Theses Doctoral

Intrasexual Competition and Reproduction in Wild Blue Monkeys

Roberts, Su-Jen

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 - specifically intrasexual competition - 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 "bachelors." 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 monkeys' 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 - group size and dominance rank - 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 within-group 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 this dissertation 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 multi-male influxes or using flexible feeding behavior, to maximize their reproductive success in the face of competition.

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More About This Work

Academic Units
Ecology, Evolution, and Environmental Biology
Thesis Advisors
Cords, Marina
Ph.D., Columbia University
Published Here
July 7, 2014