyses. Abbreviations: H / hypogaeic, I / intermediate, E / epigaeic,
N / workers from nest or migration trail, F / foraging workers.
Subgenus
Dichtadia
Rhogmus
Typhlopone
Alaopone
Dorylus
Anomma
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Species
Life-style
Sample type
Sample origin
D. laevigatus
D. sp.
D. obscurior
D. cf. vishnui
D. affinis
D. sp.
D. kohli
D. opacus
D. wilverthi
D. molestus
H
H
H
H
H
I
I
I
E
E
N
F
F
F
N
F
F
F
N
N
Sitiawan, Malaysia (482?N 10085?E, 0 m asl.)
Kibale NP, Uganda (08 34?N 308 22?E, 1500 m asl)
Wamba, Kenya (O8N, 37 E, 1350 m asl)
Kinabalu NP, Sabah, Malaysia (685?N 11683?E)
Chogoria, Kenya (0813?49??S, 37837?54?? E, 1575 m asl)
Kibale NP, Uganda (0834?N 30822?E, 1500 m asl)
Mgahinga NP, Uganda (1828?S, 29840?E, 2200 m asl)
Kibale NP, Uganda (08 34?N 30822?E, 1500 m asl)
Kibale NP, Uganda (0834? N 30822?E, 1500 m asl)
Chogoria, Kenya (0814?16??S, 37834?31??E, 1842 m asl)
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should be important in trail following behaviour, especially because Dorylus workers are eyeless. Longer
funiculi (and scapes) may enable individuals both to
follow the pheromone trails at higher speeds and avoid
collisions (Couzin and Franks 2003).
Length of the scape of the antenna (SL, 0.02%) /
Maximum straight line scape length excluding the basal
condyle. Functional significance see FL.
Mandible length (ML, 4.36%) / distance between
apex of the left mandible to the point where it touches
the head capsule when fully opened with forceps. Longer
mandibles will allow more painful biting of predators
and more efficient cutting of larger prey animals.
Mandible gape width (MGW, 5.15%) / distance
between mandible apices when both are fully opened
using forceps. A wider mandible gape allows biting of
larger objects (prey or predator).
Front leg length (FLL, 1.5%) / maximum length of
left hind leg from trochanter to tarsal tips in dorsal
view with the leg fully extended. As prey items are
carried slung beneath the body, front leg length should
limit the maximum possible size of prey items. Furthermore longer legs may allow more efficient and faster
locomotion.
Hind leg length (HLL, 0.87%) / maximum length of
left hind leg from trochanter to tarsal tips in dorsal view
with the leg fully extended. Longer legs will allow more
efficient and faster locomotion and more space for other
assisting workers during cooperative prey item transport
(Franks et al. 1999), but on the other hand increase the
cross-sectional area needed by the individual and therefore impede entering crevices (Kaspari and Weiser 1999).
The last point should be of particular significance for
species hunting in the soil.
Pronotum width (PW, 0.58%) / maximum pronotal
width in dorsal view. This character was used as another
size indicator (below).
For D. (Rhogmus ) sp. only a reduced data set (PW,
HWmax, SL, FL, HLL, DM) was recorded.
All measurements are in millimetres. Measurement
error is generally very low, but higher for MGW and ML
which reflects the fact that these measurements are
difficult to make. But the differences in MGW and ML
between species are sufficiently large as to allow meaningful conclusions despite lower accuracy in these
measurements.
While any two linear measurements can be chosen to
test for allometry and thus whether a given species
complies with Wilson’s definition (above) of a polymorphic species or not, in a comparative approach one
must have a general estimate of overall body size. DinizFilho et al. (1994) used principal component analysis in
their study of polymorphism in Camponotus rufipes to
identify which among ten morphological characters
scaled most closely to isometry and could therefore be
used as an indicator of overall size. However, when
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allometric relationships are curvilinear rather than
linear, principal component analysis cannot be employed
in a straightforward manner (Diniz-Filho et al. 1994).
We therefore used dry mass (DM, 0.02%) as an indicator
of overall size, as this should also represent a good
estimate of biomass investment and therefore allow
conclusions as to how colonies invest biomass in
workers of different proportions. Specimens were dried
at 708C for 48 h and then weighed to the nearest 0.01 mg
using a R 200 D balance (Sartorius GmbH, Göttingen,
Germany).
For the D. molestus workers engaged in various tasks
PW only was measured.
