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. The Thomas langur is a relatively well-studied colobine monkey. Their food sources, food competition, and the effect of group size on behavior have been determined. In addition, male sexual strategies have been investigated, and the dynamics of their social system are well documented. This allows an exploration of the fit and deviations in the predictions of the socio-ecological model and the importance of male sexual strategies for female behavior. Similar social dynamics are found in a number of other Asian colobines (Sterck 1998), a one-male red colobus population (Colobus badius: Marsh 1979) and mountain gorillas (Gorilla gorilla beringei: Harcourt et al. 1976; Watts 1989; Robbins and Sawyer 2007). Therefore, understanding Thomas langur’s social dynamics may suggest an explanation for the behavior of folivorous species with a similar social organization and may generate hypotheses concerning folivorous species with a different social organization. In this chapter, we review the distribution, ecology and behavior of the Thomas langur and explore how their social dynamics relate to ecology and sexual strategies.