Grandmothers care for orphans in a provisioned troop of Japanese macaques (Macaca fuscata).

This study reports cases in which grandmothers and other closely related adult females cared for orphans in the Arashiyama E-troop, a provisioned troop of Japanese macaques (Macaca fuscata). I observed the behavioral patterns of three orphans (aged 1, 2, and 3 years, respectively). The grandmothers cared for two orphans, while three other closely related adult females cared for the rest of the orphans whose grandmothers were absent. These observations differ from those of previous studies that reported that grandmothers were indifferent towards orphans. This intraspecific variation may be attributed to the demographic conditions of the troop in this study, which had only a few related nulliparous adult females, as a result of artificial birth control. In this group, adult females showed decreased fecundity and an increased interbirth interval as they received birth control treatment; thus, in the absence of their own dependent infants, these females may have invested time in the care of related orphans. Although caregiving by grandmothers is believed to be rare or insignificant in primates, this study suggests that grandmothers and other closely related adult females play an important role for orphans in troops with few related adult females.