Abstract
Play behavior, especially rough and tumble play of infant (5-9 months after birth) and juvenile (18-22 months) Japanese monkeys (B-group and J-group respectively) was studied, and its role in the development of inter-individual relationships was compared with that of other social interactions, such as grooming and mounting.The frequency of rough and tumble play among infant females was much higher than that among infant males, whereas the frequency among juvenile males was much higher than that among juvenile females.Mounting behavior, being different from rough and tumble play, is a characteristic behavior of male monkeys.This was found true even for infants, though the frequency of mounting behavior was very low as compared with that of rough and tumble play.As for the grooming relationships among individuals, both the infants as well as juveniles were most frequently groomed by their mothers.The infants and juveniles rarely performed the roles of groomers, as they were very young;being less than two years olds.Most of the grooming interactions among individuals other than mothers and their children occured among siblings, especially among elder sisters and younger siblings (elder sisters groom the younger siblings).
84.5% of rough and tumble play of B-group was observed among its members only, while the other partners were, in most of the casese, their elder siblings.Observed frequencies of rough and tumble play among individuals of the same ages or among individuals whose differences of ages were two or less than two years, were close to expected values in J-group.Rough and tumble play was rarely observed among individuals whose differences of ages were more than three years.
Kinship relationships played some role in the selection of the partners for each of the interactions i.e., rough and tumble play, mounting and grooming, as higher than expected values of frequencies of these interactions were observed among blood-related individuals.The inclination towards the blood-related individuals in selecting the partners for each of the following interactions decreased in order, in selecting the partners for grooming, in rough and tumble play and in mounting, successively.Rough and tumble play disappeared when they became late adolescent, though mounting and grooming behaviors were'still persistent and seemed to play an important role in their adult social life.
