Two subpopulations of blue monkeys (Cercopithecus mitis), located 10 km apart, were studied in Kibale Forest, Uganda, for most of a 6-yr period (1978-1984). This study was undertaken (1) to assess and evaluate the major differences in the environment and in the behavioral ecology of blue monkeys living at high and low densities and (2) to explain the differences in blue monkey densities in the two study sites. Methods included the enumeration of trees and primates, and assessments of the availability of fruits and of diets, time budgets, ranging patterns, and demographics of blue monkeys. There was a 10-fold difference in blue monkey densities between the two subpopulations. Major differences were found in the ecology, behavior, demography, and habitats of blue monkeys living at these two densities. Compared to the high-density subpopulation, the low-density subpopulation exhibited shorter tenure lengths for resident males, a greater density of nonresident males, a higher rate of group intrusions by nonresident males, a higher incidence of infanticide by new resident males, hybridization between blue monkeys and redtail monkeys (Cercopithecus ascanius), fewer adult females per adult male, and lower resident male reproductive success. From these observations I conclude that male-male competition for females was more intense where blue monkey densities were low. Predictor variables of food production and food competition indicate that more food was available for the low-density than for the high-density subpopulation of blue monkeys. The habitat containing the low-density subpopulation had a higher tree density and basal cover, greater tree species richness and diversity, and more fruit left uneaten. It also had a lower overall primate density, which, together with assessments of the kinds of foods eaten, dietary overlap and richness, foraging behavior, and home range size, suggests that intra- and interspecific competition for food both were less in the low-density subpopulation. Further evidence that food was more available for the low-density subpopulation is that both the birth rate and the population growth rate of blue monkeys were higher there than in the high-density subpopulations. The low-density subpopulation was apparently below carrying capacity and increasing in a food-rich habitat. In contrast, the high-density subpopulation appeared to be at carrying capacity, stable, and food limited. Therefore, contrary to what was hypothesized, the availability of food does not explain why blue monkey densities differed on these two areas. On this basis I attribute the low density of the one subpopulation to some unknown historical event rather than to current ecological differences between areas. I suggest that, during this study, the low-density subpopulation was recovering from a decline and that the responsible mechanism (e.g., disease) was no longer operating. This paper emphasizes: ( 1 ) the considerable variability found in the ecology and behavior of primates-even within one species in the same forest; (2) the need for long-term comparative studies of free-living primates, especially those at low densities; (3) the importance of investigating the density of primate populations relative to the carrying ca-pacities of their environments, and the influence of this relationship on behavior and ecology.
