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Kin selection plays a major role in the evolution of cooperative systems. However, many social species exhibit complex within-group relatedness structures, where kin selection alone cannot explain the occurrence of cooperative behaviour. Understanding such social structures is crucial to elucidate the evolution and maintenance of multi-layered cooperative societies. In lamprologine cichlids, intragroup relatedness seems to correlate positively with reproductive skew, suggesting that in this clade dominants tend to provide reproductive concessions to unrelated subordinates to secure their participation in brood care. We investigate how patterns of within-group relatedness covary with direct and indirect fitness benefits of cooperation in a highly social vertebrate, the cooperatively breeding, polygynous lamprologine cichlid Neolamprologus savoryi. Behavioural and genetic data from 43 groups containing 578 individuals show that groups are socially and genetically structured into subgroups. About 17% of group members were unrelated immigrants, and average relatedness between breeders and brood care helpers declined with helper age due to group membership dynamics. Hence the relative importance of direct and indirect fitness benefits of cooperation depends on helper age. Our findings highlight how both direct and indirect fitness benefits of cooperation and group membership can select for cooperative behaviour in societies comprising complex social and relatedness structures. This article is protected by copyright. All rights reserved.

Lifetime reproductive success of males is often dependent upon the ability to physically compete for mates. However, species variation in social structure leads to differences in the relative importance of intraspecific aggression. Here, we present a large comparative dataset on sexual dimorphism in skeletal shape in Carnivora to test the hypotheses that carnivorans exhibit sexual dimorphism in skeletal anatomy that is reflective of greater specialization for physical aggression in males relative to females and that this dimorphism is associated with the intensity of sexual selection. We tested these hypotheses using a set of functional indices predicted to improve aggressive performance. Our results indicate that skeletal shape dimorphism is widespread within our sample. Functional traits thought to enhance aggressive performance are more pronounced in males. Phylogenetic model selection suggests that the evolution of this dimorphism is driven by sexual selection, with the best-fitting model indicating greater dimorphism in polygynous versus nonpolygynous species. Skeletal shape dimorphism is correlated with body size dimorphism, a common indicator of the intensity of male-male competition, but not with mean body size. These results represent the first evidence of sexual dimorphism in the primary locomotor system of a large sample of mammals.

Parents should bias sex allocation toward offspring of the sex most likely to provide higher fitness returns. Trivers and Willard proposed that for polygynous mammals, females should adjust sex-ratio at conception or bias allocation of resources toward the most profitable sex, according to their own body condition. However, the possibility that mammalian fathers may influence sex allocation has seldom been considered. Here, we show that the probability of having a son increased from 0.31 to 0.60 with sire reproductive success in wild bighorn sheep (Ovis canadensis). Furthermore, our results suggest that females fertilized by relatively unsuccessful sires allocated more energy during lactation to daughters than to sons, while the opposite occurred for females fertilized by successful sires. The pattern of sex-biased offspring production appears adaptive because paternal reproductive success reduced the fitness of daughters and increased the average annual weaning success of sons, independently of maternal allocation to the offspring. Our results illustrate that sex allocation can be driven by paternal phenotype, with profound influences on the strength of sexual selection and on conflicts of interest between parents.

In most mammals, both sexes display different survival patterns, often involving faster senescence in males. Being under intense sexual competition to secure mating opportunities, males of polygynous species allocate resources to costly behaviors and conspicuous sexual traits, which might explain these observed differences in longevity and senescence patterns. However, comparative studies performed to date have led to conflicting results. We aimed to resolve this problem by first reviewing case studies of the relationship between the strength of sexual selection and age-specific survival metrics. Then, we performed a comprehensive comparative analysis to test whether such relationships exist among species of captive ruminants. We found that the strength of sexual selection negatively influenced the onset of actuarial senescence in males, with males senescing earlier in polygynous than in monogamous species, which led to reduced male longevity in polygynous species. Moreover, males of territorial species senesced earlier but slower, and have a shorter longevity than males of species displaying other mating tactics. We detected little influence of the strength of sexual selection on the rate of actuarial senescence. Our findings demonstrate that the onset of actuarial senescence, rather than its rate, is a side effect of physiological mechanisms linked to sexual selection, and potentially accounts for observed differences in longevity.

