Does home range use explain the relationship between group size and parasitism? A test with two sympatric species of howler monkeys.

Group size is related to parasite infections in primates. This relationship probably reflects the fact that group size is associated with body contact between group members and with contact with contaminated items in the environment. The latter is highly associated with range use. In the present study we hypothesized that if infection by directly transmitted parasites (DTP) is mainly determined by the exposure of individuals to parasites that accumulate in the environment, and group size correlates negatively with the intensity of home range use, then smaller groups should be more infected by DTP. Additionally, groups that share a higher proportion of their home range with other groups should be more infected. To test our hypothesis we observed and collected fecal samples of two groups of Alouatta palliata (large group size) and two groups of A. pigra (small group size) that live sympatrically in a forest fragment located in Macuspana (Mexico). Group size was positively correlated with range area size and negatively correlated with the intensity of home range use. Range use variables were not related to either DTP prevalence or load. However, there were significant differences in DTP loads between groups, which were positively correlated with group size. Our results suggest that the intensity of home range use is a poor predictor of DTP infection parameters in groups with marked differences in size. Therefore, it is possible that the individual or combined effects of other ecological (e.g., microclimate), social (e.g., contact rate), or physiological (e.g., immune function) factors are more important in the dynamics of DTP in free-ranging primates.