Insect. Soc. 51 (2004) 48–54
0020-1812/04/010048-07
DOI 10.1007/s00040-003-0704-2
© Birkhäuser Verlag, Basel, 2004
Insectes Sociaux
Research article
Between-species differences of behavioural repertoire of castes in the ant
genus Pheidole: a methodological artefact?
G. Sempo and C. Detrain
Sempo Grégory, Laboratoire d’ Eco-éthologie évolutive, CP 160/12, Université Libre de Bruxelles, Avenue F.D. Roosevelt 50, 1050 Brussels, Belgium,
e-mail: [email protected], [email protected]
Received 23 April 2003; revised 28 July 2003; accepted 1 August 2003.
Summary. This study highlights the influence of sampling
size on the interpretation of between castes division of labour
in the dimorphic ant genus Pheidole. We show that data
analyses based on rarefaction curves provide better estimates
of caste repertoire sizes. Weighted observations of the two
worker castes of Pheidole pallidula reveals that the behavioural repertoire of majors is far more extended than expected. Indeed, majors are not restricted to defence, seed milling
or food storage but can additionally participate to within nest
activities by carrying out 69% of the minors’ behavioural
repertoire including brood care. Besides, we show that interspecific variation in the size of majors’ behavioural repertoire could simply result from differences in the number of
majors observed. Therefore, the ergonomic prediction that
the repertoire size of one caste should be correlated to its
numerical representation in the colony needs to be re-examined considering between-castes differences in the sampling
effort.
Key words: Pheidole, ergonomic theory, division of labour,
caste, sampling effect.
Introduction
Division of labour based on the simultaneous performance of
different tasks by morphologically and/or ethologically specialized individuals, gives an ecological and reproductive
advantage to the colony by increasing its ergonomic efficiency (Wilson, 1968, 1980; Oster and Wilson, 1978; Gordon, 1989; Robinson, 1992). As regards polymorphic ants,
the ergonomic theory predicts that as a caste of one ant
species becomes more specialized morphologically and ethologically – becoming highly efficient in a very small subset
of tasks – it should be less represented numerically in the
colony (Oster and Wilson, 1978).
Many studies have been carried out on the genus Pheidole, amongst which most species are strictly dimorphic with
minors and majors differing notably in their body size and
head width. Being widely distributed and displaying interspecific variation in the repertory size of majors as well as in
their numerical representation within the colony, Pheidole is
usually considered as a subject of choice for testing the
ergonomic theory (Wilson, 1984; Hölldobler and Wilson,
1990). For instance, all intermediate levels of behavioural
specialization were reported between the highly specialized
majors of Pheidole guilelmimuelleri (Wilson, 1984) – that
perform a maximum of 4 different behaviours and account
for 9% of the worker force – and majors of Pheidole morrisi
(Patel, 1990) that are implicated in almost all the colony tasks
and represent 30% of the nest population.
The Old-World species, Pheidole pallidula, includes distinctive large-headed majors that are involved in nest
defence (Detrain and Pasteels, 1992; Passera et al., 1996;
Aarab and Jaisson, 1992), in foraging to large food sources
(Detrain, 1990; Detrain and Pasteels, 1991) and in food storage (Lachaud et al., 1992). In the present study, we examined the role of both minors and majors within the nest as
well as their relative implication in social activities and
brood care within the nest. The objective of this study was to
estimate the behavioural repertoire of both castes. Throughout our results, we will underline the influence of sampling
size on the estimation of the behavioural repertoire size.
These methodological considerations will lead us to reconsider the experimental validation of the ergonomic theory in
the Pheidole genus.
Materials and methods
Collection and rearing of colonies
Mature colonies of the dimorphic ant Pheidole pallidula were collected on rocky calcareous slopes of South-eastern France (locality of
Gonfaron). In the laboratory, they were housed in nest tubes (length:
Insect. Soc.
Vol. 51, 2004
16 cm; diameter: 1.5 cm) half-filled with water and fitted with a cotton-wool plug. Nest tubes were placed in plastic trays (length: 30 cm;
width: 20 cm; height: 4 cm) over which ants had a permanent access
to water and brown sugar solution (1M). Colonies were fed once per
week with dead cockroaches (Periplaneta americana). Walls of the foraging area were coated with Fluon to prevent ants from escaping.