Actuarial senescence is widespread in age-structured populations. In growing populations, the progressive decline of Hamiltonian forces of selection with age leads to decreasing survival. As actuarial senescence is overcompensated by a high fertility, actuarial senescence should be more intense in species with high reproductive effort, a theoretical prediction that has not been yet explicitly tested across species. Wild boar (Sus scrofa) females have an unusual life-history strategy among large mammals by associating both early and high reproductive effort with potentially long lifespan. Therefore, wild boar females should show stronger actuarial senescence than similar-sized related mammals. Moreover, being polygynous and much larger than females, males should display higher senescence rates than females. Using a long-term monitoring (18 years) of a wild boar population, we tested these predictions. We provided clear evidence of actuarial senescence in both sexes. Wild boar females had earlier but not stronger actuarial senescence than similar-sized ungulates. Both sexes displayed similar senescence rates. Our study indicates that the timing of senescence, not the rate, is associated with the magnitude of fertility in ungulates. This demonstrates the importance of including the timing of senescence in addition to its rate to understand variation in senescence patterns in wild populations.

The positive relationship between sexual size dimorphism (SSD) and harem size across pinnipeds is often cited as a textbook example of sexual selection. It assumes that female aggregation selected for large male size via male-male competition. Yet, it is also conceivable that SSD evolved prior to polygyny due to ecological forces. We analyzed 11 life-history traits in 35 pinniped species to determine their coevolutionary dynamics and infer their most likely evolutionary trajectories contrasting these two hypotheses. We find support for SSD having evolved prior to changes in the mating system, either as a consequence of niche partitioning during aquatic foraging or in combination with sexual selection on males to enforce copulations on females. Only subsequently did polygyny evolve, leading to further coevolution as the strength of sexual selection intensified. Evolutionary sequence analyses suggest a polar origin of pinnipeds and indicate that SSD and polygyny are intrinsically linked to a suite of ecological and life-history traits. Overall, this study calls for the inclusion of ecological variables when studying sexual selection and argues for caution when assuming causality between coevolving traits. It provides novel insights into the role of sexual selection for the coevolutionary dynamics of SSD and mating system.

Sexual selection is one of the earliest areas of interest in evolutionary biology. And yet, the evolutionary history of sexually dimorphic traits remains poorly characterized for most vertebrate lineages. Here, we report on evidence for the early evolution of dimorphism within a model mammal group, the pinnipeds. Pinnipeds show a range of sexual dimorphism and mating systems that span the extremes of modern mammals, from monomorphic taxa with isolated and dispersed mating to extreme size dimorphism with highly ordered polygynous harem systems. In addition, the degree of dimorphism in pinnipeds is closely tied to mating system, with strongly dimorphic taxa always exhibiting a polygynous system, and more monomorphic taxa possessing weakly polygynous systems. We perform a comparative morphological description, and provide evidence of extreme sexual dimorphism (similar to sea lions), in the Miocene-aged basal pinniped taxon Enaliarctos emlongi. Using a geometric morphometric approach and combining both modern and fossil taxa we show a close correlation between mating system and sex-related cranial dimorphism, and also reconstruct the ancestral mating system of extant pinnipeds as highly polygynous. The results suggest that sexual dimorphism and extreme polygyny in pinnipeds arose by 27 Ma, in association with changing climatic conditions.

Although learned mate preferences are suspected to have important effects during speciation, theoretical models have largely neglected the effects on speciation and population divergence of within-generational learning, that is, learning based upon prior experience with potential mates. Here, we use population genetic models to address this deficit. Focusing on the situation of secondary contact between populations that still hybridize, we consider models of learning by females and by males under polygyny. We assess the effects of learning to prefer conspecifics from previous conspecific encounters, learning to avoid heterospecifics from previous heterospecific encounters, and learning to prefer familiar types. We examine the amount of population divergence that results from learning in these models. We also assess the effect of learning on the spread of an allele that strengthens assortative mating in both models. We find that learning can have counterintuitive, but logical and understandable effects that differ with the version of the model assessed. In general, population divergence is expected to increase most consistently when females learn to strengthen their preferences for conspecifics from previous encounters with conspecifics. Our results also suggest that within-generational learning will generally inhibit the spread of alleles strengthening assortative mating.

