Effects of female group size on the number of males in blue monkey (Cercopithecus mitis) groups

The number of males per group varies substantially in group-living primates, both between and within species. In blue monkeys (Cercopithecus mitis), males may temporarily join groups during annual mating seasons when sexually receptive females are present. A likely determinant of the number of males per group is female group size (the number of adult females in a group). To clarify the role of female group size in driving variation in the number of males per group, we expanded on earlier observations of a wild population in the Kakamega Forest, Kenya with a larger sample of groups that varied five-fold in female group size. We found considerable flexibility in social organization, with groups experiencing multi-male episodes both during and outside mating seasons, some persisting over multi-year periods. The dichotomy between single- and multi-male mating seasons was less distinct than previously reported, suggesting greater variation in multi-male states. Across 65 group-specific conce...