Statistical analysis
As the aim of the present study was a comparison of the
relative sizes of certain body traits between workers of
different species regardless of the specific developmental
programme that generates these workers, we did not
attempted to identify physical subcastes. Instead, for
each trait we first established the best fit model for raw
linear data from all species combined over their common
size range (0.20 mg /3.67 mg) as a function of DM1/3 by
stepwise multiple regression. Higher order polynomials
were included if all coefficients were significantly different from zero (p B/0.05, Zar 1996, Fraser et al. 2000). As
the goal of conducting this multiple regression analysis
was to develop a model that best predicts variability in
the response, and there was no interest in studying the
particular relationships between the response and explanatory variables of different orders, the problems due
to multicollinearity can be ignored (Graham 2003). The
relative residuals (absolute residual divided by predicted
value) from this common regression model were then
compared using the Kruskal /Wallis Test with life style
as grouping factor and the Mann /Whitney U-test for
pairwise
comparisons
between
life-styles,
as
variances were heteroscedastic (Sokal and Rohlf 1995).
Additionally, an identical analysis was run over the
entire size range (0.05 mg /11.08 mg) displayed in our
samples.
Plots of raw data had initially indicated that the
differences between species increased with DM both
absolutely and relatively. To explore whether this is a real
effect we used two tests. For each trait we identified the
two species with the lowest and highest relative residual
values over the common size range as established by the
Mann /Whitney U-Test. We then calculated the ratios
between the values predicted by the respective best-fit
model over their common size range by determining the
function f(x) /best-fit model of species with highest
relative residual values/best-fit model of species with
lowest relative residual values. This ratio indicates the
maximum differentiation between the examined species
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for the trait under consideration as a function of DM1/3.
In addition, for each trait the coefficient of variation
(CV) was calculated for data from all species combined
in 15 equally sized intervals over the common size range.
It was then tested whether the CV increases with DM1/3.
A higher CV implies stronger differentiation.
In general, we did not use log-transformed data
because (1) log-transformation data often did not show
a normal distribution, (2) log-transformation yielded
curvilinear relationships so that statistical analyses based
on log-transformed data are as difficult to analyse as
those based on raw (and cube-root transformed) data,
and (3) a/b "/(log a)/(log b) so that the calculation of
ratios would be confounded.
To test for size differences between D. molestus
workers engaged in different tasks again the Kruskal /
Wallis Test with task as grouping factor and additionally
the Mann /Whitney U-test for pair-wise comparisons
were used (Sokal and Rohlf 1995).
Significance levels were adjusted for multiple comparisons using the Dunn-Šidák method (Sokal and Rohlf
1995).
All statistical tests were performed using STATISTICA ’99 Edition (StatSoft, Inc.).
Results
In all traits under consideration, life-style had a significant effect on the values of the relative residuals
values (Kruskal /Wallis test, pB/0.001 in all cases).
Except for pronotum width, workers of epigaeic species
always had higher values than workers of intermediate
species and these in turn showed higher values than
workers of hypogaeic species (Fig. 1). Pronotum width
values of hypogaeic workers were larger than in intermediate workers (Mann /Whitney U-test, Z/3.92, pB/
0.0001) but do not differ between the latter and epigaeic
workers (Mann /Whitney U-test, Z/ /0.16, p/0.87).
These results also hold when the entire size range,
instead of the common size range, is considered. As an
illustration of the general pattern of increase in the traits,
Fig. 2 shows bivariate plots of HLL and HWmax vs
DM1/3.
In all traits, the ratios between the species with the
highest residual values and the one with the lowest
values increased with DM1/3, but never in a simple linear
way (Fig. 3). CV increased significantly with DM1/3 for
all traits except SL (r2 /0.219 and p/0.078 for SL, r2
between 0.367 and 0.633, pB/0.02 for all others).
D. molestus workers engaged in different tasks
differed significantly in size (Kruskal /Wallis test,
pB/0.0001, Fig. 4). The groups with the highest median
values are workers transporting food item singly, leading
workers in food transporting groups, workers in defensive posture along foraging trails and workers defending
OIKOS 110:2 (2005)
the nest. These are all significantly larger than all other
groups.
Discussion
The evolution of allometric scaling relationships in the
worker caste of ants has so far received little attention.