Precopulatory sexual selection is the association between fitness and traits associated with mate acquisition. Although sexual selection is generally recognized to be a powerful evolutionary force, most investigations are limited to characters belonging to individuals. A broader multilevel perspective acknowledges that individual fitness can be affected by aspects of mating success that are characters of groups, such as families. Parental mating success in polygynous or polyandrous human societies may exemplify traits under group-level sexual selection. Using fitness measures that account for age-structure, I measure multilevel selection for mate number over 55 years in a human population with declining rates of polygyny. Sexual selection had three components: individual-level selection for ever-mating (whether an individual mated) and individual- and family-level selection for polyandry and polygyny. Family- and individual-level selection for polygyny was equally strong, three times stronger than family-level selection for polyandry and more than an order of magnitude stronger than individual-level selection for polyandry. However, individual-level selection for polyandry and polygyny was more effective at explaining relative fitness variance than family-level selection. Selection for ever-mating was the most important source of sexual selection for fitness; variation for ever-mating explained 23% of relative fitness variance.

The breeding system of social organisms affects many important aspects of social life. Some species vary greatly in the number of breeders per group, but the mechanisms and selective pressures contributing to the maintenance of this polymorphism in social structure remain poorly understood. Here, we take advantage of a genetic dataset that spans 15 years to investigate the dynamics of colony queen number within a socially polymorphic ant species. Our study population of Formica selysi has single- and multiple-queen colonies. We found that the social structure of this species is somewhat flexible: on average, each year 3.2% of the single-queen colonies became polygynous, and conversely 1.4% of the multiple-queen colonies became monogynous. The annualized queen replacement rates were 10.3% and 11.9% for single- and multiple-queen colonies, respectively. New queens were often but not always related to previous colony members. At the population level, the social polymorphism appeared stable. There was no genetic differentiation between single- and multiple-queen colonies at eight microsatellite loci, suggesting ongoing gene flow between social forms. Overall, the regular and bidirectional changes in queen number indicate that social structure is a labile trait in F. selysi, with neither form being favored within a time-frame of 15 years.

Avian plumage colors are frequently used in studies of sexual selection, yet surprisingly little is known about how these traits evolve under different mating systems. We compared historical rates of divergence in male color patterns among the oropendolas and caciques (genera Cacicus, Gymnostinops, Ocyalus, and Psarocolius), a group with both polygynous and monogamous representatives. Reconstructing the evolution of individual color patches on a molecular phylogeny showed that overall color patterns have changed much more rapidly in oropendolas, which comprise two groups that evolved polygyny independently, than in caciques, which are predominantly monogamous. None of these taxa are notably sexually dichromatic, however, suggesting that higher rates of plumage evolution occurred in both sexes rather than just males. Despite high rates of change, color patterns show few examples of convergence among taxa, similar to the lack of homoplasy in male song among oropendolas but in a stark contrast to the repeated convergence in both plumage and song patterns found in a closely related, monogamous clade, the New World orioles (Icterus). Our results support previous suggestions that display traits evolve more rapidly and with less homoplasy in polygynous mating systems, and we provide surprising evidence that these patterns may occur in both sexes.

Jarman (1974) proposed a series of relationships between habitat use, food dispersion, and social behavior and hypothesized a series of evolutionary steps leading to sexual dimorphism in body size through sexual selection in African antelope species. The hypothesis states that sexual size dimorphism evolved in a three-step process. Initially, ancestral monomorphic and monogamous ungulate species occupying closed habitats radiated into open grassland habitats. Polygynous mating systems then rapidly evolved in response to the aggregation of males and females, perhaps in relation to the clumped distribution of food resources in open habitats. Subsequently, size dimorphism evolved in those species occupying open habitats, but not in species that remained in closed habitats or retained monogamy. This hypothesis has played an important role in explaining the origins of sexual dimorphism in mammals. However, the temporal sequence of the events that Jarman proposed has never been demonstrated. Here we use a phylogeny of extant ungulate species, along with maximum-likelihood statistical techniques, to provide a test of Jarman's hypothesis.

We assess nestmate queen relatedness and the genetic similarity of neighboring nests in the polygyne (multiple-queen) social form of the introduced fire ant Solenopsis invicta using both nuclear and mitochondrial markers. We find that estimates of queen relatedness calculated with both types of markers do not differ statistically from zero. Furthermore, there is no significant relationship between the genetic similarity and geographic proximity of nests in each of six study sites. In contrast to these findings, sites show strong mitochondrial, but no nuclear, genetic differ- entiation. Our results suggest that nonnestmate queen recruitment occurs at a high frequency in introduced populations of this species. Moreover, queens within nests seem to represent a random sample of the queens within the site in which they reside. Therefore, kin selection models that rely on the recruitment of only nestmate queens to explain the persistence of polygyny in ants do not apply to polygyne S. invicta in its introduced range.

