Behav Ecol Sociobiol (2006) 61:265–275
DOI 10.1007/s00265-006-0257-2
ORIGINAL ARTICLE
A test of reproductive skew models in a field population
of a multiple-queen ant
R. L. Hammond & M. W. Bruford & A. F. G. Bourke
Received: 7 April 2006 / Revised: 16 June 2006 / Accepted: 6 July 2006 / Published online: 16 September 2006
# Springer-Verlag 2006
Abstract Determining the evolutionary basis of variation
in reproductive skew (degree of sharing of reproduction
among coexisting individuals) is an important task both
because skew varies widely across social taxa and because
testing models of skew evolution permits tests of kin
selection theory. Using parentage analyses based on microsatellite markers, we measured skew among female eggs
(n=32.3 eggs per colony, range=20–68) in 17 polygynous
colonies from a UK field population of the ant Leptothorax
acervorum. We used skew among eggs as our principal
measure of skew because of the high degree of queen
turnover in the study population. Queens within colonies
did not make significantly unequal contributions to queen
and worker adult or pupal offspring, indicating that skew
among female eggs reflected skew among daughter
queens. On average, both skew among female eggs
(measured by the B index) and queen–queen relatedness
proved to be low (meansTSE=0.06T0.02 and 0.28T0.08,
respectively). However, contrary to current skew models,
there was no significant association of skew with either
relatedness or worker number (used as a measure of
productivity). In L. acervorum, predictions of the concession
model of skew may hold between but not within populations
because queens are unable to assess their relatedness to other
queens within colonies. Additional phenomena that may help
maintain low skew in the study population include indiscriminate infanticide in the form of egg cannibalism and split
sex ratios that penalize reproductive monopoly by single
queens within polygynous colonies.
Communicated by K. Ross
Keywords Kin selection . Polygyny . Reproductive skew .
Social evolution . Social insect
R. L. Hammond : A. F. G. Bourke
Institute of Zoology, Zoological Society of London,
Regent_s Park,
London NW1 4RY, UK
Introduction
M. W. Bruford
School of Biosciences, Cardiff University,
Cathays Park,
Cardiff CF10 3TL, UK
R. L. Hammond (*)
Department of Ecology and Evolution, Biophore,
University of Lausanne,
CH 1015 Lausanne, Switzerland
e-mail: Rob.Hammond@unil.ch
Present address:
A. F. G. Bourke
School of Biological Sciences,
University of East Anglia,
Norwich NR4 7TJ, UK
The extent to which coexisting breeding individuals share
reproduction varies greatly across animal societies. Societies
with high reproductive skew are those in which one or a few
individuals monopolize reproduction; societies with low
reproductive skew occur when breeders share reproduction
more evenly. The extension by Reeve (1991) and Reeve and
Ratnieks (1993) of the original skew models of Vehrencamp
(1979, 1983) and Emlen (1982a,b) has led to a wealth of
new models aimed at explaining skew variation within and
between species, along with a growing number of empirical
tests of these models. The importance of these studies arises
because skew models offer a potential explanation for the
266
Behav Ecol Sociobiol (2006) 61:265–275
Table 1 Assumptions of models of reproductive skew and the models_ predicted correlations
Transactional models
Main
assumptions:
Compromise
models
Tug-of-war
models
Synthetic
model
1. Synthesized
assumptions of
transactional and
compromise
models
Results
of present
study
Concession
models
Resource
inheritance
models
Restraint
models
1. Dominant
controls group
membership
1. Dominant
controls group
membership
1. Dominant
controls group
membership
1. Limited control
by both parties
2. Dominant
controls skew
2. Dominant
controls skew
2. Subordinate
controls skew
2. Gaining share
of reproduction
is costly
Positive
or Negative
Positive
or Negative
No correlation
3. Subordinate
inherits
dominant_s
resource
Predictions:
1. Skew vs
relatedness
2. Skew vs
degree of
ecological
constraint
3. Skew vs
per capita
productivity
4. Relatedness vs
group
productivity
Selected
references
Positivea,b,c
or Negativeb,c
Positivea,b
Positive
or Negative
–
Negative
Negative
Negative or no
correlation
No correlation
Positivea
–
Negative
No correlation
Positive
or Negative
No correlation
No correlationa
–
No correlation
Positive
–
No correlation
Kokko and
Johnstone (1999);
Ragsdale (1999)
Johnstone and
Cant (1999b)
Reeve et al.
(1998)
Johnstone (2000)
Reeve and Ratnieks
(1993)a; Cant and
Johnstone (1999)b;
Johnstone et al.
