SELECTION WITHIN AND BETWEEN SOCIAL-GROUPS FOR INFANTICIDE

In a wide range of group-living mammalian species, immigrant adult males have been observed to kill unweaned or otherwise maternally dependent offspring. In fact, previous models have demonstrated that adult males obtain a reproductive advantage through this behavior, referred to as infanticide, relative to their non-infanticidal competitors. Infanticide is beneficial because, following the death of their infants, adult females of many mammalian species resume sexual cycles immediately or produce larger-than-usual litters during their next period of reproduction, or both. Thus, not surprisingly, most previous models have concluded that, once introduced into a population, alleles associated with infanticide by males spread within that population. However, the present model takes into account both the effects that infanticide has on the fitness of adult males and the effect that infanticide has on the other members of groups into which those males emigrate. Analysis of this model has shown a variety of conditions under which alleles favoring infanticide increase within groups, yet decrease within the population (and hence species) as a whole. Application of the model to black-tailed prairie dogs, Hanuman langurs, and lions suggests that, for all three species, the between-group components of selection are strong enough to override within-group components only when local population size is small relative to the species' norm. Nevertheless, the present model emphasizes that studies of the evolution of complex social behaviors demand relatively precise information on the migration structure of populations as well as on how the within-group frequencies of specific behavioral traits co-vary with fitness.