What are the implications of this discussion for universal sex differences and the gap between studies of human and nonhumans? Early evolutionary studies identified predicted sex roles, but failed to consider variation. Nowadays, with more sophisticated theoretical models and richer empirical evidence, we see that the roles of men and women can be highly variable. The Pimbwe study shows how some women, despite common mammalian constraints, can use multiple sequential pair bonds to out-reproduce their monogamous counterparts. Similarly, the broader discussion of the relationship between pair bonds and paternal care reveals limits to the conventional view of marriage as a trade of sex for paternal resources. Men and women have negotiable roles in marriage, for which models from behavioral ecology (beyond conventional parental investment theory) and economics can be brought to bear.
In what sense do we differ from nonhuman primates in this respect? Anthropologists once viewed regularly patterned family behavior (and particularly marriage, e.g., Levi-Strauss 1949) as what divides us from the other primates, contrasting the apparent promiscuity of our apelike cousins to, initially, our nice nineteenth century stable monogamous families, and later the wide range of family types evident in the ethnographic record. However, the gap is nowhere so big as once thought and is better viewed as continuum (e.g., Foley and Lee 1989; Rodseth et al. 1991) or just a crack. This is so in part because over the years so much has been learned about the complexity of nonhuman primate kinship and social behavior. With long-term studies of known individuals, the sophistication of primate kinship behavior is now well appreciated. The narrowing gap also reflects the fact that the human pair bond, with dad provisioning mum in exchange for sex, is now no longer viewed as necessarily the supreme human adaptation that sent us down our distinct evolutionary route. In fact, as I have tried to argue, the contemporary human pair bond is a highly variable trait which, while most likely universal in some form, functions very differently in different social and economic contexts, and has hotly contended evolutionary origins.
Another reason why crack (rather than gap) better characterizes the distinction between sex-differentiated reproductive strategies in human and nonhuman primates is that there are so many parallels between contemporary debates within primatology and human behavioral ecology. I end with a consideration of some of these.
First, nonhuman primates generally exhibit rather low levels of direct paternal care. Males can be important for protection, particularly from infanticide (e.g., Palombit et al. 1997), and males do in some cases provide direct care to their offspring (Buchan et al. 2003), but with a few exceptions (e.g., Goldizen 1987), paternal care is not extensive among primates. Furthermore, as we have seen in humans, there is lengthy debate over whether male activities are designed for improving offspring survival or enhancing access to mates (e.g., Smuts and Gubernick 1992; van Schaik and Paul 1996). In short, as in mammals more generally, male care is not extensive, and where it occurs, its relationship to paternity, and its impact on offspring survival, is debatable (Woodroffe and Vincent 1994). The situation is rather similar in the study of humans, as reviewed in this chapter. The traditional view that paternal care is central to our evolutionary trajectory is now in question, and suggestions that pair bonds originated for mate defense and avoidance of harassment are gaining attention. Humans do, however, look different with respect to the current function of pair bonds: the division of labor over the production and consumption of food among spouses is particularly developed in our species, hence the foray into economics for new theoretical tools (and see Noe et al. 2001, for similar applications of economic theory to nonhuman primates).
Secondly, related to low paternal care is the significance of female-bonded kin groups in nonhuman primates. If males are not helping, who is? In many species, amongst them cercopithecine monkeys (Silk 2007), the fertility and reproductive success of females is heavily influenced by social networks and matrilineally derived access to resources. Such social systems are increasingly being used as a model with which to think about human evolutionary origins and the importance of cooperation among kin in subsidizing the costs of childbearing (Hrdy 2005a, b). This marks a radical departure from seeing our origins in the social organization of the male-bonded apes (e.g., Rodseth et al. 1991).
A final parallel trend lies in the prevalence of multiple mating by females. Female primates commonly mate with multiple males to avoid infanticide, and rather rarely mate for resources or good genes; indeed, preferences for dominant individuals dissolve once the male loses dominance, suggesting that protection is more prominent in this preference than genetic quality (Setchell and Kappeler 2003). Interestingly, human behavioral ecologists turned initially to birds for models for mating systems (Flinn and Low 1986; Borgerhoff Mulder 1990) because of the clear importance of paternal investment in many extent societies. This, however, may have distracted us away from very different (and non paternal care-based) arguments for the origins, if not the current function, of pair bonds in our species.
In sum, there are several parallels between the arguments advanced in this chapter and current debates within primatology that support the claim that we should perhaps be minding a crack rather than a gap.
Acknowledgments Fieldwork was funded by the LSB Leakey Foundation, the Packard Foundation and UC Davis. Thanks to the editors and two reviewers for the helpful comments on the manuscript.
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