Infanticide as sexual conflict: coevolution of male strategies and female counterstrategies

Abstract

One of the earliest recognized forms of sexual conflict was infanticide by males, which imposes serious costs on female reproductive success. Here I review two bodies of evidence addressing coevolved strategies of males and females. The original sexual selection hypothesis arguing that infanticide improves male mating success by accelerating the return of females to fertilizable condition has been generally supported in some taxa--notably, some primates, carnivores, rodents, and cetaceans--but not in other taxa. One result of recent research has been to implicate other selective benefits of infanticide by males in various taxa from insects to birds to mammals, such as acquisition of breeding status or improvement of the female breeding condition. In some cases, however, the adaptive significance of male infanticide remains obscure. The second body of data I review is arguably the most important result of recent research: clarifying the possible female counterstrategies to infanticide. These potential counterstrategies span diverse biological systems, ranging from sexual behavior (e.g., polyandrous mating), to physiology (e.g., the Bruce effect), to individual behavior (e.g., maternal aggression), to social strategies (e.g., association with coalitionary defenders of either sex). Although much remains to be studied, these current data provide compelling evidence of sexually antagonistic coevolution surrounding the phenomenon of infanticide.

Figure 1.

Mean lactation/gestation (L/G) ratios for selected mammalian groups. Log-transformed L/G ratios significantly different from 1 are indicted by *(p < 0.05) or †(p < 0.01). The letter “I” denotes taxa in which deliberate sexually selected infanticide by males has been recorded. Sample sizes (number of species) are indicated in parentheses. (Modified from data in van Schaik 2000b.)