Do Foster Mothers from Socially Hierarchical Primate Species “Selfishly” Adopt Orphans?

Adoption, a particular type of alloparental care in which primary caretaking of an infant is transferred to a previously unrelated adult, has been shown to increase the likelihood of infant survival. Alloparental care and adoption behavior have been analyzed in over 120 mammalian and 150 avian species and have been especially observed in 'K-selected' species, defined by long periods of development, comparatively few progeny, and long life spans (e.g., seals, elephants, primates). Primate species feature extremely K-selected traits; however, primate non-kinship adoptions have been shown to be individually “selfish” in nature, as reflected by maternal skill improvement among juvenile and nulliparous females, reciprocal altruism among parous females, and exploitation by low-ranking males. Moreover, species-typical social structure also affects female primate fostering behavior. Here we demonstrate that adult parous female cynomolgus monkeys (Macaca fascicularis), in the absence of their social group, display exclusive adoption of their own biological offspring by failing to adopt non-kin orphan infants independent of the physical presence of their infant kin or maternal status. These findings support the hypothesis that adult parous females from socially hierarchical primate species that do not stand to gain from reciprocal altruistic effects are genetically restrictive in their adoption behaviors.