Infanticide, the killing of infants by other members of the same species, is a common phenomenon in wild animals. Adult males often kill unrelated infants and then mate with the female, whereas adult females kill strange pups to remove competition for their offspring and to acquire the nest of the victim female. Little is known about the conditions under which infanticide occurs, the mechanisms males and females use to distinguish their own from unrelated offspring, how infanticide affects population growth, and whether or not the tendency to commit infanticide is inherited. In this proposed study the principal investigator will determine (1) what cues males and females use to recognize their own offspring, and thus avoid killing related pups, (2) if there is genetic basis for the tendency to commit infanticide, and (3) how population growth and stability are affected by the presence of infanticidal and noninfanticidal animals. The frequency of infanticide in a marked, wild population of white- footed mice at high and low densities will be monitored, and breeding experiments will be conducted in the laboratory to determine the inheritance of infanticidal behavior. Previous studies by the Principal Investigator have shown that about 50% of the animals are infanticidal and 50% are noninfanticidal. Infanticidal and noninfanticidal animals will be introduced to different enclosures to determine the effects of infanticide on social interactions and population growth. The results will show how a seemingly pathological behavior can actually be adaptive and spread through a population. It will also shed light on the significance of female aggression as a mechanism to protect her pups from other members of her own species.