Colonies were maintained in the dark at 28 ± 1 °C which is a temperature maximising the production of major workers among larvae
(Passera, 1974).
Colonies were composed of 4,500 to 5,500 workers and 1 queen.
Brood covered around 25% of the nest area and consisted in eggs,
nymphs and mainly larvae belonging to all three instars of P. pallidula
larvae (Passera, 1974). Major workers accounted for 10 to 12% of the
whole population within the nest, which is a percentage close to that
observed in the field (Passera, 1977).
Building of the behavioural catalogue
A list of behavioural acts, possibly performed by P. pallidula workers,
was developed for the study of inner-nest behaviours. It was based on
behavioural repertoires previously reported for several Pheidole species
(Wilson, 1976, 1984; Calabi et al., 1983; Patel, 1990; Brown and
Traniello, 1998; Aarab, 1991).
Forty different behaviours were identified within the nest and were
classified in one of the main following categories: egg care, care of first
and second instars larvae, care of third instar larvae, pupae care, queen
care, nest cleaning, self-grooming, allogrooming, antennal contacts and
trophallaxies. Ants that were not engaged in one of these behaviours
were considered either as inactive if they stood immobile for at least 5 s
or as moving inside the nest in the other cases.
Data collection
Two months before being tested, six mother colonies were reared in an
experimental nest (20 cm ¥ 20 cm ¥ 0.2 cm). Each experimental nest
was dug in a plaster of Paris layer (3 cm depth) poured in a polyethylene box (37 cm ¥ 25 cm ¥ 7 cm). The top of the nest was covered with
a red glass plate to create the obscurity necessary for the ants’ installation, allowing observation of brood and ants’ behaviour within the nest
at the same time. This experimental nest was also covered by a grid network (20 cm ¥ 20 cm) composed of 400 numbered quadrates (1 cm ¥
1 cm), which allowed spatial localisation.
Five sessions of behavioural observations were carried out on each
colony on the same day (9 a.m., 12 a.m., 3 p.m., 6 p.m. and 9 p. m.).
Results of the 5 sessions on the same colony were pooled to obtain the
colony behavioural repertoire. Behavioural observations were made
with a Wild swing arm binocular microscope providing up to 50¥ magnification.
To prevent underestimating the behavioural repertoire size of P. pallidula by not observing the less frequent behaviours (Aarab et al., 1989),
around 1,000 observations were carried out on each caste per nest. Considering the average density of minor workers in the nest (mean worker/quadrate ± S.D. (n = 6); minor: 13.4 ± 3; major: 1.1 ± 0.3) we recorded, for each session, the behaviour of all minor workers presents in
20 randomly chosen quadrates.
Since major workers represents only 10 to 12% of the workers population, we observed, for each session, majors in 160 randomly chosen
quadrates to obtain around 1,000 observations of this caste per nest, preventing underestimation of their behavioural repertory size. As we
chose to standardize the number of monitored quadrates instead of the
number of observed individuals, differences in the total number of
observations made on each caste could be found.
Rarefaction curve of the behavioural repertoire size was computed
for each caste using EstimateS (Colwell, 2000). This software re-sampled 50 times the pool of N observation sessions, at random, plotting the
average number of the behavioural repertoire size obtained from 1,
2…N sessions in function of the average number of observations.
Research article
49
Results
Repertoire size and behavioural profile of castes in
P. pallidula
The number of observations made on minors or on majors
could be a factor modifying the observer’s perception of the
behavioural repertoire of each caste (Fagen and Goldman,
1977; Aarab et al., 1988, 1989). Indeed, in order to obtain a
complete and reliable estimate of repertoire size, the sampling effort should be adjusted in order to monitor rarely
occurring behaviours. For each caste, the estimated size of
their behavioural repertoire was drawn from the pool of our
observations and their corresponding rarefaction curve was
computed using EstimateS (Colwell, 2000). For both castes,
the estimated size of behavioural repertoire was highly sensitive to the number of observations as it started increasing
steeply to later remain stable when at least 1,500 observations were made. (Fig. 1. Minors’ equation curve: y = Sm –
Sm
∑(1 – mi)x, with mi = relative occurrence of minors’ behavi =1
iour i, Sm the maximum size of the minors’ behavioural
repertoire and x the number of active minors observed;
SM
majors’ equation curve: y = SM – ∑(1 – Mi)x, with Mi = relai =1
tive occurrence of majors’ behaviour i, SM the maximum
size of the majors’ behavioural repertoire and x the number
of active majors observed). It is noteworthy that even after an
intensive sampling, the full majors’ repertoire, approximating 27 different behaviours only, always stood below the
maximum number of 39 different behaviours performed by
minors.
