International Journal of Primatology, Vol. 17, No. 1, 1996 Mixed-Species Primate Groups in the Kibale Forest: Ecological Constraints on Association Colin A. Chapman 1 and Lauren J. Chapman 1 Received August 23, 1994;acceptedJune 30, 1995 Five species of diurnal primates in the Kibale Forest of western Uganda--red colobus (Colobus badius), black-and-white colobus (Colobus guereza), redtail monkeys (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), mangabeys (Cercocebus albigena)--oflen associate in mixed-species groups that vary in size and composition from day to day. Across this range of species, we found no consistent effect of association on feeding rate. In addition, there is no systematic difference between the species-specific individual feeding rates when animals were in mixed-species groups feeding in a specific tree on one day and when individuals of one of the same species were feeding in the same individual tree on a subsequent day. If associating in a mixed-species group lowers the risk of predation, one might expect that the number of vigilant events wouM decrease in mixed-species groups. However, the only species to exhibit a consistent decrease in vigilant behavior when in association was the red colobus. Redtail monkeys were more vigilant when in association. We predicted that the density and distribution of food resources would both constrain the frequency of association and the size of mixed-species groups. Based on 22 months of data on food resources and bimontMy censuses, we found no relationship between the frequency of association (except mangabeys) or mean mixed-species group size and the density and distribution of food resources for all species. Finally, we examined the behavior of the monkeys in and out of association before and after the playback of a crowned hawk eagle call (Spizaetus coronatus), a known predator. When more species were in associatiort, the amount of time they spent being vigilant following the playback was greater and the response more intense than when fewer species were in association or when the group was alone. The results of this study IDepartment of Zoology, University of Florida, Gainesville, Florida32611. 31 0164-029U96/0"2(}0-0031509.50/0 9 1996 Plenum Publishing Corporation