Here we show that in Dorylus army ants differences in
life-style are directly correlated with differences in the
allometric scaling relationships with an unambiguous
increase from hypogaeic to intermediate to epigaeic
species in maximum head width, anterior head width,
flagellum length, scape length, mandible length, mandible gape width, front leg length, and hind leg length.
These results provide further support for the conclusions
of other studies (Kaspari and Weiser 1999, Farji-Brener
et al. 2004) that different spatial niches in a habitat select
for specific worker ant morphologies.
As traits of identifiable functional value were selected
for our morphometric analysis, we can try to pinpoint
the selection pressures that may have favoured the
morphological changes. For workers hunting in the
leaf-litter and on the surface longer legs will convey
advantages with respect to locomotion and food transport (i.e. they should be able to cover long foraging
distances faster and/or more efficiently and carry more
food mass per unit body mass, Franks 1985, 1986), and
at the same time they will be much less constrained by
the possible costs associated with longer (hind) legs than
workers hunting in the soil. The leaf-litter stratum may
also hold many interstices yet these will be on average
larger than interstices in the soil simply because soil is
more densely packed. In the larger workers (dry mass /1
mg) the ratio between front and hind leg length increases
from hypogaeic to intermediate to epigaeic species
(Fig. 5). So the increase in hind leg length from
hypogaeic to epigaeic is stronger than the increase in
front leg length. This allows the conclusion that food
and/or brood transport is indeed also of great importance for hypogaeic species but that longer hind legs are
simply prohibitive for them when hunting in the soil.
Longer legs and faster running speeds may in turn
necessitate longer antennae to enable workers to follow
the trail pheromone and/or avoid collisions with other
workers running on the trail (Couzin and Franks 2003).
Intermediate and epigaeic life-styles require more efficient defence as colonies will be more apparent to
predators, more prone to disturbances during their raids
and finally more vulnerable to thieves, in particular
opportunistic birds which do not only catch animals
escaping from the advancing swarm raid but also steal
those that have been pinned down by the ants
(C. Schöning pers. obs.). Longer mandibles, larger
anterior head widths and as a result larger mandible
gape widths should enable these species to inflict more
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Fig. 1. Column plots of relative residuals for all traits from common regression models over common size range for workers with different life-styles (H / hypogaeic, I / intermediate,
E / epigaeic). The whiskers encompass the lower 25% and upper 75% percentile range; the square indicates the median. The results of the Kruskal /Wallis-tests are given below.
Fig. 2. Hind leg length (HLL) and maximum head
width (HWmax) vs dry mass1/3 (DM1/3) for D. wilverthi (/),
D. molestus (), D. opacus ("), D. kohli ('), D. sp. (j), D.
(Rhogmus) sp. (2), D. affinis (^), D. vishnui (k),
D. laevigatu s (m), D. obscurio r (I).
painful bites on larger animals and thus pose a formidable deterrence. On the other hand, the features associated with locomotion and defence will also allow
colonies to pin down and fragment larger prey animals.
In summary, the inferred advantages associated with
larger maximum head width, anterior head width,
Fig. 3. Ratios between the predicted values of the species with
the highest relative residuals and the predicted values of
the species with the lowest relative residuals over their common
size range. HLL (k), FLL ("), MGW (2), ML (j), FL (I),
SL (^), HWa ('), HWmax ("), PW (/).
OIKOS 110:2 (2005)
funiculus length, scape length, mandible length, mandible gape width, front and hind leg lengths are certainly
complementary, but several of them can and should be
tested. Large D. molestus workers (pronotum width /
0.9 mm, this corresponds to a dry mass of about 1.6 mg)
are disproportionately often engaged in dismembering
food, food transport, and colony defence (Braendle et al.
2003), and a similar pattern can be found in D. wilverthi
(Franks et al. 2001). As morphological differentiation
among species is also more pronounced in workers of
this size-class, this indicates that the requirement for
efficiency in these tasks might have selected for divergent
ecological niche-specific morphologies.
According to the evolutionary scenario proposed by
Gotwald (1978) the differences in life-style between
extant Dorylus species are the result of a unidirectional
shift from hypogaeic to epigaeic niches, but this scenario
did not take into account more subtle differences in
ecological niche use by Anomma species. Convergent
evolution of similar characters is the strongest type of
evidence for adaptation (Harvey and Pagel 1991). A
species-level phylogeny of the genus Dorylus is lacking,
and even the monophyly of Dorylus s.str. and Anomma
are uncertain (Barr et al. 1985). Therefore, a more
comprehensive analysis of morphological character
evolution will only be possible once the phylogeny of
the genus has been established. Species in the army ant
subfamilies Ecitoninae and Aenictinae also show prominent differences in their ecological niches (Rosciewski
and Maschwitz 1994, Gotwald 1995) and therefore offer
additional opportunities to examine the link between
scaling relationships of the morphological characters we
examined and life-style.