Minors performed all of the daily tasks and carry out a
wide variety of inner nest activities including brood care,
food management, grooming, queen care and nest cleaning.
A broad overlap was observed between their behavioural
repertoire and that of majors within the nest. The activity
Figure 1. Relationship between the size of the behavioural repertoire
of Pheidole pallidula castes and the number of observations made on
either minors () or majors (). These rarefaction curves were computed by applying the software EstimateS (Colwell 2000) on our observations of minors and majors. For further details, see the section ‘Materials and methods’
50
G. Sempo and C. Detrain
Sampling size effect on Pheidole caste repertoire
Table 1. Behavioural repertoire of minors and majors of Pheidole pallidula
Behaviour
minor
caste
major
caste
Self-groom
0.094
0.100
Allogroom
Minor worker
Major worker
0.063
0.013
0.003
0.016
Allogroomed by
Minor worker
Major worker
0.050
0.001
0.046
0.003
Brood care
Carry of roll egg
0.001
Inspect egg
0.003
Lick egg
0.004
Carry or roll first and second instars larvae
0.005
Inspect first and second instars larvae
0.009
Handle first and second instars larvae meconium 0.004
Lick first and second instars larvae
0.016
Feed first and second instars larvae
0.002
Carry or roll third instar larva
0.008
Lick third instar larva
0.041
Inspect third instar larva
0.041
Handle third instar larva meconium
0.001
Feed third instar larva liquids
0.003
Feed third instar larva solids
0.002
Carry or roll pupa
0.005
Lick pupa
0.031
Inspect pupa
0.042
0
0
0
0
0.003
0
0.001
0
0
0.004
0.022
0.001
0
0
0
0.001
0.013
Queen care
Lick queen
Antennate queen
0.003
0.004
0
0.001
Give regurgitation to:
Minor worker
Major worker
0.024
0.003
0.017
0.003
Receive regurgitation from:
Minor worker
Major worker
0.025
0.003
0.030
0.002
Feed on insect
0.019
0.003
Antennate
Minor worker
Major worker
Dead adult
Nest material
0.192
0.034
0.001
0.003
0.476
0.074
0.001
0.003
Handle
Dead adult
Nest material
0.001
0.004
0.001
0
Dig plaster
0.003
0.004
Aggression to
Minor worker
0.005
0.007
Alarm
Move inside the nest
0.001
0.239
0.001
0.197
Observed repertoire size
Total number of separated acts observed
39
4223
27
1894
Figure 2. Relative occurrence of behaviours performed by either active
minors (; n = 4223) or active majors (; n = 1894) of Pheidole pallidula. Behaviours that are functionally related are pooled
rates of both minor and major caste were statistically different (Chi-square test: c2 = 514.5, df = 1, p < 0.0001). Indeed,
58.9% of minors observed within the nest (n = 7160) were
active against only 39% of majors (n = 4858). Active minors
(n = 4223) performed all majors’ behaviours while, in contrast, active majors (n = 1894) were implicated in only 69.2%
of minors’ activities. As shown in Table 1, majors were never
seen taking care of eggs, manipulating nor transporting larvae and pupae.
Behavioural acts related to the same type of activity were
pooled in categories and their relative frequencies were computed for each caste. These relative frequencies differed
between castes (Fig. 2; Chi-square test: c2 = 653, df = 10,
P < 0.0001), this being more specifically the case for brood
care or social behaviours such as adult antennation and
allogrooming. Indeed, while brood care to eggs, larvae and
pupae account for 21.8% of all the behavioural acts performed by minors, they are 5 times less frequently observed
in majors (4.5%). The contribution of majors is generally
limited to brood antennation and sanitary tasks such as licking of larvae and removal of their meconium. Moreover,
although majors are potentially able to ensure alone brood
development when the eggs are continuously produced in
presence of a queen (Passera 1974), the care they display was
of poor quality and most often led to death. This was demonstrated by additional experiments on four colonies composed
of 100 minors or 100 majors and 60 eggs. After two weeks,
only one colony of majors reared smaller sized and less
turgescent larvae of the third instar (only 4 larvae out of 240
introduced eggs) while all colonies of minors succeeded in
rearing 15,6 ± 9,3 (mean ± S.D.) third instar larvae and
12,8 ± 6,8 pupae (mean ± S.D.). This poor rearing performance of majors is probably due to the size and toothless
shape of their mandibles, which does not allow them to feed
and manipulate small brood safely.
