Download Body size and alternative mating tactics in the beewolf Philanthus

Biological Journal of the Linnean Society (1983) 20: 175-184. With 4 figures
Body size and alternative mating tactics in the
beewolf Philanthus zebratus (Hymenoptera;
Sphecidae)
K.M. O’NEILL AND H. E. EVANS
Department of <oology and Entomology, Colorado State University, Fort Collins, Colorado,
U.S.A. 80523
Accepted 5 3uh 1982
A study of the mating behaviour of males of the beewolf Philanthus Zebratus revealed that in one
population males display variability in mating tactics and that this variability is related to male body
size. There was a tendency for large males to patrol the airspace above the nesting area while
smaller males were territorial adjacent to it. The mean sizes of the two groups of males were
significantly different, although the size ranges of the two groups overlapped. Only 2 . 5 O b of the males
were obsened to undertake both mating tactics, at different times. Observations are presented on
daily and seasonal activity patterns and on the relative location of nests, territories, and patrolling
males. A second population, with lower nest density, was observed for several days, revealing only
territorial males. It is suggested that the presence of patrolling males is related to the higher nest
density of the one population. The fact that patrolling males tend to be relatively large is possibly
related to flight energetics or simply to the ability of large males to seize females, which are usually
larger than males, in mid-air.
KEY WORDS :-Philunthus
~
body size
~
mating strategy - territoriality
-
intraspecific variation
CONTENTS
Introduction . . . . . . . . . . . .
Methods.
. . . . . . . . . . . .
. . . . . . . . . . . . . .
Results
Locations of nests, territories, and patrolling males . .
Seasonal activity patterns . . . . . . . .
. . . . . . . .
Daily activity patterns
Patrolling behaviour . . . . . . . . .
Territorial behaviour . . . . . . . . .
Mating behaviour
. . . . . . . . .
The role of body size variability in the mating system .
Some observations on a second aggregation o f P . zebratus
Discussion . . . . . . . . . . . . . .
Acknowledgements
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. . . . . . . . . . . .
References
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INTRODUCTION
Philanthus is a large genus of solitary wasps commonly called beewolves, the
females of which nest in the ground and prey upon bees and wasps. The normal
0024-4066/83/060175
+ 10$03.00/0
175
tQ
1983 The Linnean Society of London
176
K. M. O’NEILL AND H . E. EVANS
pattern in the mating system of this genus is for males to defend small perches
which they scent-mark, the pheromone evidently serving to attract females. I n 14
species of Philanthus in which males have been observed, 13 exhibit territoriality
of this basic form (Alcock, 1975a,b; Gwynne, 1978, 1980; Borg-Karlson & Tengo,
1980; Gwynne & O’Neill, 1980; O’Neill, unpubl. data). One Species, P.
albopilosus, is evidently not territorial and indeed lacks the large mandibular
glands and hair brushes associated with scent-marking (O’Neill, 1979).
We have previously reported that males in a population of P. zebmtus in
Jackson Hole, Wyoming, display a mating strategy that is unique not only in the
genus, but among sphecid wasps generally (Evans & O’Neill, 1978). Males were
found patrolling the airspace above the nesting area and attempting to intercept
females flying to and from their nests. The position of the males at a height of
3-5 m above the nesting area coincides with the altitude that females achieve on
orientation flights and on returning to nests with prey. I n this same paper, we
reported an alternative mating strategy in this species, basing this assertion on
several Colorado populations in which males were territorial, as in other species of
the genus. Since the paper was published, it has become clear that these
Colorado populations are not conspecific with zebratus but belong to the closely
related species basilarix (G. F. Ferguson, pers. comm.; O’Neill & Evans, 1978).
Further observations on P. zebrutus in Jackson Hole during the summers 1978
through 1981 revealed that both patrolling and territorial males do in fact occur
in this population, something we did not observe in 1977. Hence our assertion
that alternative strategies occur in zebratus remains valid, although originally
based on a taxonomic misinterpretation and now found to be intrapopulational
rather than interpopulational. This paper reports observations on territorial males,
further describes patrolling behaviour, and in particular presents results
concerning the relation of a male’s size to the type of mating tactic it employs.
Observations on a second Wyoming population of zebratux also suggest that
density of female nests may influence male behaviour.
