Fake twins? Two cases of intensive allomaternal care by female Japanese macaques before and after their own partum.

Allomaternal care is widely observed among primate species. Although diverse allomothering behavioral repertoire and types of caretakers have been reported, it remains unclear what social or physiological conditions may trigger such care in each individual. For a better understanding of mechanisms that give rise to allomaternal care in primates, more observational reports are needed with information on social or physiological conditions of caretakers. Here I report two cases of intensive allomaternal care in two different populations of Japanese macaques (Macaca fuscata). The two adult females provided intensive allomaternal care including carrying, grooming, and nursing towards others' infants. Their allomaternal care began 20 or 29 days prior to their own partum and lasted for 84 days at the most. This study firstly showed that nursing forms part of the allomothering behavioral repertoire, and allomaternal caretakers can be individuals that are prior to the time of their own partum in Japanese macaques. Their intensive allomaternal care may stem from their good nutritional conditions, increased concentrations of several hormones, or disappearance of the newborn infants' biological mothers.