(1999)c
Not studied
Superscripts indicate predictions varying depending on detailed assumptions. –, no prediction made or not considered in this study. See also Cant
(1998, 2006), Johnstone and Cant (1999a), Cant and Johnstone (2000), Reeve (2000), Reeve and Emlen (2000), Kokko (2003) and Zink and
Reeve (2005).
wide variation in skew observed across animal societies. In
principle, skew models also provide a means of testing kin
selection theory (Hamilton 1964), on which they are based.
The many skew models now present in the literature
differ in both their assumptions and in the predictions they
make regarding the expected genetic, ecological, demographic and social correlates of skew (Table 1). Empirical
studies of skew in both invertebrates and vertebrates
(reviews in: Keller and Reeve 1994; Emlen 1997; Reeve
and Keller 2001) have tended to lag behind the development of new models. This is because the assumptions of
skew models are either not met or are difficult to verify,
skew and its predicted correlates are not always simple to
measure and different skew models make overlapping
predictions (Clutton-Brock 1998; Magrath and Heinsohn
2000; Table 1). The social Hymenoptera have been
recognized as a particularly apt group for empirical tests
of skew models because skew varies widely even among
closely related species, a relatively large number of social
groups can be sampled and insects lend themselves to
experimental manipulation. However, conclusions from
empirical studies have been mixed. Some studies have
found support for concession models of reproductive skew
(e.g., social wasps: Reeve et al. 2000; Sumner et al. 2002;
see also Nonacs et al. 2004), some for tug-of-war models
(e.g., social wasps: Seppä et al. 2002; social bees: Langer et
al. 2004, 2006) and others have found no clear support for
any current model (e.g., social wasps: Field et al. 1998;
Behav Ecol Sociobiol (2006) 61:265–275
Fanelli et al. 2005; Liebert and Starks 2006; Nonacs et al.
2006; ants: Fournier and Keller 2001; Rüppell et al. 2002;
Hannonen and Sundström 2003a; Fournier et al. 2004). One
reason for these mixed results could be that not all tested
species share the same model assumptions (Table 1).
Whatever its causes, this situation creates a clear need for
additional empirical studies so that the balance of evidence
can be properly assessed.
The leptothoracine ants represent a group in which an
understanding of the factors underlying variation in skew
would be especially valuable because some species exhibit
multiple-queen societies in which a single queen monopolizes reproduction (i.e., functional monogyny), whereas in
other multiple-queen (polygynous) species, reproduction is
more evenly distributed among queens (Buschinger 1974;
Bourke and Heinze 1994). Leptothoracines most closely
match the assumptions of concession models of skew
(Bourke and Heinze 1994; Rüppell et al. 2002). For
example, the maintenance of functional monogyny by
aggressive dominance (Heinze and Lipski 1990; Heinze
and Smith 1990) suggests that complete control of skew by
dominants is possible (Table 1). In addition, the presence of
subordinates increases the productivity of dominant queens,
although the mechanism underlying this effect is unclear
(Heinze and Oberstadt 2003). Consistent with concession
models, broad-scale comparisons in leptothoracines (i.e., at
the between-population and between-species level) suggest
positive correlations of skew with relatedness (Heinze
1995; Heinze et al. 1995b; Bourke et al. 1997) and inferred
levels of ecological constraint (Bourke and Heinze 1994;
Felke and Buschinger 1999). In contrast, a comprehensive
study of L. rugatulus on skew variation among colonies
(which either had been kept in the laboratory for
11 months or were artificially composed) found no support
for concession models (Rüppell et al. 2002). However, to
date, no study has examined skew as a function of predicted
correlates among colonies within a field population of
leptothoracines.
We investigated the covariation of skew, relatedness and
productivity within a field population of the facultatively
polygynous Leptothorax acervorum. We did not include
ecological constraint (Table 1) in our study. This is because it
is unlikely that L. acervorum queens are able to assess
within-population levels of ecological constraint since
readopted queens in polygynous colonies almost certainly
mate near the nest and, if they disperse following the
initiation of reproduction, do so on foot (Douwes et al.
1987; Bourke and Heinze 1994; Felke and Buschinger 1999;
Hammond et al. 2001). Therefore, queens are most likely to
have evolved to adjust their reproductive output in
response to the average, population-level degree of
ecological constraint, which is invariant for queens within
a population.
267
Methods
Field collection and colony sampling
We studied L. acervorum in Thetford Forest, Norfolk, UK.