METHODS
Observations on this species were made at the same location as the previous
study (i.e. at Deadman’s Bar, 14 km SW of Moran P.O., Teton County,
Wyoming) on 7-1 1 July, 1978; 9-16 July, 1979; 4-16 July, 1980; 11-24 July,
1981. Except as otherwise indicated in the text, all observations reported were
obtained from this population. A few additional observations were made on a
second population at Huckleberry Hot Springs, Teton County, Wyoming from
17 to 19 July, 1981. This site is approximately 40 km north of the previous
locality. The results from this second population are presented for comparison.
In 1979, 79 males and in 1980, 179 males were captured, measured to the
nearest 0.1 mm (maximum head width), marked on the dorsum of the thorax
with one to three small dots of coloured enamel paint, and released. Head widths
were measured with a VWR Scientific Products Micrometer accurate to
0.05 mm. Males for dry mass analysis were dried for 7 days in an oven at 65°C
to a stable weight.
In 1979 and 1980 all individuals that were seen were captured, if possible,
regardless of their behaviour at the time. Subsequently, we recorded the type of
mating tactic (territoriality or patrolling) being displayed by marked males each
MATING TACTICS IN THE BEEWOLF
177
time they were seen again. Locations of nests and territories were recorded to the
nearest 10 cm.
RESULTS
Locations of nests, territories, and patrolling males
The main aggregation of nests at Deadman’s Bar occurred in an area of
alluvial sand, covered with sparse vegetation, along a dirt road within 100 m of
the Snake River. Nest density was high in all 5 years of study; density in 1980 is
shown in Fig. 1. Nest entrances were commonly within several centimetres of one
another, although a few outliers occurred. Females build one nest per season,
which may contain up to 17 cells. The cells vary in depth from 8 to 18 cm and
are each stocked with from four to nine prey. Prey identified included members
of 10 families and at least 46 species of Hymenoptera (Evans, 1970; Evans &
O’Neill, unpubl. data).
As in other species of Philanthus, territories are within small (i.e. approximately
1 m 2 ) areas relatively devoid of vegetation, but with at least several nearby tall
plants, usually grass stems, which are used as substrate for scent-marking.
Territories were never within the nesting area, but usually adjacent to it (Fig. 1).
00
Phtlanthus zebratus
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Figure 1. Map of P. Zebrutlcc study site at Deadman’s Bar showing locations of territories, nests, and
area patrolled by males in 1980.
Patrolling flights were generally made directly above the nesting area, but
were commonly extended beyond the immediate boundaries. Between flights
males usually landed among the nests, but would also land in that section of the
territorial area that was near the nests.
Seasonal activity patterns
In 1978 and 1980, observations of the nesting area commenced before the first
wasps emerged, so that we were able to record early season emergence patterns of
both sexes. The degree of proterandry is slight relative to many other sphecid
wasps (e.g. Lin, 1963). I n 1978, the first males were seen two days before, and in
1980 one day before the first females. I n both years, nests, territories, and patrolling
flights were all initiated on the first clear day after the females first emerged.
178
K.M.O’NEILLA N D H.E. EVANS
Thus, male activity seems to be synchronized with the beginning of female
nesting behaviour. This is confirmed by observations made on three marked
females which were observed mating. One of these females had initiated her nest
on the previous afternoon, but had yet to bring in prey. The other two marked
females mated and then initiated their nests later in the same day. Both females
caught in copula in 1981 were dissected and the condition of their ovaries
examined. I n both cases, the ovaries were relatively undeveloped, indicating that
these females were relatively recently emerged. Based on this and the above
evidence, it appears that females mate early in their nesting cycle. We have no
evidence that females mate later in the cycle.
In 1980, the number of nesting females increased each day, from 11 on 8 July,
to 151 on 16 July, when observations on this population were terminated. The
nesting area was still expanding on this last day. I n 1981, there were at least 186
active nests on 16 July.
Males made patrolling flights on all of the 7 clear days from 8 July through 16
J L I ~ Y , 1980. However, territories were present on only 4 of these days. Territorial
males were not found on 11 through 13 July 1980 although patrolling males were
present. Territories were present both before and after these dates. It is not
known why territories were not present on every day on which patrolling flights
occurred.
Daily activity patterns
Females began working at nest entrances between 10.15 and 10.50 hours
Mountain Daylight Time (MDT).T h e first prey were brought in between 10.30
and 12.54 hours. The latter time was observed on the first day of nesting in
1980. The usual beginning of the hunting period was between about 10.30 and
11.00 hours. However, a t this time most females were still digging. Another
30-60 min usually passed before most of the females were provisioning.
When males first emerged in the morning from sleeping burrows in the nesting
area (at about 10.30 to 11.00 hours) they fed on flowers near the nesting area and
perched on low plants or the ground. During this period they showed no
apparent site attachment and did not interact with conspecific males and females.
