Thomas Langurs: Ecology, Sexual Conflict and Social Dynamics

Abstract

The Thomas langur (Presbytis thomasi) is a colobine species endemic to northern Sumatra, Indonesia. Despite their limited distribution, this species may provide insights into the socio-ecology of folivorous primates. Predictions of the socio-ecological model (van Schaik 1989) suggest that colobine primates feed from nonmonopolizable food sources. Females are therefore expected to experience mainly within-group scramble competition. When this type of competition prevails, the female dominance hierarchy will not be despotic, and female coalitions against other female group members will be rare. Moreover, females may disperse between groups. Many folivorous primates, however, do not fit this predicted pattern (Sterck 1999; Chapman and Pavelka 2005). Some species are presumed to lack scramble competition because group sizes are relatively small and grouping does not seem to entail costs, also known as the folivore paradox (Steenbeek and van Schaik 2001). It has been suggested that not food competition, but male sexual strategies may limit their group size (Crockett and Janson 2000). Different connections between food competition, sexual strategies, and social behavior may exist in folivorous primates, and the Thomas langur may represent one possible connection.