Social behaviours, which include nestmates antennation,
allogrooming and trophallaxies, account for 67% of all
behavioural acts performed by majors while constituting
only 40% of minors’ activity. As regards antennations, they
occurred two times frequently in majors, what accounts for
Insect. Soc.
Vol. 51, 2004
their higher participation in social interactions than minors
are. However, these castes do not show any caste preference
in the display of their antennating behaviour. Indeed, for both
castes, the ratio between the number of antennations directed
towards a major or towards a minor (minors’ antennation
ratio = 0.18; majors’ antennation ratio = 0.16) is close to the
colony caste ratio (number of majors / number of minors in
the colony = 0.14).
Food exchange through regurgitation was carried out by
minors and majors with similar frequencies. As observed in
the case of antennation, the likelihood for an ant to regurgitate to a major or to a minor reflected the caste ratio within
the colony. Indeed, ratios of trophallaxies made to majors to
those made to minors were 0.13 for regurgitating minors and
0.18 for regurgitating majors, showing that there was no caste
related preference in food exchange. Concerning allogrooming, it was mainly performed by minors that did not show
caste preference (minor allogrooming major/minor allogrooming minor = 0.20). Conversely, majors were less prone
to allogroom a minor worker than expected from random.
Indeed, the ratio between the number of majors and minors
allogroomed by a major was 5.33, strongly differing from the
colony caste ratio. This could be explained by the spatial segregation of majors from minors within the nest.
Between species differences in the behavioural repertoires
of castes in the genus Pheidole: the influence of sampling
size
A main prediction of the ergonomic theory is that the behavioural repertoire size of one caste should be positively correlated to the fraction of this caste within the colony. In other
Research article
51
terms, for members of a given caste, the more specialized they
are, the fewer individuals are expected to be found in the
colony (Wilson, 1968; Oster and Wilson, 1978). The study of
behavioural repertoires of 10 Pheidole species (Wilson, 1984)
is usually acknowledged as the experimental validation of this
prediction: in order to compare the behavioural catalogue
built in the later study with that of other studies on Pheidole
(Wilson, 1984; Patel, 1990; Brown and Traniello, 1998), we
used the fraction of minors’ behavioural repertoire performed
by majors instead of the absolute value of their repertoire size.
Using Wilson’s data (1984) on majors‘ repertoire, a linear
relationship between the fraction of minors’ behavioural
repertoire performed by majors and the fraction of majors
within the colony could be found (Fig. 3. Spearman rank correlation coefficient r = 0.69, n = 10, p = 0.039; equation of the
regression line: y = 1.45x + 0.14). However, the same trend –
the relative increase of the behavioural repertoire performed
by majors – can be obtained in another way by plotting against
the number of observations made on majors of each Pheidole
species (Fig. 4; Spearman rank correlation coefficient r =
0.87, n = 11, p = 0.0009; equation of the regression line: y =
0.0008x + 0.1406). Differences in the estimated repertoire
size of majors among Pheidole species could therefore result
from differences in sampling sizes without any need to evoke
species-specific caste ratios and related ergonomic issues.