A positive association between plumage brightness of male birds and the degree of polygyny may be the result of sexual selection. Although most birds have a socially monogamous mating system, recent paternity analyses show that many offspring are fathered by nonmates. Extrapair paternity arises from extrapair copulations which are frequently initiated by females. Not all females will be able to mate with a male of the preferred phenotype, because of the mating decisions of earlier paired females; extrapair copulations may be a means for females to adjust their precopulation mate choice. We use two comparative analyses (standardized linear contrasts and pairwise comparisons between closely related taxa) to test the idea that male plumage brightness is related to extrapair paternity. Brightness of male plumage and sexual dimorphism in brightness were positively associated with high levels of extrapair paternity, even when potentially confounding variables were controlled statistically. This association between male brightness and extrapair paternity was considerably stronger than the association between male brightness and the degree of polygyny. Cuckoldry thus forms an important component of sexual selection in birds.

Several hypotheses have been put forward to explain the adaptive significance of interspecific variation in mating frequencies by eusocial hymenopteran queens. Four of these hypotheses assert that polyandry is advantageous to queens because of the resultant increase in genetic variability within colonies (referred to as the "GV" hypotheses). Here we compare the frequency of polyandry between monogynous (single-queen) and polygynous (multiple-queen) ant species to test the hypotheses that (1) multiple mating functions primarily to increase intracolonial genetic variability, and (2) mating has costs (such as increased energetic losses or risk of predation or venereal disease). If one of the GV hypotheses is true and mating is costly, the frequency of polyandry should be lower in polygynous species (in which the presence of multiple queens results in low relatedness among workers) than in monogynous species. As predicted by the GV hypotheses, polyandry is less common among polygynous than among monogynous species. Furthermore, it seems that the causal relationship underlying this association is that the number of matings by queens depends on the number of queens present in the colony (rather than the number of queens being influenced by the number of matings), which also supports the GV hypotheses together with the assumption that mating has costs.

We consider worker-controlled sex investments in eusocial Hymenoptera (ants in particular) and assume that relatedness asymmetry is variable among colonies and that workers are able to assess the relatedness asymmetry in their own colony. We predict that such "assessing" workers should maximize their inclusive fitness by specializing in the production of the sex to which they are relatively most related, i.e., colonies whose workers have a relatedness asymmetry below the population average should specialize in males, whereas colonies whose workers have a higher than average relatedness asymmetry should specialize in making females. Our argument yields the expectation that colony sex ratios will be bimodally distributed in ant populations where relatedness asymmetry is variable owing to multiple mating, worker reproduction, and/or polygyny. No such bimodality is expected, however, in ant species where relatedness asymmetry is known to be constant, or in cases where relatedness asymmetry is supposed to be irrelevant due to allospecific brood rearing under queen control, as in the slave-making ants. Comparative data on colony sex ratios in ants are reviewed to test the predictions. The data partly support our contentions, but are as yet insufficient to be considered as decisive evidence.

Extrapair paternity (EPP) can dramatically increase the opportunity for sexual selection if relatively few males are able to monopolize the majority of fertilizations in a population. Although recent work with birds suggests that EPP can increase the standardized variance in male reproductive success (Is) as much as 13-fold, only a male's within-pair success is typically quantified with any accuracy. In most cases, nearly half of all extrapair young are of unknown parentage. A strong, negative correlation across studies between the proportion of extrapair young for which parentage is known and the apparent effect of EPP on Is (r(s) = -0.71, P = 0.013, N = 13 studies) suggests that the incomplete sampling of extrapair sires has greatly exaggerated the influence of EPP. To achieve a more thorough accounting of EPP and its importance to variation in male fitness, we used a suite of four to six microsatellite loci to identify extrapair young and their sires in a polygynous population of Savannah sparrows (Passerculus sandwichensis). Pooling over the 2002 and 2003 breeding seasons, 79 of 116 females (68.1%) produced young outside of the pairbond and 194 of 411 offspring (47.2%) were extrapair. We identified sires for 96.4% of all young (N = 396), including sires for 92.3% of the extrapair young (N = 179), allowing us to partition Is into within-pair and extrapair components. In both years, EPP-related fitness components generated more variation in male reproductive success than the number or quality of within-pair mates. Differences among males in the number of extrapair mates alone accounted for 56.6% of Is in 2002 and for 23.6% of Is in 2003. Nonetheless, in absolute terms, the occurrence of EPP on Kent Island increased the opportunity for sexual selection less than two-fold. Averaging over the two years, Is was only 78% higher than Is,app, the variance in male reproductive success that would have occurred had EPP been nonexistent and males sired all young on their territories. Likewise, across nine socially monogamous species, we found no correlation between the extent of EPP and its effect on the opportunity for sexual selection (Is/Is,app) and only a marginally significant positive correlation between EPP and Is itself. Taken together, our results suggest that the relationship between EPP and sexual selection in birds may be much less strong and much less straightforward than commonly thought.