O n clear days the first patrolling flight was made between 11.13 and 11.55 hours
( N = 9 days). The frequency distribution of patrolling flights (during the day) for
5 days is presented in Fig. 2. The last flights were made between 16.30 and 17.12
hours. At this time, few females were bringing in prey. Most were digging at their
nest entrances.
The time at which the first territories were initiated was harder to determine
because this is a less conspicuous event. O n 5 days in 1978 and 1979, territories
were first noted between 11.58 and 13.10 hours. O n the only 2 clear days on
which the last territorial males and patrolling males were observed, both
activities ceased within 10 min of one another (at about 16.45 hours).
Patrolling behaviour
A detailed description of patrolling behaviour of males of this species is given
by Evans & O’Neill (1978). Here, we pill only briefly summarize the form of this
behaviour. From a perch on the ground in or near the nesting area, males make a
MATING TACTICS IN THE BEEM’OLF
179
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Phdanfhus zebraius
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5 days observations
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1530
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Time (hours)
Figure 2. Daily schedule of number of patrolling flights observed in 10 min intervals throughout 5
days. Data for 3 days from 1977 (Evans & O’Neill, 1978) were combined with 2 days from 1980.
quick ascent to a height of 3-5 m. Upon reaching this altitude, a horizontal flight
path is assumed, with the males hovering or flying slowly over a distance of
1-10 m. During this period, they actively pursue passing insects, including
conspecific males and females. Upon completion of the flight, the males descend
swiftly to the ground. While perching on the ground, they remain sedentary, but
may pursue another male upwards as the latter begins a patrolling flight. While
perched, males ignore females at nest entrances.
One further observation of patrolling males was made in order to determine
the relative amounts of time spent by males in flight and on the ground. O n one
day in 1981, beginning at 11.15 hours and ending at 14.15 hours, as many
patrolling flights and perch durations as possible were timed with a stopwatch (to
the nearest second). The duration of 61 flights had a mean of 9.9 s ( S . D = 3.24;
range: 3-18 s). The mean duration of 110 perches on the ground between flights
was 12.9 s (s.D. = 3.63; range: 6-23 s ) . Comparison of these means indicates that
males spent more time on the ground doing nothing that contributes directly to
obtaining females than they did actively patrolling for females (t-test for
difference in means is significant, P < 0.001 ). We may have slightly
underestimated the mean duration of high flights because of the difficulty in
following some of the longer flights to completion, but the important point is that
males undertaking patrolling tactics spend a substantial proportion of their time
not actively pursuing females.
Territorial behaviour
Few extensive observations of individual territorial males over long periods of
time were made. However, the behaviour of males on territories seems similar to
that of males of other species of Philanthus. Philanthus Zebratus males mark
territories and interact vigorously with conspecific males in contests that involve
butting and grappling. Four of seven interactions observed in 1979 resulted in
usurpation of the territory by the intruder. Occasionally, interactions between a
territorial male and a patrolling male that had landed on a territory were
observed. They were short, did not involve physical contact, and were rare since
180
K.M. O’NEILL AND H.E. EVANS
most territories were outside the patrolled area. Males also pursued passing
insects in a manner similar to that described for other species of Philanthus.
Butting of several airborne insects was observed. Territorial males did not react
to other males that were patrolling at the height of the swarm.
Mating behaviour
From 1977 through 1980, 11 matings by patrolling males and none by
territorial males were observed. It is true that the initiation of matings on
territories might be much less conspicuous. The 11 copulations occurred between
12.39 and 15.05 hours (Fig. 2).
Pairing of males and females probably occurred primarily within the aerial
swarm, after which the pair descended to the ground. Nine of the 11 mating pairs
were found after they were already at ground level. The other two were first
observed a t a height of 2 and 5 m. After coupling they completed the copulation
on low vegetation in and around the nesting area.
The role of body sire variability in the mating system
Males of this species collected in 1979 and 1980 ranged from 2.4 to 4.1 mm in
head width (2 = 3.20 m m ; S.D. = 0.25; .N = 258). The range of this sample is
misleading because of the presence of one male with a head width of 4.1 mm.
