Precopulatory intimidation by male chimpanzees can only be fully understood in the context of postcopulatory sexual conflict in the form of infanticide. Muller et al. (2009) argue that sexual coercion, particularly as practiced by high-ranking males, is a counterstrategy to limit female promiscuity, and that promiscuity is itself a counterstrategy to male infanticide (see also Stumpf et al. 2008). This scenario highlights the nature of sexually antagonistic coevolution: male infanticide favors female promiscuity, which favors male sexual coercion, etc.
Infanticide figures prominently in Smuts and Smuts' (1993) original discussion of sexual coercion, but it does not fit easily within Clutton-Brock and Parker's (1995) more general harassment-intimidation dichotomy. It is initially difficult to appreciate that male infanticide might constitute incidental harm to females, since an infant's death seems so directly detrimental to the mother's fitness. But this proposition becomes clearer when we consider that the specific "problem" that lactating females pose to reproducing males is a straightforward consequence of primate biology: a nursing infant is, in the words of Altmann et al. (1978: 1029), a "perfect contraceptive." The function of sexually selected infanticide, then, is to disrupt this contraceptive system, not to harm the mother (or reduce her fitness) per se. Thus, following the broader theoretical logic of Partridge and Hurst (1998) and Lessells (2005), if, speculatively, males possessed other means of effectively counteracting the contraceptive - e.g., by manipulating the mother's hormonal state or accelerating weaning - and if the costs of such a strategy did not exceed the costs of infanticide, then males would not be selected to kill infants (but could still achieve the same reproductive benefit). Under such conditions, the death of infant, if it occurred, would be an incidental by-product of the male manipulative strategy, not a necessary harmful component of that strategy. This is not to say that male attacks on infants can not, in principle, function as sexual intimidation, if their mothers' mating compliance forestalls further aggression directed at them. As Clutton-Brock and Parker (1995) point out, however, this mechanism of indirect sexual intimidation predicts that male threats and attacks will also extend to juveniles, which is neither predicted by the sexual selection hypothesis nor is a common correlate of infanticidal behavior (Hrdy 1974; van Schaik and Janson 2000).
Male infanticide is still, however, a drastic form of sexual conflict. It reflects adaptive harm (sensu Johnstone and Keller 2000) insofar as infanticidal males exploit a preexisting feature of female reproductive plasticity, such that infant loss often accelerates resumption of ovulatory cycling. Although the adaptive significance of infanticide in primates continues to be debated, the available evidence is more consistent with the sexual selection argument (Borries et al. 1999; Soltis et al. 2000; van Schaik 2000b) than with competing hypotheses, such as the generalized aggression model (Bartlett et al. 1993) and the social pathology argument (Dolhinow 1977). Thus, infanticide appears a likely manifestation of post-copulatory sexual conflict in primates, as well as, arguably, the most studied form of sexual conflict (Hausfater and Hrdy 1984; van Schaik and Janson 2000).
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