We investigated sex allocation in a Mediterranean population of the facultatively polygynous (multiple queen per colony) ant Pheidole pallidula. This species shows a strong split sex ratio, with most colonies producing almost exclusively a single-sex brood. Our genetic (microsatellite) analyses reveal that P. pallidula has an unusual breeding system, with colonies being headed by a single or a few unrelated queens. As expected in such a breeding system, our results show no variation in relatedness asymmetry between monogynous (single queen per colony) and polygynous colonies. Nevertheless, sex allocation was tightly associated with the breeding structure, with monogynous colonies producing a male-biased brood and polygynous colonies almost only females. In addition, sex allocation was closely correlated with colony total sexual productivity. Overall, our data show that when colonies become more productive (and presumably larger) they shift from monogyny to polygyny and from male production to female production, a pattern that has never been reported in social insects.

Although mammalian mating systems are classically characterized in terms of male competition and polygyny, it is becoming increasingly apparent that alternative male strategies and female choice may play important roles. For example, females who mate with males from a dominant dynasty risk producing inbred offspring. Many pinnipeds are highly polygynous, but in some species alternative male strategies such as aquatic mating appear to be important, even when behavioral observations suggest strong polygyny. Here, we analyze male reproductive success in the Antarctic fur seal Arctocephalus gazella, an otariid described behaviorally as being highly polygynous, by combining a microsatellite paternity analysis spanning seven consecutive breeding seasons with detailed behavioral data on both sexes. Territorial males fathered 59% of 660 pups analyzed from our study colony. Male reproductive skew was considerable, with a quarter of all paternities assigned to just 12 top individuals on a beach where mean annual pup production was 635. Most males were successful for only a single season, but those able to return over successive years enjoyed rapidly increasing success with each additional season of tenure. We found no evidence of alternative male reproductive tactics such as aquatic or sneaky terrestrial mating. However, paternity was strongly influenced by maternal status. Females observed on the beach without a pup were significantly less likely to conceive to a sampled territorial male than equivalent females that did pup. In addition, their pups carried combinations of paternal alleles that were less likely to be found on the study beach and exhibited lower levels of shared paternity. Thus, from a territorial male's perspective, not all females offer equal opportunities for fertilization.

The evolution of a quantitative genetic trait under stabilizing viability selection and sexual selection is modeled for a polygynous species in which female mating preferences are acquired by sexual imprinting on the parents and by exposure to the surviving population at large. Stabilizing viability selection acts equally on both sexes in the case of a sexually monomorphic trait and on males only in the case of a dimorphic trait. A genetically fixed sensory or perceptual bias defines the origin of the scale on which the trait is measured, and the possibility is incorporated that female preferences may deviate asymmetrically from the familiar-either toward or away from this origin. When viability selection is strong relative to sexual selection, the models predict that the mean trait value will evolve to the viability optimum. With intermediate ratios of the strength of viability to sexual selection, a stable equilibrium can occur on either side of this viability optimum, depending on the direction of asymmetry in female preferences. When viability selection is relatively weak and certain other conditions are also satisfied, runaway selection is predicted.

The parasite pressure exerted by the slavemaker ant Protomognathus americanus on its host species Leptothorax longispinosus was analyzed demographically and genetically. The origin of slaves found in colonies of the obligate slavemaker was examined with nuclear and mitochondrial DNA markers to make inferences about the frequency and severity of slave raids. Relatedness of enslaved L. longispinosus workers in the same nest was very low, and our data suggest that, on average, each slavemaker nest raids six host colonies per season. Therefore, the influence of slavemaker species on their hosts is much stronger than simple numerical ratios suggest. We also found that slave relatedness was higher in small than in large slavemaker nests; thus, larger nests wield a much stronger influence on the host. We estimated that in the study population, on average, a host nest has a 50% chance of being attacked by a slavemaker colony per year. Free-living Leptothorax colonies in the vicinity of slavemaker nests did not represent the source of slaves working in P. americanus colonies, which suggests that raided nests either do not survive or migrate after being raided. Colony composition and intranest relatedness of free-living L. longispinosus colonies differed markedly between areas with slavemakers and those that are parasite-free. In the presence of slavemakers, host colonies were less likely to be polygynous and had fewer workers and a higher relatedness among worker brood. Host nests with slavemaker neighbors allocated more resources into sexuals, possibly caused by these shifts in nest demography. Finally, enslaved Leptothorax workers in P. americanus nests appeared to be less efficient than their counterparts in free-living colonies. Thus, slavemakers exert a much stronger impact on their hosts than had previously been suspected and represent an unique system to study parasite-host coevolution.

