Models predicated on the differential postzygotic investment of males and females (Trivers 1972) have dominated the study of sexual and reproductive strategies in most mammals, and provided a theoretical context for the classic finding that males benefit more from multiple matings than do females. Key to this discussion has been the regression of reproductive success on mating success, known as "Bateman's gradient (Bateman 1948). Whichever sex has the steepest gradient is the sex that experiences the stronger sexual selection pressure on traits that enhance mating success (Andersson and Iwasa 1996).
In mammals, gestation and lactation fall exclusively to females, paternity certainty is never assured, and paternal care is provided facultatively. Therefore male fitness is seen as limited by competition over mates, and female fitness by access to resources that can in some but not all cases, be acquired through males (Emlen and Oring 1977; Wrangham 1980). Thus, the reproductive strategies of each sex, in particular decisions over mating effort and parenting effort, are analyzed as a product of sex differences in parental investment. Trivers' model (in an expanded form that deals more explicitly with the operational sex ratio and potential reproductive rates, Clutton-Brock and Vincent 1991) can in fact predict much of the variation in sexual selection across taxa and has important implications for sex roles. As noted earlier, its successes in predicting human sex differences in reproductive strategies brought prominence to the new discipline of evolutionary social science (Borgerhoff Mulder et al. 1997).
In the intervening years, theoretical and empirical work in behavioral ecology has taken a richer and more dynamic approach to sex roles (reviewed in Borgerhoff Mulder 2009a). First, there has been a rethinking of the internal logic and consistency of Trivers', and specifically Maynard Smith's (1977) model (Queller 1997; Houston and McNamara 2005; Kokko et al. 2006). These revisions do not change the basic prediction that the caring sex is more likely to be choosier and the object of more competition, but fundamentally alters the evolutionary sequence. In the conventional sequence, differences in prezygotic investment determine potential reproductive rates which then shape payoffs to postzygotic care. In the revised sequence, prezygotic investment generates the conditions for sexual selection as numerically abundant male gametes compete for access to rare female gametes. This lowers the confidence of males in paternity and, given male-male competition for access to females (and/or female choice), creates an elite subset of males that are more eligible to mate (Kokko and Jennions 2003). This revised logic gives more salience to sex differences in competition over mates and less to sex differences in parental care.
Second, and independent of these revisions, both theoretical and empirical work shows that anisogamy does not always produce classic sex roles (Gowaty 2004) and that competition and choice are not mutually exclusive (Kokko et al. 2006), as indeed long recognized in empirical studies of nonhuman primates (Hrdy 1986). In other words, choosiness is not simply a function of operational sex ratios, with the limiting sex enjoying the luxury of choice; it is also dependent on variance in quality among potential mates (Owens and Thompson 1994; Johnstone et al. 1996), the costs of reproduction (Kokko and Monaghan 2001; Maness and Anderson 2007), and extrinsic survival rates (Gowaty and Hubbell 2005).
Third, there is evidence that there are some species in which females are the principal caregivers, but compete more frequently and more intensively with each other than do males. In meerkats (Suricata suricatta, Clutton-Brock et al. 2006) and many other cooperatively breeding vertebrates (Holekamp et al. 1996; Hauber and Lacey 2005), females gain greater reproductive benefits from dominance than do males (e.g., Engh et al. 2002, for spotted hyenas, Crocuta crocuta), and accordingly are more competitive with one another, thereby demonstrating that sex differences in parental investment are not the only mechanism capable of generating sex differences in reproductive competition. Finally in some species, notably cooperative breeders with single breeding pairs, sex differences in fitness variances are unrelated to differences in mate number, thus providing evidence that counters Bateman's gradient (the idea that males benefit more from multiple mates than do females, Hauber and Lacey 2005). Higher female than male variance in fitness is also observed in sex role-reversed species such as dusky pipefish, Syngnathus floridae (Jones et al. 2000) and wattled jacanas (Jacana jacana) (Emlen and
Wrege 2004). Recognition of these additional selective considerations generates a much richer picture of how competition and choice can figure in the strategy of each sex and how these may vary over the life time and across populations.
In short, contemporary perspectives within behavioral ecology provide a broader framework within which to study the great diversity of sex differences in nature than that afforded by the simple parental investment model that guided seminal work in the evolutionary social sciences until the late 1990s.
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