In this population, approximately 20–50% of colonies are
polygynous with a mean of two to five related queens per
colony, 95% of which are singly mated (Chan and Bourke
1994; Heinze et al. 1995a; Bourke et al. 1997; Hammond et
al. 2001–2003). Sex ratios are split, with monogynous
colonies producing mainly females and polygynous
colonies producing mainly males (Chan and Bourke
1994; Chan et al. 1999; Hammond et al. 2002). Collections
of colonies were made in 1999 (FSD99_ colonies) and 2000
(FSD00_ colonies). The SD99 colonies (n=46, collected on
June 3 and June 10) are the same as those whose collection
is described by Hammond et al. (2001) and for which we
have previously presented genetic analyses of traits other
than reproductive skew (Hammond et al. 2001–2003). The
SD00 colonies (n=100, collected between July 27 and
August 24) came from a site approximately 1 km away. We
have not previously presented data on the SD00 colonies.
All SD99 colonies and 39 SD00 colonies were located by
random searching (the remaining 61 SD00 colonies were
located by searching in the area defined by a circle of 2-m
radius centred on each of the focal 39 colonies). Only data
from the 39 SD00 colonies located by random searching are
presented in the current paper. After discovery, all colonies
in both samples were collected using methods described in
Chan and Bourke (1994). All adults and brood were extracted
from their twigs within a few days of collection and frozen
for later genetic analysis. Colony composition therefore
reflected that found in the field at the time of collection.
We investigated reproductive skew in a subset of 17
polygynous colonies (9 SD99 colonies and 8 SD00 colonies). Polygynous colonies were defined as those containing
more than one dealate, mated queen (henceforth, Fqueens_;
dealate queens are those that have shed their wings). We
determined the insemination status of queens by noting the
presence of a full or empty sperm receptacle upon ovarian
dissection (Bourke 1991; Hammond et al. 2001). We
selected 9 polygynous colonies with 2–8 queens per colony
(meanTSD=3.6T1.8 queens) from the SD99 sample (i.e., the
9 polygynous colonies in Table 1 of Hammond et al. 2001).
In the SD00 sample, 11 of the 39 focal colonies proved to
be polygynous, but we omitted 3 of these 11 colonies from
our skew analysis. In two of the omitted colonies, parentage
analyses could not be conducted because both colonies
contained high numbers (16 and 17) of closely related
queens, which therefore shared many alleles. Omitting
these colonies is unlikely to have biased our results since,
in the remaining colonies, relatedness varied across the
whole spectrum of likely values (j0.01–0.89), indicating
268
Behav Ecol Sociobiol (2006) 61:265–275
that the exclusion of these two colonies did not truncate
variability in relatedness. The third SD00 colony was
omitted because it contained no eggs. In the eight SD00
colonies in which we measured skew, there were 2–7
queens per colony (meanTSD=3.4T2.0 queens).
relatedness values calculated previously (Hammond et al.
2001–2003). In the SD00 colonies, we calculated Queller
and Goodnight_s (1989) regression relatedness from
genotype data with the program RELATEDNESS 5.07
(Goodnight Software: http://www.gsoft.smu.edu/Gsoft.html).
Molecular methods
Colony productivity
We genotyped individuals using the microsatellite loci
LXAGT1, LXAGA1, LXAGA2 (Bourke et al. 1997),
MYRT3 (Evans 1993), LXGT223 (Hamaguchi et al.
1993) and L18 (Foitzik et al. 1997), using methods
described by Hammond et al. (2001). In the Thetford
Forest population, these loci have a mean expected
heterozygosity of 0.88 and a mean of 23 alleles per locus
(Hammond et al. 2001). In the SD99 colonies, a total of 32
queens and the contents of 29 corresponding sperm
receptacles (3 were lost during dissection) were genotyped
at a mean of 6.0 loci (range=4–6), as described in
Hammond et al. (2001). In the SD00 colonies, a total of
27 queens and the contents of 27 corresponding sperm
receptacles were genotyped at a mean of 3.6 loci (range=2–4).
Genotyping of the contents of sperm receptacles had a high
failure rate (38% in SD99 samples and 30% in SD00
samples). In SD99 colonies, as described in Hammond et al.
(2001), we also typed a per colony mean of 12.8 adult
workers (Fold workers_, range=0–15), 13.9 callow workers
or worker pupae (Fnew workers_, range=0–19), 10.3 adult
males (range=0–19) and 6.7 alate (winged) queens or queen
pupae (Fnew queens_, range=0–21) at a mean of 6.0 loci.
Finally, across both sets of colonies, we genotyped a mean of
42.6 eggs per colony at a mean of 2.0 loci (range=1–4).