The mechanisms allowing coexistence of different
army ant species in a given habitat are poorly understood. Resource partitioning according to food item size
has been shown to be an important mechanism in
harvester ant assemblages (Davidson 1977) but is
unlikely to be a relevant phenomenon here because
hypogaeic, intermediate and epigaeic Dorylus spp. all
display extensive polymorphism. Rather, we suggest that
species are using different strata of the same habitat for
foraging and thereby reduce competition with each
other. This, however, does not explain how different
species displaying the same life-style can be sympatric.
Detailed analyses of the food spectra of sympatric
species hunting in the same stratum may reveal whether
niches are partitioned according to prey type as has been
suggested for neotropical army ants (Rettenmeyer et al.
1983).
Foraging stratum use is not completely static. During
their swarm raids in the leaf litter, workers of D. opacus
and D. (Anomma ) sp. for example sometimes also come
up on the leaf litter surface (but do not climb vegetation). On two occasions we observed antagonistic
interactions between the epigaeic D. wilverthi and these
237
Fig. 4. Division of labour in D. molestus. Size (measured as
pronotum width) of workers engaged in the following tasks
(n given in brackets along with a more precise definition where
necessary): (A) dropping soil particles on a slimy prey animal
(144); (B) running on foraging trail (687); (C) removing soil
particles from foraging trench during wet weather conditions
(249); (D) participating in swarm raid (777, workers were
collected from the front 20cm of the swarm); (E) running on
migration trail towards new nest (1667); (F) defending foraging
trail after disturbance (308, workers biting into the observer’s
finger after blowing onto the trail with the finger placed 5 cm
away from trail); (G) carrying soil particles out of nest during
dry weather conditions (299)); (H) transporting food items in
groups (294, all workers of the group combined); (I) carrying
soil particles out of nest after heavy rain during previous night
(273); (J) transporting food item alone (136); (K) transporting
food item in group (113, front runners only); (L) standing
alongside foraging trail in defensive posture (357); (M) cutting
up earthworms (104); (N) defending nest after disturbance
(142, workers biting into the observer’s hand after beating onto
nest).The box encompasses the lower 25% and upper 75%
percentile range; the internal line is the median; whiskers
indicate the minimum and maximum values. The groups
engaged in the various tasks are arranged according to
increasing median values.
two sympatric intermediate species. In both cases, workers of the intermediate species immediately withdrew
to their usual hunting stratum into the leaf litter and
into tunnels in the ground when the D. wilverthi swarm
raids were approaching. This behaviour is different
from the one displayed by colonies of the epigaeic
species D. molestus during intraspecific encounters
Fig. 5. Front leg length/hind leg length ratios vs dry mass for
workers of hypogaeic, intermediate and epigaeic species.
238
which simply continue the swarm raid on the surface
in another direction (C. Schöning pers. obs.). Thus
species appear to be specialized for a particular foraging
stratum and avoid competition with other specialized
species when they leave it.
In conclusion, our results show that morphological
diversification in Dorylus army ants was most likely
influenced by niche shifts to new foraging strata. Longer
legs, antennae, and mandibles as well as wider heads are
probably adaptations for efficient foraging and colony
defence in intermediate and epigaeic microniches, respectively. The use of different foraging strata allows the
complementary niche exploitation and thus coexistence
of several army ant species in the same habitat.
Acknowledgements / We would like to thank Randolf Menzel
for making this study possible, Washington Njagi for steadfast
help during field work, William Gotwald, Jr. for kindly
identifying several ant specimens, Stefanie Berghoff for
providing specimens, Daniel Kronauer for helpful comments
on the manuscript and Scott Powell, Xavier Espadaler and
Bernhard Seifert for fruitful discussions. We are grateful to the
government institutions of Kenya (Kenya Wildlife Service,
Ministry for Science, Education and Technology) and Uganda
(Uganda Wildlife Authority, National Council for Science and
Technology) for granting research permission. Financial
support was provided through doctoral fellowships by the
Federal State Berlin and the DAAD to C.S.
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