"Social Organization and Ecology of Mentawai Leaf Monkeys (Presbytis potenziani)" by Sasimar Sangchantr

Sasimar Sangchantr

Deposited 2004

Abstract
Mentawai leaf monkeys (Presbytis potenziani) are endemic to the Mentawai Islands, Indonesia. There is little documentation of their social behavior or ecology. Prior to this project, Mentawai leaf monkeys had never been habituated nor been the subject of any study exceeding 200 hours of visual contact. Previous studies suggest that they live exclusively in one-male, one-female social groups throughout their range and are the only Old World monkey species to do so.

Mentawai leaf monkeys were found to live in one-male, multi-female and multi-male, multi-female groups, as well as one-male, one-female groups. Different types of group composition were observed in different populations occupying different types of habitat. Mentawai leaf monkeys may have lived mostly or exclusively in one-male, one-female groups at one time, but new selective pressures (i.e. increased feeding competition, predation risk, and/or infanticide risk) resulting from the depletion of their habitat in recent years may have caused the present variation in social organization. Mentawai leaf monkeys may exemplify a case of extreme flexibility in social organization among nonhuman primates.

Habituated Mentawai leaf monkeys in secondary forest at Muntei, North Pagai Island, were observed from June 2000 to October 2001. Systematic data were collected on their general activity, feeding and ranging behavior, social interactions within and among groups, and vocalizations.

Mentawai leaf monkeys at Muntei lived in stable one-male, one-female; onemale, multi-female; and multi-male, multi-female groups. Intragroup social interactions were rare; they spent 33% of their diurnal time feeding, 56% resting, 10% traveling, and 1% socializing. Fruit comprised a high proportion of their diet; they spent 70% of their feeding time eating fruit and seeds, 22% leaves, 7% flowers, and 1% fungi. They occupied home ranges 17 to 28 hectares in size. Their average day range was 800 meters. Intergroup encounters were generally agonistic, and neighboring groups consistently defended areas of range overlap. Their vocal repertoire included duet calls, documented in some Mentawai leaf monkey populations but not others, and female loud calls, not documented previously in any other Mentawai leaf monkey population or any other colobine species.