I present a model demonstrating that, in social Hymenoptera, split sex allocation can influence the evolution of reproductive partitioning (skew). In a facultatively polygynous population (with one to several queens per colony), workers vary in their relative relatedness to females (relatedness asymmetry). Split sex-ratio theory predicts that workers in monogynous (single-queen) colonies should concentrate on female production, as their relatedness asymmetry is relatively high, whereas workers in the polygynous colonies should concentrate on male production, as their relatedness asymmetry is relatively low. By contrast, queens in all colonies value males more highly per capita than they value females, because the worker-controlled population sex ratio is too female-biased from the queens' standpoint. Consider a polygynous colony in a facultatively polygynous population of perennial, social Hymenoptera with split sex ratios. A mutant queen achieving reproductive monopoly would gain from increasing her share of offspring but, because the workers would assess her colony as monogynous, would lose from the workers rearing a greater proportion of less-valuable females from the colony's brood. This sets an upper limit on skew. Therefore, in social Hymenoptera, skew evolution is potentially affected by queen-worker conflict over sex allocation.

We used microsatellites to study the fine-scale genetic structure of a highly polygynous and largely uni-colonial population of the ant Formica paralugubris. Genetic data indicate that long-distance gene flow between established nests is limited and new queens are primarily recruited from within their natal nest. Most matings occur between nestmates and are random at this level. In the center of the study area, budding and permanent connections between nests result in strong population viscosity, with close nests being more similar generically than distant nests. In contrast, nests located outside of this supercolony show no isolation by distance, suggesting that they have been initiated by queens that participated in mating flights rather than by budding from nearby nests in our sample population. Recruitment of nestmates as new reproductive individuals and population viscosity in the supercolony increase genetic differentiation between nests. This in turn inflates relatedness estimates among worker nestmates (r = 0.17) above what is due to close pedigree links. Local spatial genetic differentiation may favor the maintenance of altruism when workers raise queens that will disperse on foot and compete with less related queens from neighboring nests or disperse on the wing and compete with unrelated queens.

For social insect species, intraspecific variation in colony social structure provides an opportunity to relate the evolution of social behavior to ecological factors. The species Myrmica punctiventris is a cavity-dwelling forest ant that exhibits very different colony structures in two populations in the northeastern United States. Combined data from seasonal censuses, allozyme electrophoresis, and worker hostility tests showed that a population of M. punctiventris in Vermont was strictly monogynous and seasonally polydomous. The same procedures showed that a population of M. punctiventris in New York was facultatively polygynous and predominantly monodomous. Genetic relatedness among colony-mates was not different from Hamilton's expected values in the Vermont population and was consistent with little exchange of ants between colonies and single-mating of queens. In contrast, relatedness was lower in New York, and examination of nest-mate genotypes revealed exchange of ants between colonies, high rates of colony loss and replacement of queens, or multiple-mating of queens. The genetic structure of the Vermont population was consistent with no inbreeding, but in New York, the population genetic structure reflected microgeographic subdivision and inbreeding. Previous study of the ant communities at these sites implicates nest-site limitation in New York as a primary constraint on social structure.

The effective population size (Ne ) depends strongly on mating system and generation time. These two factors interact such that, under many circumstances, Ne is close to N/2, where N is the number of adults. This is shown to be the case for both simple and highly polygynous mating systems. The random union of gametes (RUG) and monogamy are two simple systems previously used in estimating Ne , and here a third, lottery polygyny, is added. Lottery polygyny, in which all males compete equally for females, results in a lower Ne than either RUG or monogamy! Given nonoverlapping generations the reduction is 33% for autosomal loci and 25% for sex-linked loci. The highly polygynous mating systems, harem polygyny and dominance polygyny, can give very low values of Ne /N when the generation time (T) is short. However, as T is lengthened, Ne approaches N/2. The influence of a biased sex ratio depends on the mating system and, in general, is not symmetrical. Biases can occur because of sex differences in either survival or recruitment of adults, and the potential for a sex-ratio bias to change Ne is much reduced given a survival bias. The number of juveniles present also has some influence: as the maturation time is lengthened, Ne increases.

