Date of Award

2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Biology

First Advisor

Dorothy L. Cheney

Abstract

Group-living carries significant costs: disease transmission, resource competition, reproductive interference by conspecifics, and increased conspicuousness to predators. These costs can be substantial for dispersing males, as they are not offset by any benefits of sociality to kin. Although intragroup affiliative and aggressive interactions likely impact male reproduction and physiology, assessment of these relationships is complicated by a multitude of non-social factors that affect male behavior and physical condition in wild populations. Rhesus macaques (Macaca mulatta) of the Caribbean island Cayo Santiago live in naturally-structured groups, free of predation or food scarcity. The population's social complexity and simplified ecology offer a unique opportunity to evaluate the reproductive and physiological correlates of social behavior, dominance rank, and age in group-living male primates.

In the first chapter, I investigated the relationship between socio-demographic factors and male mating success within a reproductive season. Dominance rank, age, and intrasexual aggression emerged as principal determinants. Affiliative interactions with adult conspecifics were not related to the average mating rate. Male-female dyads that groomed more mated less than non-grooming dyads.

The second chapter focused on glucocorticoid and androgen correlates of male rank, age, and sociality. I found a significant correlation between glucocorticoid and androgen levels in fecal samples across reproductive seasons. Both hormones peaked in the first half of the mating season when the levels were closely related to male-male aggression rates. There was no significant difference between hormone concentrations in the second half of the mating season and the birth season. The results indicated the role of inter-individual differences in male endocrinology.

The third chapter evaluated a novel hair-based assay to assess cortisol levels. In young and middle-aged males, this technique revealed a positive correlation between birth season hair cortisol concentrations and average glucocorticoid values extracted from feces. Male-male affiliative and aggressive interactions were closely related to hair cortisol levels. As a long-term indicator of hormone levels, hair holds a promising potential for field endocrinology.

In the fourth chapter, I reviewed potential caveats of the feces-based assessment of glucocorticoid and androgen levels in primates, and emphasized the importance of species-specific validation of this technique.

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