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Inclusive fitness theory predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry (relative relatedness to females and males) of the individuals controlling sex allocation. In monogynous ants (with one queen per colony), assuming worker control, the theory therefore predicts female-biased sex investment ratios, as found in natural populations. Recently, E.O. Wilson and M.A. Nowak criticized this explanation and presented an alternative hypothesis. The Wilson– Nowak sex ratio hypothesis proposes that, in monogynous ants, there is selection for a 1 : 1 numerical sex ratio to avoid males remaining unmated, which, given queens exceed males in size, results in a female-biased sex investment ratio. The hypothesis also asserts that, contrary to inclusive fitness theory, queens not workers control sex allocation and queen–worker conflict over sex allocation is absent. Here, I argue that the Wilson–Nowak sex ratio hypothesis is flawed because it contradicts Fisher’s sex ratio theory, which shows that selection on sex ratio does not maximize the number of mated offspring and that the sex ratio proposed by the hypothesis is not an equilibrium for the queen. In addition, the hypothesis is not supported by empirical evidence, as it fails to explain ‘split’ (bimodal) sex ratios or data showing queen and worker control and ongoing queen–worker conflict. By contrast, these phenomena match predictions of inclusive fitness theory. Hence, the Wilson–Nowak sex ratio hypothesis fails both as an alternative hypothesis for sex investment ratios in eusocial Hymenoptera and as a critique of inclusive fitness theory.

Inclusive fitness theory predicts that, other things equal, individuals within social groups should direct altruistic behaviour towards their most highly related group-mates to maximise indirect fitness benefits. In the social insects, most previous studies have shown that within-colony kin discrimination (nepotism) is absent or weak. However, the number of studies that have investigated within-colony kin discrimination at the level of individual behaviour remains relatively small. We tested for within-colony kin discrimination in the facultatively multiple-queen (polygynous) ant, Leptothorax acervorum. Specifically, we tested whether workers within polygynous colonies treated queens differently as a function of their relatedness to them. Colonies containing two egg-laying queens were filmed to measure the rate at which individually marked workers antennated and groomed or fed each queen. Relatedness between individual queens and workers was calculated from their genotypes at four microsatellite loci. The results showed there were no differences in the rates at which workers antennated or groomed/fed their more related queen and their less related queen. Workers interacted preferentially with their potential mother queen with respect to grooming/feeding but not with respect to antennation. However, because of high queen turnover, the frequency of adult workers with their potential mother queen still present within the colony was relatively low. Overall, therefore, we found no evidence for within-colony kin discrimination in the context of the average worker's treatment of queens in polygynous L. acervorum colonies.

Worker policing (mutual repression of reproduction) in the eusocial Hymenoptera represents a leading example of how coercion can facilitate cooperation. The occurrence of worker policing in "primitively" eusocial species with low mating frequencies, which lack relatedness differences conducive to policing, suggests that separate factors may underlie the origin and maintenance of worker policing. We tested this hypothesis by investigating conflict over male parentage in the primitively eusocial, monandrous bumblebee, Bombus terrestris. Using observations, experiments, and microsatellite genotyping, we found that: (a) worker- but not queen-laid male eggs are nearly all eaten (by queens, reproductive, and nonreproductive workers) soon after being laid, so accounting for low observed frequencies of larval and adult worker-produced males; (b) queen- and worker-laid male eggs have equal viabilities; (c) workers discriminate between queen- and worker-laid eggs using cues on eggs and egg cells that almost certainly originate from queens. The cooccurrence in B. terrestris of these three key elements of "classical" worker policing as found in the highly eusocial, polyandrous honeybees provides novel support for the hypothesis that worker policing can originate in the absence of relatedness differences maintaining it. Worker policing in B. terrestris almost certainly arose via reproductive competition among workers, that is, as "selfish" policing.

Foraging distance is a key determinant of colony survival and pollination potential in bumblebees Bombus spp. However this aspect of bumblebee ecology is poorly understood because of the difficulty in locating colonies of these central place foragers. Here, we used a combination of molecular microsatellite analyses, remote sensing and spatial analyses using kernel density estimates to estimate nest location and foraging distances for a large number of wild colonies of two species, and related these to the distribution of foraging habitats across an experimentally manipulated landscape. Mean foraging distances were 755 m for Bombus lapidarius and 775 m for B. pascuorum (using our most conservative estimation method). Colony-specific foraging distances of both species varied with landscape structure, decreasing as the proportion of foraging habitats increased. This is the first time that foraging distance in wild bumblebees has been shown to vary with resource availability. Our method offers a means of estimating foraging distances in social insects, and informs the scale of management required to conserve bumblebee populations and enhance their pollination services across different landscapes.

