Infanticide, the killing of conspecific young, has been documented in numerous species of mammals and is considered an adaptive behavioral strategy to enhance the reproductive success of the perpetrator. The potential benefits of committing infanticide for males are obtaining nutritional gain and mating partners, and for females are acquiring access to resources such as food and nest sites. Some costs are associated with committing infanticide such as additional energy expenditure, risk of injury, and exposure to predation. However, the major costs associated with infanticide are borne by the victim Female and the sire male in loss of fitness. In response to this selection, males and females use a variety of counter-strategies to protect their young from infanticide. We summarize the published accounts and theory associated with infanticide and the occurrence of counter-strategies in a variety of mammalian groups in order to explain how infanticide may influence individual behavior as well as the social systems of mammals. We focus on the behavioral strategies used, primarily by females, to deter major losses in reproductive success. These strategies include aggression, female choice of dominant males, and promiscuity to confuse paternity as defense against males, and territoriality, association with kin, reproductive suppression, and reproductive synchrony as defenses against females. Male counter-strategies are less well known, but intrasexual territoriality may in part function as defense against infanticide. The costs associated with the different male and female counter-strategies are likely to vary, but may include increased energy expenditure, exposure to predators and injury for both sexes, as well as increased competition for resources, limited mate choice, and postponed reproduction for females. We propose that the occurrence of infanticide does not only have the potential to affect the behavior of individuals (e.g. aggression. spacing and mate choice), but may also have consequences for the shaping of mammalian mating systems.