Results obtained on P. pallidula species in the present
study, showed that the behavioural repertoire of majors
includes 69% of the minors’ repertoire instead of the expected 30% overlap predicted by the Wilson’s ethocline (see
Fig. 3) for a fraction of 12% majors in the colony (percentage close to that observed in the field). However, P. pallidula should not be considered as an ‘unusual’ species in which
the major caste would show an unexpectedly large repertoire
Figure 3. Behavioural repertoire of majors as a function of their fraction in the colony. This repertoire of majors is expressed as the fraction of the total
repertoire of minors in order to allow comparison of our data () with those of different Pheidole species (). The regression line is calculated from
Wilson’s data (1984). Full scientific names of tested Pheidole species are abbreviated as follows: P. dentata (dent), P. distorta (dist), P. embolopyx (emb),
P. guilelmimuelleri (guil), P. hortensis (hort), P. megacephala (meg), P. mendicula (mend), P. minutula (min), P. pubiventris (pub), P. sp. indet. (sp A)
52
G. Sempo and C. Detrain
Sampling size effect on Pheidole caste repertoire
Figure 4. Behavioural repertoire of majors as a function of the number of observations made on majors. The regression line (solid line) is calculated from
pooled data from Brown and Traniello’s study (1998) on P. morrisi (mor) and Wilson’s study (1984) for the other Pheidole species. The rarefaction curve
(dotted line) of P. pallidula is drawn from repertoires described in this paper. Abbreviations of scientific names are the same as described in Figure 3
in relation to its caste ratio. Indeed, the deviation of P. pallidula data from Wilson’s ethocline (1984) fades when considering sampling size instead of majors’ fraction in the
colony (Fig. 4). Moreover, the increase of majors’ behavioural repertoire with sampling size is similar when either based
on studies on different Pheidole species (Wilson, 1984; Patel,
1990; Brown and Traniello, 1998) or exclusively on our
P. pallidula data. Indeed, the regression line obtained for the
latter species (Fig. 4; y = 0.0008x + 0.1406) and the rarefaction curve (Fig. 4; y = 0.1448 ln(x) – 0.3889) computed by
EstimateS out of our sessions of observations on P. pallidula
colonies have the same slope (Data Ln transformation. Student t test: |t| = 0.76, n = 19, p > 0.05) and the same elevation
(Data Ln transformation. Student t test: |t| = 1.30, n =19,
p > 0.05). The increase of majors’ repertoire size that was
originally based on inter-species comparisons can thus be
drawn out of data originating from one species only. There is
therefore no need to implicate evolutionary issues to account
for inter-species differences in repertoire size: the number of
observations carried out on a caste is prevailing in the assessment of its repertoire size.
Similarly, the reported increase of majors’ repertoire size
in colonies deprived of minors (Wilson, 1984) can be
explained not exclusively by a behavioural ‘elasticity’ but
also by the higher number of acts observed. For instance, in
majors alone colonies, the fraction of minors’ behavioural
repertoire performed by majors of P. pubiventris increases
from 0.40 to 0.54 when the number of majors observed
increases (from 233 to 819) which is close to the expected
values of 0.40 and 0.58 drawn from rarefaction curve of
P. pallidula for the same number of observations.
Discussion
Repertoire size of castes and division of labour within the
nest in P. pallidula
In P. pallidula, majors perform a large subset of the minor’s
behavioural repertoire. This result contrasts with the commonly accepted idea that majors of Pheidole species are
specialized in a few sets of tasks such as colony defence,
seed milling or food storage, with the behavioural repertoire
of majors overlapping weakly that of minors (Wilson, 1976,
1984; Calabi et al., 1983; Droual, 1983; Fowler, 1984; Wilson and Hölldobler, 1985). In most of these Pheidole
species, minors are considered as the main or even exclusive brood tenders. In the present study, we show that majors of P. pallidula are also involved in some brood care
acti-vities like antennal tipping or licking of larvae. However, these majors were never seen transporting or feeding brood. Regarding brood transport, majors do not participate in the spatial arrangement of larvae instars within
the colony except during nest emigration (Droual, 1983)
or experi-mental removal of larvae outside the nest (personal observation). Concerning brood feeding, colonies
comprising only P. pallidula majors systematically failed
to rear an adult worker out of a limited number of eggs,
demonstrating the incompetence of majors in handling
successfully the whole brood development cycle. Occasionally, a rearing of brood to maturity by Pheidole majors alone
can occur but only from late brood instars (Wheeler and
Nijhout, 1984; Wilson, 1984) or from a constantly renewed
eggs supply and with a very low rate of success (Passera,
1977).
Insect. Soc.
Vol. 51, 2004
It is also noteworthy that social interactions with nestmates (allogrooming, trophallaxies and antennations)
account for 40 and 67% of the activity of minors and majors
respectively. Outside a foraging or agonistic context, ants do
not preferentially interact with nestmates belonging to the
same caste, except the allogrooming between majors. Knowing the role of allogrooming in nestmate recognition in ants
(Soroker et al., 1995; Lenoir et al., 2001a, b), the preferential
allogrooming of majors to individuals belonging to the same
caste could enhance the intra-caste social cohesion and could
help maintaining the spatial aggregation of majors within the
nest (personal observations).