These consisted of a mean (range) of 54.9 (36–89) eggs from
the SD99 colonies (the samples described in Hammond et
al. (2003) plus a few additional SD99 eggs) and a mean
(range) of 28.8 (20–38) eggs from the SD00 colonies. Eggs
were genotyped at loci found to be diagnostic for parentage
analyses on the basis of the genotypes of queens and the
queens_ mates (the genotypes of the queens_ mates being
deduced from those of the contents of the sperm receptacles or
those of female progeny). Within colonies, we attributed
female (diploid) eggs to one of the queens using exclusion
criteria. The high degree of genetic variation per locus,
together with the absence of relatedness between L. acervorum queens and their mates and between mates of coexisting
queens (Hammond et al. 2001), meant that almost all eggs
could be assigned to individual queens (see FParentage
analysis_ under FResults_).
In both the SD99 and SD00 colonies, we used the number of
adult workers as a surrogate measure of colony productivity.
This was justified because, based on previous data from
polygynous colonies in the study population in Chan et al.
(1999), we found colony sexual production (measured as
biomass of either new queens, or males or new queens and
males combined) to be highly correlated with number of
adult workers (all Pearson_s r>0.47, all n=30, all P<0.009).
Previous work has shown that polygynous colonies of L.
acervorum in the study population have a high rate of queen
turnover, with large proportions of old workers, new queens
and males being unattributable to resident queens (Bourke et
al. 1997; Hammond et al. 2001). In the present study, we
estimated the degree of queen turnover in SD99 colonies
only since no adults or pupae were genotyped in SD00
colonies. We estimated the genetically effective turnover of
queens (τ) across pairs of female age cohorts using Eq. 4 in
Pedersen and Boomsma (1999). This defines 100% turnover
as occurring when all queens contribute to one cohort only
and 0% turnover as occurring when all queens contribute to
both cohorts. The variables used in the estimation of τ are
the relatednesses within and between the two age cohorts
being compared. We measured relatednesses within and
between female eggs, new workers, old workers and new
queens. Relatednesses were averaged across colonies for the
estimation of population-level turnover. Since L. acervorum
workers overwinter once as larvae, whereas queens usually
overwinter twice (Buschinger 1973), we assumed that,
relative to eggs, new workers were 0–1 years older, old
workers were >1 year older and new queens were 2 years
older. These relative ages are approximate because some
workers may eclose in the year that they were laid as eggs,
the longevity of adult workers is unknown, and some queens
may overwinter as larvae only once. Nonetheless, comparing
relatedness within and between female eggs and these
cohorts allowed us to estimate queen turnover across an
ever-increasing age interval.
Relatedness
Reproductive skew
We estimated regression relatedness among coexisting queens
in the SD00 colonies and, for the SD99 colonies, used
The likely occurrence of high queen turnover meant
that reproductive skew could not easily be calculated
Queen turnover
Behav Ecol Sociobiol (2006) 61:265–275
from the genotypes of adult or pupal progeny. We
therefore measured skew in samples of female eggs
(SD99 colonies: mean=44.8 female eggs per colony,
range=28–77; SD00 colonies: mean=25.0 female eggs per
colony, range=20–33). This assumed that individual
queens within colonies did not differ in the queen-toworker ratio among their progeny (we consider other
assumptions in FDiscussion_). We were able to test this
assumption by comparing the proportion of either caste
attributable to each queen in four SD99 colonies with
sufficient numbers of new workers and new queens. In one
of these colonies (SD99.54), we included as a maternal queen
an individual that was not present at collection but whose
existence could be inferred from the genotypes of her
daughters, i.e., a Flost_ queen (Hammond et al. 2003). In
each colony, a minority of new workers and new queens
could be not be assigned to either resident or lost queens. We
therefore grouped these progeny into a Fqueen unknown_
parentage class.
We did not investigate reproductive skew among male
eggs for two reasons. First, the sample sizes for male eggs
in each colony were small because in L. acervorum, only a
small minority of eggs laid by queens (16%) are male and
workers lay very few male eggs (Hammond et al. 2002,
2003). Second, parentage assignment of males, being
haploid, was much more difficult than for females because
queens often shared a high proportion of alleles (frequently,
it was the paternal alleles in females that allowed parentage
assignment). However, in five SD99 colonies, the sample
size of adult, queen-produced males was large enough and
parentage assignment was possible. In these colonies, we
investigated whether reproductive skew differed between
sexes of progeny by comparing the proportion of adult
males and new workers attributable to each queen within
the colonies. We did not compare adult males and new
queens because such a comparison would have been
confounded by year since usually queens take 2 years to
develop, whereas workers and males take only 1 year
(Buschinger 1973). As in our comparison of skew between
castes, progeny that could not be assigned to either resident
or lost queens were grouped into a Fqueen unknown_
parentage class.
