Constraints on group size in red colobus and red-tailed guenons: Examining the generality of the ecological constraints model

The ecological constraints model proposes that an increase in group size will increase intragroup feeding competition and thereby constrain group size. Although this model has received wide acceptance, tests of it are based only on a few studies of species that have similar ecological requirements and social organizations, and there are reasons to question the widespread acceptance of the assumptions underpinning it. Via a 2-year study, we explored determinants of group size in species that feed on markedly different types of foods: the folivorous red colobus (Procolobus pennantii) and the frugivorous/insectivorous red-tailed guenon (Cercopithecus ascanius). We established 4 study sites approximately 15 km apart in Kibale National Park, Uganda, to examine the relationship between average group size and food availability. In both species, we quantified interdemic variation in diet, density of food trees, rate of travel, and group size. Red colobus at all sites relied heavily on leaf resources (75.5%-86.9%), but fruit (6.4%-13.9%) and flowers (2.0%-13.9%) were important in some populations. In general, red-tailed guenons fed on fruit (35.7%-59.7%), insects (14.5%-17.6%), and young leaves (12.2%-32.8%), brit the amount of rime allocated to these foods varied among sites. Average monthly density of trees bearing food items ranged among sites from 45 to 79 trees/ha for red colobus and from 19.6 to 67.3 trees/ha for red-tailed guenons. For both species, rate of travel was similar among sites, with one exception for red colobus. Average red colobus group size varied among sites from 14 to 40 (28 groups counted). Red-tailed guenon group size varied among sires from 11 to 24 (16 groups counted). As predicted by the ecological constraints model, group size increased with food tree density across sites for both species.