SPACING PATTERN OF PROBOSCIS MONKEY GROUPS AT SLEEPING SITES

Abstract

Proboscis monkeys (Nasalis larvatus) are endemic to Borneo Island inhabiting mangrove, peat swamp or riverine forests. Proboscis monkeys have a multi-level social structure which is rare among Primates. The basic social unit is a one-male group (OMG), consisting of one adult male, several adult females, and their offspring. Since one-male groups regularly associate with each other at sleeping sites, this monkey has been considered to have a multi-level society, the OMG and the band. To reveal the multi-level society of this monkey, I observed the spacing pattern of used sleeping site by proboscis monkey in riverine forests along the Menanggul River which is a tributary of the Kinabatangan River, Sabah, Malaysia from April 2000 to October 2001. I identified eight one-male groups and one all-male group in this study site. The analysis of spacing pattern of used sleeping-site detected that the one-male groups were frequently associated with each other. The one-male groups inhabited at study site divided into two bands. However, these bands were not exclusive each other. Sometimes one-male group which was belonged to a band associated with another band. The inter-group association degree was fluctuated with low from October or November to March including rainy season (January-March). Male replacement occurred in three one-male groups during study period, and the home range of two new one-male groups were extended. Moreover one of the new one-male groups showed band change while the other two did not. The band structure of proboscis monkeys appears to be rather loose.