Reconsidering the experimental evidence of the ergonomic
theory within the genus Pheidole
The ergonomic theory predicts that the behavioural repertoire size of one caste should be positively correlated with the
fraction of this caste in the colony (Wilson, 1968; Oster and
Wilson, 1978). The ethocline obtained by Wilson (1984)
from the study of ten Pheidole species (Fig. 3), is usually
considered as an experimental validation of this prediction.
This study estimates completeness of a behavioural repertoire size by using the Fagen and Goldman (1977) method.
This method extrapolates back to the origin the lognormal
Poisson distribution fitting the behavioural-abundance data
curve. The extrapolated value of the distribution gives the
missing zero-abundance class of behaviours and, when added
to the number of behaviour types actually observed, provides
an estimate of the true repertory size of a species. However,
according to Fagen and Goldman (1977), their method lacks
accuracy when the distribution has its mode at abundance
one and fails to estimate repertoire size when it is composed
of many rare behaviours as it is the case for Pheidole majors.
Moreover, this method requires a sufficiently long period of
sampling in order to push the mode of behavioural abundance to the right, decreasing the likelihood of missing rare
behaviour together with the error associated to the estimated
repertoire size. Based on a study performed by Wilson and
Fagen (1974) on the ant Leptothorax curvispinosus, a total
number of 2,000 observations is considered as adequate to
get a reliable estimate of ant’s behavioural repertoire.
In previous studies on Pheidole species (Wilson, 1984;
Patel, 1990; Brown and Traniello, 1998), this recommended
sampling effort was approached or reached only for minors
(1,000–3,600 acts observed). By contrast, a 3-to-30 fold
lower number of observations were performed on majors
(60–650 acts observed), probably leading to an underestimation of the behavioural capacities of this caste. As a correlate, a too unequal sampling effort performed on the two
castes could give the impression of a higher behavioural specialisation of majors: in this respect, Aarab et al. (1989)
demonstrate that the weighing of sampling on minors and
majors of P. pallidula reduces inter-caste differences in
repertoire size. Knowing this sampling size effect, one can
also explain between-studies divergences in the estimated
size of majors’ repertoire within the same P. morrisi species.
Research article
53
Indeed, Patel (1990) reported an overlap between the behavioural repertoires of minors and majors 1.5 times greater than
Brown and Traniello (1998): such difference could be due to
the 1.9 fold higher number of majors observed in Patel’s
study (1990).
A broader consequence of these sampling-based considerations is that they provide a new interpretation of the ethocline
reported by Wilson (1984). Indeed, inter-specific differences
in majors’ specialisation (fraction of minors’ repertoire performed by majors) are not necessarily due to inter-species
changes in caste ratios (Fig. 3) since the same linear relationship can be obtained when plotting the absolute number of
observations made on majors on the X-axis (Fig 4). Furthermore, our results showed that, within the same species, the
repertoire size of P. pallidula majors increases with sampling
size in a similar way (Fig. 4). Therefore, an alternative explanation to behavioural differences among related Pheidole
species is that they could simply reflect sampling-size effects
instead of stages of an evolutionary trend.
This methodological consideration is added to other concerns about the experimental validation of the ergonomic
prediction relating behavioural and morphological specialization to caste ratio within the genus Pheidole. Firstly, the
choice of one species-specific caste ratio does not take into
account the natural variation of the fraction of majors due to
colony intrinsic or extrinsic factors (Passera, 1974, 1977;
Oster and Wilson, 1978; Johnston and Wilson, 1985; Gibson,
1989; Patel, 1990). Secondly, up to now, Pheidole species
tested are not sufficiently representative of Pheidole genus:
they showed similar degrees of morphological specialization
of castes, measured by the ratio of minor to major body sizes
(Patel, 1990). Further work would be needed on additional
Pheidole species with wider range of minor:major ratios if
one attempt to validate the ergonomic prediction.
The results obtains in the present study strongly suggest
that studies aiming to describe the relationship between the
behavioural repertoire of Pheidole majors and their caste
ratio should be reconsidered by taking into account the effect
of sampling size. We do not wish to rule out the potential for
caste optimisation in polymorphic ant species, we suggest
rather that these concepts still need definite experimental
validation.
