The evolution of infanticide: genetic benefits of extreme nepotism and spite

Abstract

Among patterns of naturally occurring infanticide, one of the most interesting is the killing of young by lactating adult females in several mammalian species. In different species, the litters of either distantly related mothers or of close relatives have been documented as victims. We examined possible genetic benefits of infanticide by adult females using models of inclusive fitness and coancestry. Infanticide is viewed as a form of nepotism, in which offspring that are closer relatives of the infanticidal marauder have a greater chance of recruiting into a breeding group following infanticide. In this context, the genetic relationship of marauders and the young they kill is based upon the competitive social environment within breeding groups of adults. Two models were developed, based on linear and curvilinear changes in the fitness of mothers that either protect their litters or leave their litters unprotected while they maraud the litters of other adult females. Forms of these models that describe genetic polymorphisms of infanticide and ESS (evolutionary stable strategy) frequencies of infanticide yield quantitatively similar results. The models suggest that a balance should occur within populations between marauding and protection of young. At equilibrium, infanticide appears to be a spiteful behavior because it lowers the reproductive success of both the marauder (due to leaving their litters unattended and thus subject to other marauders) and mothers that try to protect their litters (because at least some maraudings are successful). Further, high levels of coancestry among females in a social breeding group should not change the frequency of infanticide when the population is at the equilibrium frequencies of marauding and protecting. Thus, in breeding groups made up of matrilines of highly philopatric females, infanticide of young genetically related to the marauder might be expected. Conclusions drawn from the models are supported by patterns of infanticide in ground-dwelling squirrels, where extensive information on infanticide exists and the social structure of breeding groups varies among species.