Male diploidy was studied in natural populations of the fire ant, Solenopsis invicta, in order to find an explanation from population genetics for the apparently common occurrence of this phenomenon in some North American populations. The association of male diploidy with polygyne (more than one queen per colony) populations in this species led us to expect that the two phenomena are causally linked. We proposed three hypotheses, based on current knowledge of sex-determining mechanisms in the Hymenoptera, to explain the loss of genetic diversity associated with high rates of diploid male production in S. invicta: a) allelic diversity was reduced during colonization of North America by a small founder group; b) allelic diversity was reduced during subsequent subfounding of the several polygyne populations; and c) genetic structuring of polygyne populations due to local inbreeding caused reduced allelic diversity and/or increased homozygosity. An extensive survey revealed that diploid males are common in all four polygyne (P) populations studied, and that none occur in the monogyne (M) population in north-central Georgia. On the other hand, newly mated (i.e., colony-founding) queens from this same M population produced diploid males in the laboratory, and in the Georgia M and P populations, the frequencies of queens that produce diploid males were shown to be similar. We conclude that the frequent production of diploid males in S. invicta has resulted from a loss of allelic diversity during colonization of North America rather than from any special genetic attribute of P populations. We found no evidence for genetic structuring of the Georgia P population or for decreased allelic diversity relative to the Georgia M population. Thus, the exclusive occurrence of diploid males in P populations does not reflect a fundamental difference between P and M populations in the genetic determinants of male diploidy but is, rather, the result of differential mortality of diploid-male-producing colonies of the two forms. In view of these results, the common occurrence of male diploidy in the P form of S. invicta cannot be taken as evidence of a role for kin selection in the evolution of polygyny. (In this paper, "polygyny" is used to refer to a multiple-queen society.) Studies of the progeny of single queens with allozyme markers demonstrated that diploid male S. invicta are produced biparentally, as in other hymenopteran species. Diploid males were found to take part in the normal summer mating flights in the Georgia P population, although the significance of this behavior is unknown. Males sampled from two P populations exhibited bimodal size distributions, with the diploid males consistently larger than haploids. Assuming a single locus sex determinant, we estimate 15 alleles at this locus for both forms of the ant in North America.

Avian mating systems and their evolution continue to be the subject of attention in behavioral and ecological literature (e.g., Wiley, 1974; Wilson, 1975; Wittenberger, 1976; Emlen and Oring, 1977). We define polygyny, the subject of this paper, as the simultaneous possession of more than one mate by a male; systems in which pair bonds are successive or in which no male-female bond is formed and the sexes meet only for copulation are excluded from our definition. Lack (1968) concluded that polygyny is the mating system of only 2% of the world's bird species; Verner and Willson (1969) found that 14 of 317 North American passerines are polygynous, although recent investigations have increased that number slightly (see below). A model for the evolution of polygyny in birds was suggested by Verner (1964) and Verner and Willson (1966) and was developed into a general theory for the evolution of avian and mammalian mating systems by Orians (1969). This model has been widely accepted (e.g., Emlen, 1973; Brown, 1975; Wilson, 1975), elaborated on (Wittenberger, 1976; Altmann et al., 1977), and incorporated into more comprehensive theories about mating systems (Emlen and Oring, 1977). However, acceptance of it has not been universal (Wiley, 1974). The purpose of this study was to investigate the Verner-Wilson-Orians (hereafter VWO) model in two ways. Primarily, the objective was to test the model's ability to predict polygyny in a population of an altricial bird species whose mating system was unknown. Reasons for selecting the indigo bunting (Passerina cyanea) as the subject are presented below. Secondly, data were collected on the breeding biology and population dynamics of the indigo bunting in order to learn whether these would conform to predictions of the model. The study was conducted from 1973 through 1976. Preliminary results were reported earlier by Carey and Nolan (1975). The rarity of avian polygyny is usually attributed to the lowered fitness to be anticipated for a female that chooses an already-mated male. A male having two or more altricial broods simultaneously dependent on him will reduce, of necessity, the parental attention given to each. Other disadvantages may arise from density effects, such as attraction of predators to, or depletion of food resources on, a populous territory. However, according to the VWO hypothesis, all such disadvantages may be compensated for if critical resources are distributed sufficiently unequally among male territories and therefore controlled in disparate amounts by males (see Fig. 1). Given a wide enough range in territory quality (i.e., a range exceeding the "polygyny threshold" of Fig. 1), then a female's expected productivity may be greater if she chooses a mated male on a high-quality territory rather than a bachelor on a poor territory. Orians (1969) emphasized two circumstances that may affect the probability of occurrence of polygyny: 1) range in quality of territories and 2) extent to which female productivity is dependent on male aid in parental care. (Other conducive circumstances have been pointed out by various authors-e. g., asynchronous formation of sexual bonds [Emlen and Oring, 1977] and dissolution and reformation of bonds in midseason [Nolan, 1978].) To consider Orians' points, under two conditions a

