Brief report: Newborn adoption in a confined group of Japanese macaques.

In a group of Japanese macaques, a multiparous high-ranking female gave birth to an infant and, two days later, adopted a neonate abandoned right after birth by a primiparous low-ranking female. Both infants were reared successfully. Whereas the "selfish" explanation does not accord with the evidence from the present case, the "altruistic" explanation cannot be discarded definitively. However, the context and the consequences of the adoption suggest reproductive error on the part of the adoptive mother as the most likely explanation.