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 conception periods, female group size strongly influenced how often multiple sexually active females and multiple males were present in a group. The number of sexually active females present on a given day related closely to the number of males in the group that same day, especially during conception periods. Results suggest that males join and remain in larger groups where mating opportunities are greater and costs of joining or staying may be lower than in smaller groups. This longitudinal study highlights intraspecific social variation within and across groups while confirming that female group size influences the number of males.

Data were collected from 2006-2014 inclusive via field observations of wild, individually-recognized blue monkeys living in multiple groups in the Kakamega Forest, Kenya, as described in detail in the associated publication.

Dataset 1 (Gao_Cords_2020_IJP_Dataset1_Daily_data) indicates how many sexually active females (count) and how many males (count) were present in each study group on each date (range: 01-Jan-2006 to 31-Dec-2014), whether the date was part of the population-wide mating season (June 1 to Oct 15 each year, coded yes/no), and whether it was part of a group-specific conception period (see publication for details on this coding, also coded yes/no). Each record also includes adult female group size (total count of adult (parous) females) on the given date. Missing values (group not observed) are coded as ND (no data). For Mating Season, codes of ND were assigned to groups Gsa and Gsb from 03-Oct-2008 to 31-Oct-2008 and to groups Gsaa and Gsc from 11-Oct-2009 to 31-Oct-2009, because we did not include these brief end-October periods, which occurred just after the groups were formed by group fission, in the mating season analyses (see publication).

Dataset 2 (Gao_Cords_2020_IJP_Dataset2_Changes_in_female_sexual_activity_and_changes_in_number_of_males_in_group) indicates, for a given day (Date) and group and individual female whether her sexual activity status on the subsequent calendar day (=Date+1) was  "active" or "inactive", and whether male numbers changed ("increased" or "decreased") or did not change ("no change") on the next calendar day (=Date+1).  Females were included in this data set only if their sexual activity status on "Date" was "inactive". Data extracted from field records span dates from 2006-Jan-1 to 2014-Dec-31.