Infanticide Versus Adoption: An Intergenerational Conflict

Considerable attention has been paid to the phenomenon of infanticide in recent years. Five functional categories of infanticide have been defined. Here I concentrate on those that either have been described as the outcome of possible competition for limited resources or have by default been classified as the result of social pathology. Many of the species that show infanticide of this nature also show adoption of unrelated young at fairly high frequencies. I suggest that the possibility of caring for nonfilial offspring creates an intergenerational conflict, or arms race, whereby offspring separated from their parents or receiving parental care of substandard quality (insufficient for their survival) should be selected to solicit care from adults other than their parents and the potential adoptive parents are selected to avoid giving such care. Evidence suggests that most examples of supposedly pathological infanticide, or resource-based infanticide, are the result of potential foster parents killing unrelated offspring when these offspring can clearly be identified as nonrelatives. Support for this idea comes from observations that (1) such infanticide is most common in group-living or colonial species, where chances of encountering wandering offspring are high; (2) infanticidal individuals come almost exclusively from the sex that bears the primary costs of adoption; (3) such infanticide occurs only under conditions where victims can clearly be identified as nonfilial; and (4) in species with little or no cost to adoption, adoption is common, but infanticide is nonexistent.