Adoption of 5- and 12-month-old infants by infantless, multiparous females in the Katsuyama free-ranging, provisioned group of Japanese macaques.

Adoption of motherless infants-when such orphans receive continuous care from other individuals-is a widespread, albeit infrequently observed behavior in mammals. Most reported macaque (Macaca) cases of adoption that include nursing involve a recently postpartum female (within a few weeks) adopting a recently orphaned infant less than a month old. However, in a free-ranging group of Japanese macaques (M. fuscata) we observed two infantless, multiparous females adopting a 5- and a 12-month-old infant who had lost their mother one and seven months earlier, respectively. Interactions between adoptive mothers (AMs) and their adopted infants (AIs) were quantitatively similar to those between biological mothers (BMs) and offspring (BIs). Both AIs were observed mouthing their AM's nipples at least a few days or more after first being held by them. When 13 to 15 months old, one adoptee attempted nipple contact again after his AM gave birth to a new offspring. The other adoptee continued to cling to his AM's back well into the second half of his second year, longer than most normally reared infants. These observations suggest that both AIs, having presumably developed strong bonds with their BMs during their first 4 to 5 months, suffered psychological stress, delaying establishment of nipple contact with their AMs, as well as their independence. The AIs and AMs appeared to form mutual bonds, ensuring that the former obtained warmth, transportation, and nutrition, while AMs obtained at least some thermal benefit.