Infanticide by Males and Its Implications (review)

Infanticide by Males and Its Implications, edited by C.P. van Schaik and C.H. Janson. Cambridge, UK: Cambridge University Press, 2000. 584 pp. $43.60 (paperback); $116.50 (hardback).

The claim that infanticide is part of evolved male reproductive strategies has probably engendered more heated argument than any other proposal about behavioral evolution in primates, as Sommer reminds us in the opening chapter of this volume. Yet, as the contributors show, the sexual selection hypothesis for the evolution of infanticide by males enjoys widespread empirical support, and infanticide has been fundamentally important for social evolution in primates, some other mammals (notably carnivores and rodents), and birds. The volume contains several review chapters that summarize the supporting data and several case studies. Importantly, many chapters present comparative analyses that seek to explain regularities and variation in the presence, risk, and rate of infanticide and in female counter-strategies. These lead to conceptual advances, and raise many issues that warrant further study, such as whether the length of female periods of sexual receptivity (van Noordwijk and van Schaik; van Schaik, Hodges, and Nunn) or the prevalence of female dispersal (Sterck and Korstejns) vary systematically with infanticide risk.

The sexual selection hypothesis holds that infanticide is advantageous to perpetrators (so long as they do not kill their own infants) when it reduces interbirth intervals and increases their chances of siring the females' next infants. It predicts that female reproductive biology and behavior show evidence of evolved counter-strategies—for example, that infanticide risk leads to extended estrus period, to mating with multiple males, and to reliance on "protector" males who have high paternity probabilities. Van Schaik (chapter 2) reviews the range of conditions under which infanticide would increase male fitness and concludes that these generally hold for wild nonhuman primates in which infanticide has been documented. He argues (chapter 3) that the ratio of lactation to gestation is the most important life history variable affecting infanticide risk in mammals generally. Ratios of greater than one preclude postpartum estrus, and the higher the ratio, the greater the probability that infanticide hastens a female's return to estrus; this helps to explain the relatively high prevalence of infanticide in primates, sciurognath rodents, carnivores, and perhaps cetaceans.

Van Noordwijk and van Schaik (chapter 14) examine the implications of infanticide risk for female reproductive behavior. In particular, female primates and carnivores in "vulnerable" species are especially likely to mate with multiple males and to engage in postconception mating, tactics that could confuse paternity, although rodents do not show the same pattern. Van Schaik, Hodges, and Nunn (chapter 15) also examine possible female counterstrategies in primates and find mixed support for the hypotheses that the length of the follicular phase of female cycles increases, hence, female ability to confuse paternity increases, in association with the potential for male sexual coercion and with the mean number of males per group.

Janson and van Schaik (chapter 19) summarize data on infanticide rates in primates, and reinforce arguments from other chapters that females have multiple counterstrategies, some of which may act in opposing directions (e.g., confusing paternity versus gaining protection from powerful individual males). Their comparative analysis shows that the replacement rate for breeding males (positive effect) and the number of males per group (negative effect) independently influence infanticide risk. Sterck and Korstejns (chapter 13) conclude that female dispersal sometimes functions partly as an infanticide avoidance tactic, and Steenbeeck (chapter 7) gives an excellent review of long-term data on Thomas langurs (Presbytis thomasi) that support this proposal and show that females often desert "late tenure" males whose protective abilities are waning. However, female dispersal and infanticide risk are not obviously systematically related across taxa (Sterck and Korstejns; Janson and van Schaik).

Crockett and Janson (chapter 4) use long-term data on demography and infanticide in a population of red howler monkeys to argue that infanticide risk can sometimes limit group size in primates: infanticide rate increases with the number of females per...