Moreover, the repertoire size – that means just the total
number of different behaviours – is not sufficient to evaluate
caste specialization and to carry out reliable inter-specific
comparisons. Even though the two castes show the same
behavioural repertoire, you can still argue that the major
caste is quite specialized when looking at the relative frequencies of behaviours. Indeed, most of acts performed by
the major caste can represent only a few different behaviours
whilst the minor caste may show a more uniform distribution
of frequencies between behaviours. The behavioural repertoire of castes have thus to be analysed in relation with the
relative weight of observed behaviours before drawing conclusions on castes specialization.
All considerations presented in this study show that the
social regulation of work in the genus Pheidole should be
addressed through adequate sampling effort. Moreover, em-
54
G. Sempo and C. Detrain
phasis should be put on quantitative changes in the relative
occurrence of behaviour, in activity rates as well as in other
task related factors like the spatial organisation of castes.
Acknowledgments
We would like to thank J.-P. Lachaud for valuable discussion on the
manuscripts, B. Danis for revising the English manuscript and to
L. Passera and an anonymous referee for constructive comments on this
manuscript. We are also grateful to J.-L. Deneubourg for helping in data
processing and for his helpful suggestions. This work was supported by
the Belgian Fund for Joint Basic Research (grant n° 2.4510.01) and
funded by a F.R.I.A. (Fonds pour la formation à la Recherche dans l’Industrie et dans l’Agriculture) PhD grant. C. Detrain is a research associate from the Belgian National Fund for Scientific Research.
References
Aarab, A., 1991. Polyéthisme, régulation sociale et éthogénèse chez les
deux sous-castes morphologiques de la fourmi Pheidole pallidula.
Ph.D. thesis, Université Paris-Nord, Villetaneuse.
Aarab A., J.P. Lachaud and D. Fresneau, 1988. Flexibilité du répertoire
comportemental chez les ouvrières major de Pheidole pallidula.
Actes Coll. Insectes Sociaux 4: 135–140.
Aarab, A., J.P. Lachaud and D. Fresneau, 1989. Effets de la variabilité
interindividuelle sur la taille des répertoires comportementaux des
deux sous-castes ouvrières de Pheidole pallidula (Formicidae,
Myrmicinae). Actes Coll. Insectes Sociaux 5: 251–258.
Aarab, A. and P. Jaisson, 1992. How to become a good ant-soldier : the
effect of solicitation by minors on the development of aggressive
behaviour in soldiers. Anim. Behav. 44: 593–595.
Brown, J.J. and J.F.A. Traniello, 1998. Regulation of brood-care behavior in the dimorphic caste of the ant Pheidole morrisi
(Hymenoptera: Formicidae): effects of caste ratio, colony size, and
colony needs. J. Insect Behav. 11: 209–219.
Calabi, P., J.F.A. Traniello and M.H. Werner, 1983. Age polyethism: its
occurrence in the ant Pheidole hortensis, and some general considerations. Psyche 85: 395–412.
Colwell, R.K., 2000. EstimateS: statistical estimation of species richness and shared species from samples (Software and User’s Guide),
Version 6. http://viceroy.eeb.uconn.edu/estimates.
Detrain, C., 1990. Field study on foraging by the polymorphic ant
species Pheidole pallidula. Insect. Soc. 37: 315–332.
Detrain, C. and J.M. Pasteels, 1991. Caste differences in behavioral
thresholds as a basis for polyethism during food recruitment in the
ant, Pheidole pallidula (Nyl.) (Hymenoptera: Myrmicinae). J.
Insect. Behav. 4: 157–176.
Detrain, C. and J.M. Pasteels, 1992. Caste polyethism and collective
defense in the ant, Pheidole pallidula: the outcome of quantitative
differences in recruitment. Behav. Ecol. Sociobiol. 29: 405–412.
Droual, R., 1983. The organization of nest evacuation in Pheidole
desertorum Wheeler and P. hyatti Emery (Hymenoptera: Formicidae). Behav. Ecol. Sociobiol. 12: 203–208.
Fagen, R.M and R.N. Goldman, 1977. Behavioural catalogue analysis
methods. Anim. Behav. 25: 261–274.
Fowler, H.G., 1984. Recruitment, group retrieval and major worker
behavior in Pheidole oxyops Forel (Hymenoptera: Formicidae).
