Group size in folivorous primates: ecological constraints and the possible influence of social factors

Abstract

The ecological-constraints model assumes that food items occur in depletable patches and proposes that an increase in group size leads to increased day range due to more rapid patch depletion. Smaller groups become advantageous when an increase in travel costs is not repaid by an increase in energy gained or some other fitness advantage. On the other hand, we also know that group size can be influenced by social factors. Here we contrast the diet and group size of red colobus (Procolobus badius) and black-and-white colobus (Colobus guereza) in Kibale National Park, Uganda to consider how ecological and social factors are affecting their group sizes. Subsequently, we examine whether the insights gained from this detailed comparison can provide an understanding of why the social organization and group size of mantled howlers (Alouatta palliata) and black howlers (A. pigra) differ. Two groups of red colobus and two groups of black-and-white colobus were studied over 10 months. Red colobus groups were larger (48 and 24) than black-and-white colobus groups (9 and 6). The two groups of red colobus overlap home ranges with the two groups of black-and-white colobus; 75% and 95% of their home ranges were within red colobus's home range. There was a great deal of similarity in the plant parts eaten by the two species and both species fed primarily on young leaves (red colobus 70%, black-and-white colobus 76%). In terms of the actual species consumed, again there was a great deal of similarity between species. The average dietary overlap among months for the two neighboring groups of red colobus was 37.3%, while the dietary overlap between the red colobus and the black-and-white colobus group that had its home range almost entirely within the home range of the red colobus groups averaged 43.2% among months. If ecological conditions were responsible for the difference in group size between the two colobine species, one would expect the density of food trees to be lower in the home ranges of the black-and-white colobus monkeys, since they have the smaller group size. We found the opposite to be true. Both black-and-white colobus groups had more food trees and the cumulative size of those trees was greater than those in the red colobus's home ranges. We quantify how these differences parallel differences in mantled and black howlers. The average group size for mantled howlers was 12.9 individuals, and for black howlers it was 5.3 individuals. We explore possible social constraints, such as infanticide, that prevent black-and-white colobus and black howlers from living in large groups.

Variation in diet and ranging of black and white colobus monkeys in Kibale National Park, Uganda

“…For each food species, we used its percentage basal area measures, for each of the eight groups' core areas, to predict groups' percentage feeding scores on those species for months, or combinations of 2 months (in cases where groups were followed for <15 days for a single month), with comparable phenology scores. Because guerezas at Kibale generally prefer young leaves (YL) to mature leaves (ML; Oates 1977;Chapman et al 2004;Chapman and Pavelka 2005), we analyzed monthly phenology scores for young leaves. However, if any group devoted more than 10% of its feeding on a given species to fruit (FR) or flowers (FL), we analyzed monthly combined YL + FL or YL + FR scores.…”

Socioecological influences on the dispersal of female mountain gorillas—evidence of a second folivore paradox

“…b Potential scenario for a second folivore paradox, if observed group sizes are smaller than optimum, leading to extra infanticide risks even while feeding competition remains minimal. Adapted from Crockett and Janson (2000) and Chapman and Pavelka (2005) found little evidence to support such expectations (unpublished data). Regardless, inbreeding avoidance cannot explain most cases of secondary dispersal (Stewart and Harcourt 1987) or most destinations for either type of transfer, which are the focus of this paper.…”

“…Thus, larger groups of females are more likely to be multimale (Lindenfors et al 2004). Multimale groups can provide better protection against infanticide (Janson and van Schaik 2000), so the effect of group size on infanticide may reach a turning point when its females cannot be monopolized by a single male (Crockett and Janson 2000;Chapman and Pavelka 2005).…”

Small folivorous primate groups exhibit behavioral and physiological effects of food scarcity

“…• Folivorous primates and food scarcity 53 monkeys), which inhabit the same areas in Kibale National Park and have very similar diets (Chapman CA, Chapman LJ, and Gillespie 2002;Rode et al 2003;Chapman and Pavelka 2005), are so different from one another. Red colobus groups in Kibale are multimale, whereas a majority of guereza groups are unimale, and red colobus groups are much larger, ranging in size from 12 to 127 individuals (Struhsaker 1975;Snaith and Chapman 2008).…”