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Biology, M.A. 2005)
reasons for natal dispersal in blue monkeys
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
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.
See Ekernas & Cords 2007, Animal
Behaviour, for the publication derived from this thesis.
After working as a researcher for Wild
Metro in NYC and in India, Stefan started a Ph.D. program in 2007 at
the Department of Organismal Biology and Ecology, University of Montana.