Determinants of group size in the red colobus monkey (Procolobus badius): an evaluation of the generality of the ecological-constraints model

The ecological-constraints model proposes that increased group size increas es within-group feeding competition, necessitating increased travel and, co nsequently, constraining group size. Previous studies have supported the mo del for frugivores, but its applicability to folivores remains untested. Th is study evaluated the generality of the model by re-examining the relation ship between day range and group size for a folivorous species for which pu blished accounts have not found a relationship between these factors. This study differs from earlier studies by accounting for variation in food avai lability, which may drive changes in day range. We quantified the relations hips among food availability, day range, and group size for two red colobus groups at Kibale National Park, Uganda. Mean day range and home range were significantly greater for the group of 48 individuals compared to the grou p of 24 individuals. The large group traveled more and rested less than the small group. The large group also traveled more rapidly than the small gro up. Food availability significantly predicted mean day range for the large group, but not for the small group. Analyses of covariance demonstrated tha t the large and small group responded differently to changes in food availa bility. These observations suggest that the large red colobus group experie nces greater levels of within-group feeding competition than the small grou p. This study provides added support for the generality of the ecological-c onstraints model and contributes toward an understanding of the mechanisms controlling feeding competition and social organization in primates.