We used Nonacs_s B index (Nonacs 2000, 2003) to
quantify skew in female eggs. Using the program SKEW
CALCULATOR 2003 (Nonacs 2003), we calculated B, the
95% confidence limits of B (using 10,000 randomizations)
and, given the queen number in each colony, the maximum
(where only one queen reproduces in each colony) and
minimum (where all queens reproduce equally) possible
values of B. To control for the maximum and minimum
values of B varying across colonies (P. Nonacs, personal
communication), we also calculated an adjusted B index
(Badj). We calculated this as the absolute difference between
269
the observed and minimum B values divided by the
absolute difference between the maximum and minimum
B values.
Statistical methods
Queen–queen relatedness (Frelatedness_), colony size
(Fworker number_, i.e., number of old workers), reproductive skew (as measured by B or Badj) and the number of
queens per colony (Fqueen number_) were all normally
distributed (Kolmogorov–Smirnov tests, all P>0.05). There
was no significance difference in any variable across
sampling years (SD99 vs SD00: all t15<1.07, all P>0.30),
so data were pooled across years for further analyses. We
tested relationships predicted by the skew models (Table 1)
in two general linear models (GLM). First, in Fskew GLM_,
we tested whether skew (B or Badj) varied with relatedness,
queen number and worker number (as a measure of
productivity). We also tested whether skew varied with
productivity per queen by testing in these analyses for an
interaction between queen and worker number. Second, in
Fproductivity GLM_, we tested whether worker number
varied with relatedness. In addition, in Fqueen number
GLM_, we tested whether relatedness varied with queen
number. All GLM analyses were carried out with SPSS
(version 12.0.0). To compare skew between castes (new
queens vs new workers) and sexes (males vs new workers),
we used exact tests calculated by the program RC [Miller
MP (1997) RC: a program for the analysis of contingency
tables, 1.0 edn. Department of Biological Sciences,
Northern Arizona University, Flagstaff]. Where multiple
tests were performed on the same data, we applied
sequential Bonferroni correction (Rice 1989).
Results
Relatedness and queen turnover
Average queen–queen relatedness (meanTSE) was
0.28T0.08 (n=59 queens from 17 colonies). The genetical
effective turnover of queens (τ) was 50.3% (comparing
eggs vs new workers, n=8 colonies), 43% (comparing eggs
vs old workers, n=8 colonies) and 67.2% (comparing eggs
vs new queens, n=5 colonies). Therefore, queen turnover
was substantial and, as expected, showed evidence of rising
as the age interval between cohorts increased (with the
reversal of values between eggs vs new workers and eggs
vs old workers, relative to those expected, presumably
stemming from high overlap between the ages of new and
old workers and from sampling error in the underlying
relatedness estimates). These results, coupled with the
complementary finding that a mean of 27% of new
270
Behav Ecol Sociobiol (2006) 61:265–275
workers, new queens and old workers were not assignable
to resident queens (see below) and with previous findings
from the same population (Bourke et al. 1997), provided
support for measuring skew among eggs in this study.
Queens coexisting within the same colony therefore
differed in their relative success at producing male and
female offspring, with some concentrating on male production and others on female production.
Parentage analysis
Average reproductive skew
We could successfully attribute 91% (550 out of a total
n=603) of all female eggs to individual resident queens.
The remaining 9% were either not attributable to any
resident collected queen (8%) or were equally likely to be
the offspring of two or more resident queens (1%). Almost
half of the eggs that were unattributable to resident queens
were found in two colonies (SD99.74 and SD99.96);
omitting these raised the proportion of successfully
assigned eggs to 95%. In our analysis of skew (see below),
we ignored the small fraction of unattributed eggs and
analyzed skew in the large majority of eggs whose
maternity we could deduce. Furthermore, we included all
colonies since the omission of the two colonies with the
highest proportions of unattributable eggs did not alter the
results. Compared to female eggs, a lower proportion of
adult males (82%), new queens (64%), new workers (69%)
and old workers (85%) could be attributed to resident
queens.
Reproduction was relatively equitably distributed among
queens, with only 4 of 59 resident queens (6.8%) failing to
contribute any female eggs (Fig. 1). The highest proportion
of eggs contributed by a single queen was 69.2% in a
colony with 3 queens (colony SD00.129; Fig. 1). Average
skew across colonies was low, with the overall mean (TSE)
of B equalling 0.06T0.02 and of Badj equalling 0.12T0.03
(Fig. 2). The mean level of B (randomization test,
P<0.0001) and skew in 5 of 17 colonies (randomization
test, P<0.004, corresponding to an overall alpha of 0.05)
was significantly greater than that expected by chance
(Fig. 2). However, even in the 5 colonies with significant
skew, skew estimates were nearer their minimum than their
maximum level (Fig. 2).
