Sexual conflict is inevitable and ubiquitous: the question is not whether it occurs, but how and when, and to what degree sexually antagonistic coevolution has acted, compared with other mechanisms of sexual selection (Hosken and Snook 2005: S1, Andersson and Simmons 2006). Sexual conflict theory situates explanations in the "arms race" perspective previously reserved for more conventional coevolutionary adversaries, such as predators and prey (Dawkins and Krebs 1979). The Red Queen hypothesis, that any gain in fitness by one unit of evolution is balanced by equivalent losses in fitness by others (van Valen 1973), may provide the most appropriate framework for analyzing reproductive strategies as a zero-sum game between opposing males and females (Chapman and Partridge 1996; Rice and Holland 1997). This does not mean, however, that conflict universally characterizes the phenotypic expression of male and female interaction. Affiliation and intersexual cooperation may be one outcome of this coevolutionary conflict, as suggested for chacma baboon friendships. Indeed, the chacma baboon and chimpanzee together highlight the view of universals as process, rather than as pattern. Current evidence suggests that sexually selected infanticide has generated two distinct modes of female counterstrategy in these species: promiscuity and association with males. The patterns are different, but the underlying process that generates the patterns is the same: sexual conflict. This chapter has focused mostly on sexual conflict over mating, but it may also occur at the level of sex chromosomes, gamete interaction, parental investment, group size and composition, and group dynamics (Table 3.1).
Sexual coercion via intimidation/punishment is likely to be a common, if not universal feature of life among animals that live gregariously and modify their behavior through learning (Clutton-Brock and Parker 1995). The attention following the publication of the Smuts and Smuts (1993) model has enlarged the data base for male mating aggression to females. Somewhat surprisingly, however, relatively few studies have rigorously tested the full set of constituent predictions (but see Muller and Wrangham 2009) or differentiated analytically between sexual harassment and intimidation. Costs to females are often an assumed rather than measured consequence of overt aggression, or are assessed qualitatively (e.g., as an "injury"). A key goal for future studies is quantitative measurement of these costs (as Muller et al.  do). These data will help address some other questions: do females do worse reproductively when mating with more coercive or persistent males, as predicted by theory? The hypothesis that females may derive benefits from coercion also merits greater study. Likewise, the costs of coercion to males are virtually ignored, but may be significant. For example, the seminal fluids of bushcrickets inhibit receptivity of females to further mating in a manner similar to D. melanogaster, but males that deliver greater quantities of these fluids also experience longer sexual refractory periods themselves (Vahed 2007). Information on costs to males, combined with data addressing covariation in male coercion and fitness, will help to clarify the trade-offs of coercion or manipulation of females versus alternative mating strategies. Most primate studies of sexual conflict have focused on sexual coercion, but male manipulation in the form of antagonistic seduction, and concomitant females resistance (Holland and Rice 1999) merits more attention.
The life history of primates, as well as the practical constraints on an experimental study of them, significantly limit the kinds of data that can be collected. Nevertheless, there are compelling reasons to study sexual conflict in primates. Until fairly recently, much of the research on sexual conflict was conducted on (invertebrate) taxa that conform more or less to the Bateman (1948) principle that males are selected to mate and females not (Partridge and Hurst, 1998, Tregenza et al. 2006). Our understanding of the full significance of sexual conflict will be improved by greater study of systems violating this assumption, i.e., taxa in which remating is potentially beneficial to females. Additionally, as Clutton-Brock and Parker (1995) emphasize, models of sexual conflict have generally focused on relatively simple social contexts. The study of highly social species promises to reveal important and subtle influences of social relationships on the economic trade-offs of sexual coercion and resistance. In spite of the methodological difficulties they pose, primates are excellent subjects to achieve all of these goals.
In summary, conflict among genes is "a universal feature of life" (Burt and Trivers 2006, p 3). This is true not only for genes within a genome, but also for genes residing in the genomes of the interacting entities we call "male" and "female."
Acknowledgments I am extremely grateful to Peter Kappeler and Joan Silk for their generous cooperation and advice in the preparation of the manuscript. Field research was funded by NSF (BCS-0117213), the Leakey Foundation, the Wenner-Gren Foundation and the Center for Human Evolutionary Studies (Rutgers University).
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