There are three general approaches to studying sexual conflict. The first method is exemplified by the now classic study of seminal proteins in the fruit fly (Drosophila melanogaster) (Rice 1996; Holland and Rice 1999). These proteins originate in accessory glands, are transferred (with sperm) to female mates, and influence females in a number of ways that benefit males, such as: (1) increasing the rate of female egg-laying (Chen 1984); (2) decreasing female receptivity to additional matings (Ravi Ram and Wolfner 2007); and (3) improving sperm competition by displacing the sperm of previous copulators (Harshman and Prout 1994; Clark et al. 1995). Seminal fluids are apparently toxic, such that prolonged exposure to them elevates female mortality (Chapman et al. 1995; Clark et al. 1995; Lung et al. 2002). In order to test the prediction that monogamous mating systems engender less sexual conflict than polygynous systems, Holland and Rice (1999) randomly assigned individual D. melanogaster to one of two population treatments: imposed monogamy versus the (control) polygynous ancestral condition. After 47 generations, the monogamous lineage was characterized by lower toxicity of male seminal fluids and lower female resistance to seminal fluids (see also Rice et al. 2005). These data exemplify a key (though not inevitable) corollary of interlocus sexual conflict: sexually antagonistic coevolution. This historical approach, tracking changes over evolutionary time, can provide particularly compelling evidence of sexual conflict and sexually antagonistic coevolution, but it is feasible primarily with relatively short-lived animals that can be manipulated in the laboratory.
A second approach, based on quantitative genetics, defines sexual conflict as negative covariance between the sexes in fitness, particularly over generations (Rice and Chippindale 2001; Shuster and Wade 2003; Pizzari and Snook 2003, 2004). For example, red deer (Cervus elaphus) stags with greater lifetime reproductive success sired less successful daughters and more successful sons than stags with lower lifetime fitness (Foerster et al. 2007). The negative correlation between the fitness of males and females suggests opposing optimal genotypes for males and females, i.e., sexually antagonistic coevolution. Again, this method is impractical for primates because we know relatively little about lifetime reproductive success, particularly for males, and even less about the selection coefficients and heritability of characters related to fitness.
A third approach considers how certain behavioral, anatomical, or physiological aspects of reproductive strategies among members of one sex impose costs on the other sex, and how phenotypic features of the second sex may function to mitigate those costs (as coevolutionary counterstrategies). The relevant data are collected over relatively short time periods, rarely long enough to demonstrate the effects of sexual conflict on the lifetime reproductive success of individuals. These kinds of analyses can reveal the extent and form of sexual conflict, but they can only indirectly imply the action of sexually antagonistic coevolution. This approach is the only one that is now tractable for studies of nonhuman primates.
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