The conservation genetics of bees is of particular interest because many bee species are in decline, so jeopardizing the essential ecosystem service of plant pollination that they provide. In addition, as social haplodiploids, inbred bees may be vulnerable to the extra genetic load represented by the production of sterile diploid males. Using microsatellite markers, we investigated the genetic structure of populations of the Great Yellow Bumblebee (Bombus distinguendus Morawitz) in the UK, where this species has undergone a precipitous decline. By means of a mixture of analytical methods and simulation, we also extended--and then applied--genetic methods for estimating foraging distance and nest density in wild bees. B. distinguendus populations were characterized by low expected heterozygosity and allelic richness, inbreeding coefficients not significantly different from zero, absence of detected diploid males, absence of substantial demographic bottlenecking, and population substructuring at large (c. 100+ km) but not small (10s of km) spatial scales. The minimum average effective population size at our sampling sites was low (c. 25). In coastal grassland (machair), the estimated modal foraging distance of workers was 391 m, with 95% of foraging activity occurring within 955 m of the nest, and estimated nest density was 19.3 nests km(-2). These findings show that B. distinguendus exhibits some genetic features of scarce, declining or fragmented populations. Moreover, B. distinguendus workers appear to forage over above-average distances and nests remain thinly distributed even in current strongholds. These considerations should inform future conservation actions for this and similar species.

Queens in some species of ant with several queens per colony are believed to inhibit one another's fecundity pheromonally. This paper describes a repeat of an experimental test of this idea in the multiple-queen ant Leptothorax acervorum. The three most fecund queens from a field-collected colony were housed together in an experimental nest. This was then filmed for 10 h per day over 36 consecutive days (i.e. for 360 h in total). Over the first 12 d, all queens were present in the colony. For the middle 12 d, the two most fecund queens were removed, but were returned for the final 12 d. The queen that was observed throughout the experiment failed to show a rise in fecundity when the other queens were removed. Instead, her egg-laying rate fell, although this drop was not significant. These results confirm previous findings that short-term, strong pheromonal inhibition of fecundity does not occur among L. acervorum queens. The phenomena of pheromonal inhibition in other species may be a result of workers treating queens differentially in response to rapid, short-term changes in the numbers of queens and eggs. If so, the results presented here could stem from the relative infrequency of such changes in L. acervorum.

Social insects (ants, bees, wasps and termites) as a group are species rich and ecologically dominant. Many are outstanding “ecological engineers”, or providers of “ecosystem services”, or potential bioindicator species. Few social insects are currently formally classified as Threatened, but this is almost certainly due to a lack of information on population sizes and trends in scarce species. The main influence that sociality has on threats faced by social insects is in reducing effective population sizes, increasing population genetic subdivision and possibly reducing levels of genetic variation relative to solitary species. The main influence that sociality has on threats from social insects is via its role in the ecological success of invasive species, which frequently pose a major hazard to native biotas. In some cases, social features underpinning ecological success in the original range almost certainly contribute to the success of invasive social insects. However, recent studies show or strongly suggest that, in some of the most notoriously invasive populations of ants, bees and wasps, novel social traits have arisen that greatly enhance the rate of spread and ecological competitiveness of these populations. Sociality can therefore represent either a liability or an asset in its contribution to the persistence of social insect populations.

Multiple mating by queens (polyandry) and the occurrence of multiple queens in the same colony (polygyny) alter patterns of relatedness within societies of eusocial insects. This is predicted to influence kin-selected conflicts over reproduction. We investigated the mating system of a facultatively polygynous UK population of the ant Leptothorax acervorum using up to six microsatellite loci. We estimated mating frequency by genotyping 79 dealate (colony) queens and the contents of their sperm receptacles and by detailed genetic analysis of 11 monogynous (single-queen) and nine polygynous colonies. Results indicated that 95% of queens were singly mated and 5% of queens were doubly mated. The corrected population mean mating frequency was 1.06. Parentage analysis of adults and brood in 17 colonies (10 monogynous, 7 polygynous) showed that female offspring attributable to each of 31 queens were full sisters, confirming that queens typically mate once. Inbreeding coefficients, queen–mate relatedness of zero and the low incidence of diploid males provided evidence that L. acervorum sexuals mate entirely or almost entirely at random. Males mated to queens in the same polygynous colony were not related to one another. Our data also confirmed that polygynous colonies contain queens that are related on average and that their workers had a mixed maternity. We conclude that the mating system of L. acervorum involves queens that mate near nests with unrelated males and then seek readoption by those nests, and queens that mate in mating aggregations away from nests, also with unrelated males.