Su-Jen Roberts
Intrasexual competition and reproduction in wild blue monkeys
Dissertation Abstract: Competition and cooperation with conspecifics
affect the costs and benefits of group living and the evolution of
social organization and mating systems. Understanding the role of
competition Ð specifically intrasexual competition Ð in determining
reproductive success thus informs models explaining the diverse types
of social organization seen across animal species. The research
presented in this dissertation combines molecular, demographic, and
social behavior data to explore patterns of reproduction in a
population of blue monkeys (Cercopithecus mitis stuhlmanni) in western Kenya.
Blue monkeys typically live in one-male/multi-female groups and
resident males are presumed to have a reproductive advantage over
non-resident Òbachelors.Ó I used fecal samples from 60 resident and
bachelor males and 126 offspring born in 8 study groups over a 10-year
period to quantify resident siring success. Residents sired at most 61%
of offspring conceived in their groups, a percentage that is less than
most other mammals living in one-male groups and may be linked to blue
monkeysÕ unusually dynamic social organization. In the study
population, some groups in some years experience influxes of competitor
males; these influxes are most likely to occur in years when many
females are mating simultaneously. I found a significant and negative
effect of female reproductive synchrony and the number of male
competitors on resident siring success. These results suggest that it
is difficult for a resident male to defend access to multiple sexually
receptive females, which may be further complicated by the presence of
many competitors trying to steal matings. Resident male blue monkeys
lost a substantial proportion of reproduction (39% of infants sired) to
outside males, which challenges the presumed reproductive advantage of
residency. Even though rival males are, by definition, less often
nearby in one-male groups than in multi-male groups, they pose a
competitive threat to resident male blue monkeys.
I used the paternity assignments to identify the factors affecting the
siring success of extra-group males, including resident males in
adjacent groups and bachelors. When a resident male was unable to
monopolize reproduction in his own group, resident males in adjacent
groups tended to be more likely to sire offspring than bachelors.
Neither bachelor dominance rank nor time spent in a group was a
significant predictor of siring success, suggesting that bachelor
siring success may reflect a highly opportunistic mating tactic, which
succeeds in a visually opaque habitat where estrous females, who mate
rarely, are often widely dispersed.
Comparing the success of alternative reproductive tactics provides a
more complete understanding of the evolution of mating systems. I used
rates of resident and bachelor siring success and home range overlap to
compute the number of years the hypothetical average bachelor would
have to pursue the bachelor tactic to sire as many offspring as the
hypothetical average resident during one or two periods of tenure. In
most cases, a bachelor would not live long enough to match resident
siring success. If, however, a bachelor was able to reproduce at the
average rate in the average number of groups for several years, he may
be able to sire as many offspring as a resident male with a short
period of residency, especially if that resident was in a small group.
These results suggest that the resident male tactic may not always
result in the highest reproductive success. The calculation used here
is a simple way to estimate and compare the success of alternative
reproductive tactics, which is important for understanding the
evolution of social organization and mating systems. This study calls
for future research that tracks individual males over the course of
their lifetimes to determine how often males switch between residency
and bachelorhood, to estimate the length of male reproductive
lifespans, and thus to assess variance in lifetime reproductive success.
Female blue monkeys face competition with other group members for
access to food resources, and such competition may affect fitness. I
tested the effect of two indicators of within-group competition Ð group
size and dominance rank Ð on the probability that a female conceived.
The probability of conception was highest for females in medium-sized
groups containing about 31 individuals, suggesting the existence of an
optimal group size. This optimal size may occur if individuals in small
groups do not obtain the full benefits of group living, including
decreased predation risk and increased foraging success, and
individuals in large groups have lower quality diets or face time
constraints that reduce their nutrient intake. Dominance rank had no
effect on the probability of conception, which may reflect the use of
behavioral tactics like spreading out during feeding and readily
switching food resources to minimize withingroup contest competition. A
relaxed dominance hierarchy may promote group cohesion and increase
success in between-group contest competition. These results emphasize
the potential disconnect between behavioral proxies and reproduction;
specifically, the existence of a dominance hierarchy and the absence of
a relationship between group size and travel distance were not good
indicators of the effect of rank and of group size on reproduction.
When possible, researchers should examine the effects of rank and group
size on measures of reproduction directly.
The results of my research emphasize the value of long-term studies of
individually-identified subjects when investigating patterns of
reproduction in long-lived animals. My findings indicate that
intrasexual competition affects reproduction in both sexes and suggests
that individuals use behavioral tactics, such as participating in
multimale influxes or using flexible feeding behavior, to maximize
their reproductive success in the face of competition.