Variation in skew as measured across castes and sexes
There were no significant differences between the proportion of new queens and new workers attributable to each
resident queen in all four SD99 colonies where comparisons were possible (Table 2), supporting our assumption
that skew in female eggs provides an accurate estimate of
reproductive skew in new queens. However, there were
significant differences between the proportion of males and
new workers attributable to each resident queen in all five
SD99 colonies where comparisons were possible (Table 3).
Relationship of reproductive skew with relatedness, worker
number and queen number
We found no significant relationship of skew with
relatedness, queen number or worker number (skew GLM,
B: F4,12=0.99, P=0.45; Badj: F4,12=0.73, P=0.59; Fig. 3).
There was also no significant interaction between queen
number and worker number (B: F1,12=2.91, P=0.11; Badj:
F1,12=2.52, P=0.14). Bivariate linear regressions showed
that our analyses had relatively high power because the
standard deviations of the relevant regression coefficients
were small (B vs relatedness: bTSD=j0.03T0.06,
F 1,15 =0.18, r 2 =0%, P=0.67; B vs queen number:
bTSD=0.01T0.01, F1,15=0.96, r2=0%, P=0.34; B vs worker
number: bTSD=0.00T0.00, F1,15=0.08, r2=0%, P=0.79; B vs
Table 2 Number of new queens and new workers attributable to individual queens in four polygynous L. acervorum colonies
Colony
SD99.53
SD99.54
SD99.55
SD99.94
New
New
New
New
New
New
New
New
queens
workers
queens
workers
queens
workers
queens
workers
A
B
C
D
E
F
G
H
U
Totals
P value
0
0
0
1
2
3
1
1
2
1
2
3
4
5
0
2
7
4
0
0
4
5
0
2
2
3
10
4
–
–
0
3
–
–
–
–
–
–
0
1
–
–
–
–
–
–
4
2
–
–
–
–
–
–
0
0
–
–
–
–
–
–
0
1
4
7
9
11
5
2
0
3
15
15
21
19
15
15
5
15
0.630
0.247
0.681
0.196
Individual queens (differing across colonies) are labelled A, B, C, etc. The final column (U) in each sequence per colony includes progeny
of unknown parentage (i.e., progeny that could not be attributed to any queen within the colony). The queens in SD99.54 also include one Flost_
queen whose presence was inferred from progeny genotypes (see FReproductive skew_ under FMethods_). P value is from exact probability
tests of the null hypothesis that, within colonies, individual queens do not differ in the ratio of new workers and new queens produced.
Behav Ecol Sociobiol (2006) 61:265–275
271
Table 3 Number of males and new workers attributable to individual queens in five polygynous L. acervorum colonies
Colony
SD99.55
SD99.61
SD99.64
SD99.74
SD99.78
Males
New workers
Males
New workers
Males
New workers
Males
New workers
Males
New workers
A
B
C
D
E
U
Totals
P value
0
3
0
0
10
0
0
1
0
2
9
5
7
14
0
7
0
1
0
8
0
5
–
–
2
8
0
1
17
4
–
–
–
–
–
–
1
4
–
–
–
–
–
–
–
–
6
0
–
–
6
2
9
1
2
8
0
8
0
1
15
15
16
15
14
23
7
15
17
15
0.008
0.005
<0.0001
<0.0001
<0.0001
Individual queens (differing across colonies) are labelled A, B, C, etc. The final column (U) in each sequence per colony includes progeny
of unknown parentage (i.e., progeny that could not be attributed to any queen within the colony). The queens in SD99.64 and SD99.74 also
each include one Flost_ queen whose presence was inferred from progeny genotypes (see FReproductive skew_ under FMethods_). P value is from
exact probability tests of the null hypothesis that, within colonies, individual queens do not differ in the ratio of males and new workers
produced. All P values are also significant after sequential Bonferroni correction.
worker number per queen: bTSD=0.00T0.00, F1,15=0.21,
r2=0%, P=0.65; Badj vs relatedness: bTSD=j0.03T0.09,
F 1,15 =0.12, r 2 =1%, P=0.74; B adj vs queen number:
bTSD=0.00T0.02, F1,15=0.68, r2=1%, P=0.80; Badj vs
worker number: bTSD=0.00T0.00, F1,15=0.01, r2=0%,
P=0.95; B adj vs worker number per queen: bTSD=
0.00T0.00, F1,15=0.09, r2=1%, P=0.77). We also found that
relatedness did not vary significantly with worker number
(productivity GLM: F1,15=0.04, P=0.84) or queen number
(queen number GLM: F1,15=0.01, P=0.91).
