Asian colobine social structure: Ecological and evolutionary constraints

Both ecological and phylogenetic factors and their interaction influence primate social structure. Food resources, in particular, affect social structure parameters such as sex/age composition, group size, home range size, and individual density. As a group, the Asian colobines may be partially buffered from the impact of food constraints on social structure, given their specialized digestive physiology. Asian colobines are able to subsist on mature foliage, a relatively abundant and non-patchily distributed resource, when preferred foods are not available. In food limited populations, increases in group size or density should cause increases in home range size. We did not find this relationship for Asian colobines, however, and thus it would appear that the majority of Asian colobine populations are below carrying capacity. Food may be less of a limitation to populations than other factors, such as social stress. The formation of bands (associations between relatively stable groups) has been documented for bothNasalis andRhinopithecus, and based on recent data for severalPresbytis andTrachypithecus species, we predict that band formation is the norm for the Asian colobines, includingPygathrix. Group home ranges forPresbytis andTrachypithecus species typically overlap and both intergroup tolerance and intergroup aggression are observed. This suggests that groups form differentiated relationships, tolerating some groups but not others. Such differentiated relationships are the foundation of band formation.