Excluding this male, the upper size limit of males was 3.7 mm. It is possible that
the 4.1 mm male represented a mistake in sex determination, whereby a male
(unfertilized) egg was laid in a cell provisioned for a female (fertilized) egg. This
has been previously discussed in relation to such a phenomena in bees of the
genus Osmia (Raw & O’Toole, 1980). Females of this species were larger than
males on the average ( 2 = 3.81 mm: S.D. = 0.18; N = 120; t-test for difference
P<c 0.00 1). This difference is typical of the direction of sexual size dimorphism
in Philanthus (O’Neill, 1981). Head width of males is highly significantly
correlated with dry mass ( r = 0.93; P < 0.001).
Of 1 1 matings observed (including four from the previous study), the size of six
males was determined, one of which was observed to mate twice. The six males
ranged in size from 3.2 to 3.6 mm (ii = 3.41 mm; S.D. = 0.17; ”V = 7). Although
this sample is too small to allow for statistical analysis, it is interesting to note
that they were all of mean size or larger.
Alcock, Jones & Buchmann (1977) reported that males of the bee Centris pallida
undertake alternative mating strategies, and that large males tended to be
patrollers and small males tended to be ‘hoverers’ (i.e. territorial). This
population of P. rebratus was investigated in order to determine if there was any
relationship between the mating tactic that a male undertook and his size.
In 1979 and 1980, 78 different males were caught after they were seen to make
patrolling flights. They had a mean head width of 3.36 mm (s.D. = 0.20; Fig. 3).
O n the other hand, 44 different males found to be territorial had a mean head
width of 3.06 mm (s.D. = 0.26). The means of all patrolling males and territorial
males were significantly different (t-test, P<c 0.001). The difference between
3.36 mm and 3.06 mm males becomes more apparent and biologically significant
when one considers mass rather than linear dimensions such as head width.
The regression of dry mass on head width for P. eebratus males indicates that a
male of 3.36 mm should weigh about 39(y0 more than a 3.06 mm male.
MATING TACTICS IN T H E BEEWOLF
I
181
High Flight Moles
f = 3.36 mm
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.
,
.
,
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Head width (mm)
Figure 3. Size-frequency distributions of patrolling males and territorial males of P. zebrutus in
Jackson Hole from 1979 and 1980. Hollow portions of bars in lower histogram indicate the size of
three males that both held territories and made high flights.
I t is also instructive to compare the range of values for the head widths of the
two groups. Only one male (2.9 mm) with a head width of less than 3.0 mm was
seen to undertake a patrolling flight, although 40 males in this size range were
found in the population (16% of all males). I n contrast, although 58 males (22O/,,,
of all males) had head widths greater than 3.3 mm, only four of these males were
found to hold territories. Males descending from high flights were easy to follow
against the backdrop of spruce trees near the nesting area, so we feel confident
that the above is not a function of differential visibility of males of different sizes.
Only three of the 122 males were seen to undertake both territoriality and
patrolling flights (i.e. at different times). These males had head widths of 3.3, 3.4
and 3.6 mm (Fig. 3). Two of these males were observed to be territorial before
switching to patrolling, while the third male held a territory between two
observations of it making patrolling flights. The behaviour of these males while
they were territorial was not obviously different from that of other territorial
males.
Seven interactions between territory residents and intruders, in which the size
of at least one of the males was known, were observed. The smaller of two males
never won a fight (.N=5). The mean size of the winners was 3.23 mm
(s.D. = 0.11; . V = 7 ) . The mean size of the losers was 2.77 mm (S.D = 0.21;
.N=6;Mann-Whitney test for difference; PGO.001). Even with such a small
sample of observed fights we are confident of the advantage of large males in
fights given results from other species of Philanthus. In 133 territorial contests
observed in P. basilaris, P. pulcher, and P. crabroniformis, between males of different
size, the larger male of the pair won 131 contests (O'Neill, 1981 and unpubl.
data).
Some observations on a second aggregation of P. zebratus
As noted earlier, we made a brief study of a population about 40 km north of
our major study area, at Huckleberry Hot Springs. These studies began at
midseason and lasted only 3 days. This was a much smaller aggregation and the
K. M. O’NEILL AND H. E. EVANS
182
Philantbus zebrotus
t
0
= Territory
= Nest
10 rn
Figure 4. Map of P. Zebratus study site at Huckleberry Hot Springs.
nests were much more scattered; hence it is of interest to note differences in male
behaviour which may be related to population size and nest density.
Although nests were scattered along the centre and sides of a little-used dirt
road and in a sandy field adjacent to the road, they were relatively conspicuous
and we feel that we found most that were active during the study period. We
found 23 nests, separated by distances of from 0.5 to 45 m (Fig. 4). As in the
other population, territories were usually adjacent to the nests, though here they
seemed more intimately associated. We also found four territories about 50 m
from the nests. Territorial males scent-marked grass stems and interacted with
intruding males and other insects. We saw no evidence of patrolling flights above
the nests.