Complete skew
Upper 95% CL
Observed B
Lower 95% CL
No skew
1.00
1.0
52 31 41 25 41 48 28 68 23 26 22 18 32 26 25 24 20
Skew (B)
0.8
0.60
0.40
0.20
0.6
0.00
0.4
SD99.53
SD99.54
SD99.55
SD99.61
SD99.64
SD99.74
SD99.78
SD99.94
SD99.96
SD00.75
SD00.76
SD00.121
SD00.126
SD00.129
SD00.143
SD00.144
SD00.171
Proportion of eggs attributable to each queen
0.80
0.2
Colony
SD99.53
SD99.54
SD99.55
SD99.61
SD99.64
SD99.74
SD99.78
SD99.94
SD99.96
SD00.75
SD00.76
SD00.121
SD00.126
SD00.129
SD00.143
SD00.144
SD00.171
0.0
Colony
Fig. 1 Relative proportions of female eggs produced by queens
within 17 polygynous Leptothorax acervorum colonies. Alternating
black and white segments of each bar represent the proportion of
female eggs attributable to different queens within a given colony.
Numbers above bars equal the number of female eggs per colony that
were successfully attributed to resident queens (total n=550)
Fig. 2 Reproductive skew, as estimated by the B index among female
eggs, in 17 polygynous L. acervorum colonies. For each colony, upper
horizontal bar (F—_) denotes the maximum value of B (i.e., value if
one queen monopolizes all reproduction), given the observed number
of queens in the colony; upper F_ denotes the upper 95% confidence
limit of B, based on 10,000 randomizations; filled circle denotes
observed B; lower F_ denotes lower 95% confidence limit of B,
based on 10,000 randomizations; lower horizontal bar (F—_) denotes
the minimum value of B (i.e., value if all queens share reproduction
equally), given the observed number of queens in the colony.
Following Bonferroni correction, skew was significantly greater than
expected by chance in five colonies (SD99.54, SD99.55, SD99.94,
SD00.126 and SD00.129) and marginally greater than expected by
chance in a sixth colony (SD00.143)
272
Fig. 3 Variation in reproductive skew, as estimated by the B index
among female eggs, in 17 polygynous L. acervorum colonies as a
function of queen–queen relatedness within colonies (upper figure),
queen number (middle figure) and number of old (adult) workers
(representing a measure of productivity; lower figure)
Discussion
Using parentage analyses based on microsatellite markers,
we measured skew among female eggs in a sample of 17
polygynous colonies taken from a field population of the
ant L. acervorum. Skew tended to be low, consistent with
previous data showing that nearly all queens participate in
egg laying (Bourke 1991) and that the maternity of new
queens is shared (Bourke et al. 1997). However, despite
Behav Ecol Sociobiol (2006) 61:265–275
wide variation in queen–queen relatedness, we found no
significant association of skew with relatedness. We also
found no significant association of skew with productivity
(as measured by worker number) or per capita productivity.
These findings were contrary to the predictions of the
concession model of reproductive skew (Table 1). Furthermore, contrary to the prediction of the tug-of-war model
(Table 1), we found no significant association of relatedness
and productivity (as measured by worker number). The
absence of a significant association between relatedness and
queen number was consistent with our previous findings
from an overlapping data set (Hammond et al. 2001).
Overall, our results failed to match any single current skew
model predicting an association of skew with the variables
that we investigated (Table 1). In this, our results are
consistent with the majority of similar, within-population
studies of the expected correlates of skew in other social
Hymenoptera, especially ants (see FIntroduction_).
Our principal measure of reproductive skew was the
degree to which coexisting queens shared the parentage of
female (diploid) eggs. We selected this measure because of
the high turnover among L. acervorum queens in the study
population (Bourke et al. 1997; present study), which
meant that offspring sampled as adults would no longer
have been assignable to resident queens. This measure of
skew assumed that egg-to-adult survival of female eggs
does not vary with parentage, that queens do not differ in
the chances of their female eggs developing into adult
queens or workers and that skew among female progeny
matches skew among male progeny. The first of these
assumptions is plausible because, although differential
mortality stemming from nepotism has been found in the
polygynous ant Formica fusca (Hannonen and Sundström
2003b), L. acervorum queens, although known to eat eggs,
do not discriminate eggs by their maternal origin (Bourke
1994). Likewise, as regards the second assumption,
although coexisting queens have been shown to differ in
their relative contributions to worker and queen progeny in
some polygynous ant species (Ross 1988; Pamilo and
Seppä 1994), in the present study, we found that
L. acervorum queens contributed similar shares to worker
and queen progeny (see also Rüppell et al. 2002). This
finding is consistent with queen–worker caste fate being
environmentally determined in Leptothorax (Wesson 1940).
