Re-evaluating the Ecological Constraints model with red colobus monkeys (Procolobus rufomitratus tephrosceles)

The Ecological Constraints model was proposed to explain limits to group size and the observation that larger groups of primates often travel farther per day than smaller groups. It argues that larger groups are forced to travel to more patches of food to compensate for more rapid patch depletion. While the evidence from highly frugivorous primates is consistent with this argument, evidence from folivorous primates is contradictory. Many folivorous species, including the extremely folivorous red colobus (Procolobus rufomitratus tephrosceles), have long been thought to show no group size-adjusted changes in daily travel distance (DTD). More recent research suggests, however, that larger groups of red colobus do travel farther per day than smaller groups, and it has been argued that earlier research obscured real effects of group size on DTD because of differential but unacknowledged habitat quality. This study challenges that explanation with a comparison of red colobus feeding and ranging behavior and activity budgets in two distinct compartments of Kibale Forest, Uganda, one of which was selectively logged 11 years earlier. Logging would have accentuated natural floristic differences that might have existed between compartments, thus increasing the likelihood that DTD would differ significantly between compartments. Findings reveal, however, that although there were compartmental differences in canopy cover, understory density, size of trees, and food tree species, the group did not respond to these differences by altering DTD (or their activity budgets). If the floristic differences between compartments are interpreted as insufficient to affect DTD, then it weakens the explanation that earlier studies were confounded by habitat differences. Alternatively, if the differences between compartments are interpreted as theoretically or ordinarily sufficient to affect DTD (e.g., in primates that routinely adjust DTD to group size), then the mechanism underlying the Ecological Constraints model must be questioned for red colobus. Feeding behavior coupled with group movements suggest that the group did not feed in discrete patches, an important element of the Ecological Constraints model. Instead, the group typically fed from multiple tree species per 0.25-ha quadrat and moved slowly, suggesting that their food trees were more uniformly distributed. Estimates from botanical transects and behavioral data also support the interpretation of a more uniform food distribution. Nearly all stems were food tree species and food trees occurred at a density much higher than for folivorous primates that do show a group size effect on DTD. The group increased its DTD more when funneling across gaps in the forest. Funneling is a group movement that is expected to occur more frequently during travel when group size is large, travel routes are few and narrow, and target locations are sparse or patchy and small relative to group size. It is proposed here to be more fundamental than patch depletion as a mechanism to explain group size-adjusted DTD in other primates.