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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.

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.

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.

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.

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 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.

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

Evolution of polygyny in a species whose young require parental care, to which the male can contribute directly, presents certain interesting problems. With each additional mate, the male may have more young to care for simultaneously; less time being devoted to each may influence their survival. Even if polygamy increases the male's total contribution to future generations, as compared to monogamy, the reverse would probably be true for each of the females mated to him. Since it is generally accepted that in birds females select mates from among available males, it seems unlikely that selection would favor those females that bred with males already mated. This has probably led to the general belief that polygamy in avian mating systems is a by-product of an unbalanced sex ratio (cf. Chapman, 1928; Kendeigh, 1941a; Mayr, 1941; Ryves and Ryves, 1934; Skutch, 1935; Williams, 1952), an assumption which is not necessarily true but which has probably led to a stagnation of thought concerning evolution of polygamy in birds. Fisher (1958) and Kolman (1960) indicate that sex ratio at fertilization would be adjusted by selection so that average parental expenditure to rear males equals that to rear females. Willson and Pianka (1963) point out that if sexual selection results in sexual dimorphism expressed during the period of parental care, there may be a change in average expenditure per sex with a consequent change in sex ratio. Thus, mate preference, which in my opinion should have a profound influence on the mating system, may alsohave an influence on the sex ratio. This does not preclude the possibility that sex ratios may