It also suggests that the workers_ biasing of caste fate that
we have previously described in L. acervorum (Hammond
et al. 2002) is exercised randomly with respect to female
parentage. Contrary to our third assumption, we found that
coexisting L. acervorum queens exhibited significant
variation in their relative contributions to female and male
progeny (cf. Fournier and Keller 2001). In general, skew
among male progeny was higher than among female
progeny (B=0.56 and 0.15 among males and new workers,
Behav Ecol Sociobiol (2006) 61:265–275
respectively, and Badj=0.67 and 0.28 among males and new
workers, respectively; data from five colonies in Table 3),
from which it follows that skew among sexual progeny as a
whole would be higher than skew estimated among females
only. However, this does not necessarily affect our main
conclusions regarding the lack of association between skew
and its predicted correlates across colonies. This is because
it seems unlikely that skew in males would vary as a
function of variables with which we found skew among
females to be uncorrelated.
Previous evidence suggests that leptothoracine ants
exhibit a positive relationship of skew with relatedness at
the between-population and between-species level, as the
concession model of skew evolution predicts (see
FIntroduction_). However, within-population studies reveal
either no relationship (present study) or a negative
relationship (Rüppell et al. 2002). Applied within populations, skew models assume that coexisting breeders are
capable of assessing within-group relatedness and adjusting
their share of reproduction accordingly. At first sight, two
pieces of evidence suggest that L. acervorum queens in
polygynous colonies could be capable of assessing queen–
queen relatedness. The first is the negative relationship of
skew and relatedness in L. rugatulus (Rüppell et al. 2002).
The second is the existence of worker-controlled sex ratios
associated with variation in relative colony-level relatedness asymmetry (relative relatedness to the sexes) in
L. acervorum and other ant species (Chan and Bourke
1994; Sundström 1994; Evans 1995; Sundström et al.
1996), a precondition for which is workers_ assessment of
relatedness asymmetry. However, in L. rugatulus (but not
L. acervorum), queens are dimorphic, with small-bodied
queens (microgynes) producing relatively more sexuals
than large-bodied queens (macrogynes) (Rüppell et al.
2002). Furthermore, microgynes tend to be less related
than macrogynes (Rüppell et al. 2001). Therefore, unlike
L. acervorum queens, L. rugatulus queens may have a
physical cue (frequency of nestmate microgynes) that
covaries with relatedness and, at least partly, predicts
nestmates_ share of reproduction. As regards workers_
assessment of relatedness asymmetry, it is likely that such
assessment, which occurs on the basis of chemical cues
(Boomsma et al. 2003), is an easier chemosensory task than
queens_ assessment of queen–queen relatedness since it
almost certainly requires discriminations on a coarser scale.
Therefore, it remains possible that L. acervorum queens in
polygynous colonies are unable to assess within-colony
relatedness and that this is why they fail to adjust their
levels of skew as a function of relatedness varying within
populations.
In addition to processes in the concession models
operating at the population level, other factors may
contribute to the maintenance of low skew in the study
273
population of L. acervorum (and other polygynous ants
sharing its biology). One is indiscriminate infanticide in the
form of indiscriminate egg cannibalism (Bourke 1991,
1994). Coupled with low costs of offspring (egg) production, this is predicted to promote low skew by a model of
Johnstone and Cant (1999a). Another is split sex ratios
(Chan and Bourke 1994; Chan et al. 1999; Hammond et al.
2002). These could interact with skew evolution to reward
the maintenance of low skew in polygynous colonies, given
that queens achieving reproductive monopoly within
polygynous colonies should lose fitness through their
workers rearing less-valuable daughters from the sexual
brood (Bourke 2001; Nonacs 2002). Future work in
L. acervorum and other species should therefore concentrate on (a) further testing of which assumptions of the
differing skew models are applicable, (b) greater integration
of the differing skew models into a single comprehensive
framework (e.g., Johnstone 2000) and (c) the experimental
testing of the models_ predictions (e.g., Langer et al. 2004).
Acknowledgements We thank Peter Nonacs, Seirian Sumner and an
anonymous referee for helpful comments and advice. The Forestry
Commission kindly granted permission to collect samples. This work
was supported by a grant (GR3/11792) from the UK Natural
Environment Research Council to AFGB and MWB. Rob Hammond
was supported by Swiss National Science Foundation grants awarded
to Laurent Keller while this paper was written. The work in this paper
complies with current UK law.
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