Ecological constraints on the grouping of wild orang-utans (Pongo pygmaeus) in the Gunung Leuser National Park, Sumatra, Indonesia

Abstract

The orang-utan (Pongo pygmaeus)is an interesting subject on which to base an evaluation of the costs and benefits of social life in apes, since grouping in this heavy, arboreal frugivore is facultative. In the Ketambe area of Sumatra, Indonesia, the food sources of wild orang-utans display a clear seasonality. The two main food types— figs and other, nonfig, fruits— fulfill different functions. The huge fig trees meet the high caloric requirements of the ape and are therefore visited regularly throughout the year. Nonfig fruits are an additional food source but become a really important alternative when figs are in short supply. Since fig trees are relatively rare, it is only in the nonfig fruiting season that food is relatively abundant. Two types of orang-utan groupings could be distinguished. During lean periods, groups formed in the few productive fig trees available can be explained as forced aggregations taking place in spite of centrifugal forces caused by competition. Only when a temporary surfeit of food slackens these centrifugal forces (i.e., during the fruit season) does the tendency to form spontaneous social groups (the second type of grouping) reveal itself. It is suggested that the development of social skills is an important aspect of grouping in adolescent orang-utans. For the adult male, safeguarding his reproductive success by protecting females against sexually aggressive subadult males is probably the only reason for being in groups.