Abstract
Effects of the presence of older sibling on the social relationships of juveniles with other monkeys were assessed in a free-ranging group of Japanese monkeys at Katsuyama, Okayama Prefecture, by comparison between fifteen 2-year-old juveniles with older siblings (3-6year old) and nine 2-year-old juveniles without older siblings. The juveniles' proximity and playmate relationships with other monkeys in the group were analyzed in a situation where the juveniles were apart more than 2m from their mothers. The juveniles without older siblings tended to maintain more frequent proximity relationships with same-aged monkeys than did the juveniles with older siblings, while the juveniles with older siblings were in more frequent proximity with adults than did the juveniles without older siblings. These results suggest that the juveniles with older siblings had more opportunities to interact with elder monkeys through agent of their older siblings than did the juveniles without older siblings. The juveniles without older siblings tended to interact more frequently with same-aged monkeys than did the juveniles with older siblings, due to the lack of such opportunities. The male juveniles with older brothers and the female juveniles with older sisters were in more frequent proximity with each other than were those who had the older siblings of the opposite sex. The male juveniles had opportunities to interact with elder males, when they were in proximity with their older brothers.