Rev. Brasil. Biol. 44: 21–24.
Sampling size effect on Pheidole caste repertoire
Gibson, R.L., 1989. Soldier production in Camponotus novaeboracensis during colony growth. Insect. Soc. 36: 28–41.
Gordon, D.M., 1989. Caste and change in social insects. In: Oxford Surveys in Evolutionary Biology (P.H. Harvey and L. Partidge, Eds),
Oxford University Press. pp. 67–72.
Hölldobler, B. and E.O. Wilson, 1990. The Ants. The Belknap Press of
Harvard University Press, Cambridge, Mass. 732 pp.
Johnston, A.B. and E.O. Wilson, 1985. Correlates of variation in the
major/minor ratio of the ant, Pheidole dentata (Hymenoptera:
Formicidae). Ann. Entomol. Soc. Am. 78: 8–11.
Lachaud, J.P., L. Passera, A. Grimal, C. Detrain and G. Beugnon, 1992.
Lipid storage by major workers and starvation resistance in the ant
Pheidole pallidula (Hymenoptera, Formicidae). In: Biology and
Evolution of Social Insects (J. Billen, Ed.), Leuven University
Press, Leuven. 390 pp.
Lenoir, A., D. Cuisset and A. Hefetz, 2001a. Effects of social isolation
on hydrocarbon pattern and nestmate recognition in the ant
Aphaenogaster senilis (Hymenoptera, Formicidae). Insect. Soc. 48:
101–109.
Lenoir, A., A. Hefetz, T. Simon and V. Soroker, 2001b. Comparative
dynamics of gestalt odour formation in two ant species Camponotus fellah and Aphaenogaster senilis (Hymenoptera : Formicidae).
Physiol. Entomol. 26: 275–283.
Oster, G.F. and E.O. Wilson, 1978. Caste and Ecology in the Social
Insects. Monographs in Population Biology Vol 12. Princeton University Press, Princeton, NJ. 372 pp.
Passera, L., 1974. Différenciation des soldats chez la fourmi Pheidole pallidula Nyl. (Formicidae Myrmicinae). Insect. Soc. 21: 71–
86.
Passera, L., 1977. Production des soldats dans les sociétés sortant
d‘hibernation chez la fourmi Pheidole pallidula (Nyl.) (Formicidae, Myrmicinae). Insect. Soc. 24: 131–146.
Passera, L., E. Roncin, B. Kaufmann and L. Keller, 1996. Increased soldier production in ant colonies exposed to intraspecific competition. Nature 379: 630–631.
Patel, A.D., 1990. An unusually broad behavioral repertory for a major
worker in a dimorphic ant species: Pheidole morrisi (Hymenoptera,
Formicidae). Psyche 97: 181–191.
Robinson, G.E., 1992. Regulation of division of labor in insect societies. Annu. Rev. Entomol. 37: 637–665.
Soroker, V., C. Vienne and A. Hefetz, 1995. Hydrocarbon dynamics
within and between nestmates in Cataglyphis niger (Hymenoptera,
Formicidae). J. Chem. Ecol. 21: 365–378.
Wheeler, D.E. and H.F. Nijhout, 1984. Soldier determination in Pheidole bicarinata: inhibition by adult soldiers. J. Insect. Physiol. 30:
127–135.
Wilson, E.O., 1968. The ergonomics of caste in the social insects. Am.
Nat. 102: 41–66.
Wilson, E.O., 1976. Behavioral discretization and the number of castes
in an ant species. Behav. Ecol. Sociobiol. 1: 141–154.
Wilson, E.O., 1980. Caste and division of labor in leaf-cutter ants
(Hymenoptera: Formicidae: Atta). II. The ergonomic optimization
of leaf cutting. Behav. Ecol. Sociobiol. 7: 157–165.
Wilson, E.O., 1984. The relation between caste ratio and division of
labor in the ant genus Pheidole (Hymenoptera: Formicidae). Behav.
Ecol. Sociobiol. 16: 89–98.
Wilson, E.O. and R.M. Fagen, 1974. On the estimation of total behavioral repertories in ants. J. N.Y. Entomol. Soc. 82: 106–112.
Wilson, E.O. and B. Hölldobler, 1985. Caste-specific techniques of
defense in the polymorphic ant Pheidole embolopyx (Hymenoptera:Formicidae). Insect. Soc. 32: 3–22.