I collected data on the social behavior of juvenile and subadult blue monkeys (Cercopithecus mitis stuhlmanni) in three study groups in Kakamega Forest, Kenya, to identify the proximate reasons for natal dispersal. I tested five hypotheses: 1) Juvenile males are evicted by non-resident males briefly entering the group during periods of male influx; 2) Juvenile males are evicted by the resident male or by a newly established resident male; 3) Juvenile males disperse because they are sexually attracted to non-kin estrous females; 4) Juvenile males disperse because they have few or no social ties to hold them in the natal group; and 5) Juvenile males disperse to minimize reproductive and survivorship risks.

I sampled all 14 juveniles of dispersal age, 5 females and 9 males, as well as 2 subadult males that had dispersed from their natal group 6 months before I began collecting behavioral data.  Of my 9 male focal subjects, 6 dispersed within 5 months (and 3 of those within 1 month) after I finished collecting behavioral data. I compared the social behavior of juvenile females to juvenile males dispersing males to remaining males, and pre- and post-dispersal males to assess the five hypothesized proximate reasons for natal dispersal in blue monkeys. I also tested these hypotheses by investigating the relationship between the timing of dispersals and various social and ecological variables taken from all 22 natal dispersals that have taken place in the study population since 1997.

My results indicate that blue monkey juveniles are not evicted from their natal group and that sexual attraction was not a strong causal factor in natal dispersals. There were no significant differences in the rate or type of aggression received by juvenile females vs. males or by dispersing vs. remaining juvenile males. Males did not disperse more during the conception season or during male influxes than expected by chance.

My results were consistent with the hypothesis that males disperse because they are poorly socially integrated in the natal group. Juvenile females spent eight times as much time as juvenile males grooming or sitting in contact with other group members. Few significant differences were found in the social integration of dispersing vs. remaining males, however, indicating that social integration did not decline in the last three months prior to dispersal.

I tentatively suggest that blue monkey males also disperse to minimize reproductive and survivorship risks.  Males favored dispersing during the three months when rainfall was lowest, and they avoided dispersing during the three months when fruit availability was lowest.  Males may have suffered from a lack of access to mates in the natal group, as they have never been witnessed copulating with a natal female.  Age at dispersal coincided approximately with a local peak in life expectancy for females (the only sex that adult survivorship data are available for), suggesting that males dispersed when cumulative life-long risks were minimized.

Lastly, I suggest that in short-term studies, sex differences may be a more reliable indicator of causal factors for natal dispersal than differences between dispersing and remaining males.  In short-term studies it is difficult to know the timeframe over which factors causing dispersal act, which can obscure differences between remaining and dispersing males. This problem is avoided by comparing males and females in species where only one sex disperses.

After working as a researcher for Wild Metro in NYC and in India, Stefan completed his Ph.D. in 2016 in Wildlife Biology at the University of Montana. He is now Rocky Mountain/Great Plains Program Director at the Denver Zoo.