The Savannah Sparrow (Passerculus sandwichensis) is a widespread and common North American bird that shows both geographic variation and sexual dimorphism in size. I used information from 24 measurements on 1,791 individuals from 51 populations to test two hypotheses (sexual-selection and niche-partitioning) about the evolution of sexual dimorphism. Throughout their range male Savannah Sparrows are larger, on average, than females. This doubtless reflects Darwinian sexual selection, for territorial fights usually involve males, many of whom fail to obtain mates. In some parts of their range, Savannah Sparrows are commonly polygynous, whereas in others they are characteristically monogamous. Among species of American sparrows (subfamily Emberizinae) sexual size dimorphism is generally greater in polygynous species than in monogamous ones. However, I did not find a similar trend among populations of Savannah Sparrows. The amount of dimorphism in all populations of Savannah Sparrows is equivalent in magnitude to that of other species of sparrows that are commonly or regularly polygynous, and it is greater than that of other sparrow species that are characteristically monogamous. The amount of sexual dimorphism, either in overall size or in bill size, does not correlate with species diversity and does not differ between island and mainland populations. These results do not support the niche-variation hypothesis. Size dimorphism is relatively great in populations of Savannah Sparrows that are resident in southwestern salt marshes, and these birds are the only sparrow-like birds that generally breed in these marshes. Dimorphism is, in general, relatively great in marsh-dwelling species in the family Emberizidae. These species are commonly, but not always, polygynous; the mating systems of the salt-marsh Savannah Sparrows are not known. There are no significant differences in the extent of dimorphism among populations of salt-marsh sparrows, and there are few among the non-salt-marsh ones, probably reflecting conservatism in the evolution of size dimorphism.

In most species of birds adult males are larger than adult females. The two major hypotheses proposed to explain this pattern of sexual size dimorphism are appropriately termed the sexual selection and competition avoidance hypotheses. The sexual selection hypothesis states that large size gives males an advantage in intrasexual competition for females or in epigamic displays, activities not engaged in by females. Males will thus be larger than females, and the difference should be greater among polygynous and promiscuous species where there is greater variability in male reproductive success and thus stronger sexual selection (Darwin, 1871; Huxley, 1938; Fisher, 1958). The sexual selection hypothesis, then, explains the relationship between pronounced sexual dimorphism in size and nonmonogamous mating systems in birds (Darwin, 1871; Selander, 1958, 1972; Amadon, 1959; Orians, 1961), a relationship particularly evident among the Icteridae (Selander, 1958). The competition avoidance hypothesis states that sexual size dimorphism is favored because it reduces competition between males and females of the same species (Selander and Giller, 1963; Selander, 1966, 1972; Johnston and Selander, 1973). If size affects the range of resources used (e.g., larger individuals eat larger food items), then those males differing most in size from females would overlap in resource utilization with the fewest individuals and thus have the most resources available. This hypothesis may be especially applicable to monogamous, ter

macrogyna: females and males larger, the largest sexual females with the head dark brown above and orange-brown below the eyes, with longer, more tapering ovarioles, of which the terminal filaments account for only onethird the total length in virgins, eyes less prominent but of the same size, colonies usually monogynous with relatively large aggressive workers, the average worker head-width less than the average female head-width, colony reproduction by dissemination of fertile females which found colonies either alone or in small aggregates; microgynca: females and males smaller, the smallest sexual females with head brown above and yellow-brown below the eyes, with shorter, less tapering ovarioles of which the terminal filaments account for two-fifths of the total length in virgins, eyes more prominent but of the same size, colonies polygynous with relatively small and docile workers, the average worker head-width similar to the average female headwidth, fertile females return to their nest (and possibly other nests) and colonies reproduce by fission.