In this population a larger area containing far fewer nests was involved, and
patrolling may be much less effective under these conditions. We had earlier
suggested (Evans & O’Neill, 1978) that under the conditions of high population
density in the Deadman’s Bar population, the cost of territorial defence within the
aggregation may be too great. This was presumably not true at Huckleberry Hot
Springs, and males were territorial. However, because of the brevity of our study
we cannot be certain that patrolling flights do not sometimes occur in this
population.
We measured the head width of 12 different males that held territories in this
population. They ranged from 2.8 mm through 3.4 mm and had a mean size of
3.13 mm (s.D. = 0.18). This mean was not significantly different from that
obtained for the 44 territorial males in the Deadman’s Bar population (t-test;
t = 1.07).
DISCUSSION
Territoriality and scent-marking appear to be ancestral forms of mating
behaviour in male Philanthines. Not only do most Philunthus spp. display these
strategies, males of the related genus Eucerceris have also been observed behaving
in this manner (Alcock, 1975b; Evans, unpubl. data). Therefore, it is likely that
MATING TACTICS I N T H E BEEWOLF
183
the patrolling behaviour in the Deadman’s Bar population of P. zebratus is a
derived tactic.
Under conditions of high nest density P. rebrutus males probably practise a
type of conditional strategy based upon size. The presence of at least three males
that performed both beha\iours in the 1980 population indicates that there is not
a fixed polymorphism in which males of different genotypes are programmed to
exhibit totally different mating behaviours. Patrolling males collected and
dissected in 1977 had the large mandibular glands associated with scent-marking
(Evans & O’Neill, 1978) and all males examined had mandibular and abdominal
hair brushes typical of Philanthus males. We can infer that, relative to
territoriality, the patrolling tactic is more successful, at least when nests are closely
aggregated. We know from this and especially other species of Philanthus that
large males are much more successful at the territorial tactic than are small
males. If there is not a fixed polymorphism, and if large males have been selected
to abandon territoriality for patrolling, it follows that the latter tactic is more
profitable in this population.
Patrolling is a form of scramble competition polygyny (Alcock, 1980) which
may be more profitable than territoriality because patrolling males intercept
receptive females before the latter reach territories. Because of the high nest
density, the cost of defending a territory and repeatedly soliciting non-receptive
females in the nest area would be correspondingly high. A similar situation may
be occurring in the bee Centris pallidu (Alcock et al., 19771. This is the only other
reported case of size-related alternative behaviours in the solitary Hymenoptera.
In that species, large males patrol the emergence area and copulate with
emerging females. By virtue of their large size, they are able to fend off other
males attempting to usurp possession of the female. Many of the smaller males
hover near plants outside the emergence area and intercept females that have
not copulated upon emerging from the ground. It should be noted that, although
a size-conditional strategy is the most parsimonious explanation for size-related
male behaiiour in Philanthus, without genetic analysis i t is not possible to rule out
a stable polymorphism in which size and a tendency to behave in a particular
manner are genetically predetermined (Cade, 1980).
The question arises as to why small Philanthus males do not also switch to
patrolling beha\riour under high nest density. The fact that males spend more
time on the ground between patrolling flights, on a\rerage, than they do in the
air, suggests that the flights are energetically expensi\.e to undertake. It may be
that, during flight, large males are energetically more efficient than small males
and can, therefore, sustain longer flights or more flights per day. Between species
of insects comparisons do suggest that in flight, larger insects have a lower ‘cost of
transport’ than smaller insects (Tucker, 1975), but it is not known whether this
analysis can be extended to intraspecific body size comparisons. Again, as
suggested by interspecific comparisons (Bonner, 1965), larger males may have
higher flight speeds. A simpler reason for sex-related behavioural variability may
be that large males are better able to seize the usually larger females in mid-air.
ACKNO\VLEDGEhlENTS
Keith Christian contributed valuable help with the field work. FVe thank
Kenneth L. Diem of the University of Wyoming, National Park Service Research
184
K. M. O’NEILL AND H. E. EVANS
Centre and Robert Wood of Grand Teton National Park for their co-operation
and hospitality. Robert Longair commented on the manuscript. Financial aid
was supplied by the National Science Foundation Grant Nos BNS 76-09319 and
BNS 79-26655 to Howard E. Evans and by the National Park Service.
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y,
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