Doctoral Dissertation Improvement: Social and Hormonal Mechanisms of Male Reproductive Skew in Black Howler Monkeys (Alouatta pigra)

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This 12-month study will investigate the social and hormonal mechanisms underlying the partitioning of reproduction among males in two troops of black howler monkeys (Alouatta pigra) at Palenque National Park, Mexico. Specifically, the PIs will examine the relationships between male social interactions and fecal testosterone and cortisol levels using systematic focal samples on all adult troop members to assess whether male reproductive strategies correlate with their testosterone and cortisol levels. Understanding these relationships will provide valuable insights into the biology underlying primate social and sexual behavior. The distribution of reproduction among primate males in multimale groups can range from highly skewed, when one male monopolizes all reproductive opportunities during his residence, to less skewed when reproduction is more evenly distributed among males in a group. Groups with more than one reproductively active male could result from either (1) the inability of the dominant male to exclude other males from reproducing (tug-of-war model), or (2) the dominant male's concession of reproductive opportunities to subordinate males as a staying incentive, with the dominant male benefiting from the presence of subordinate males through increased reproductive success (concession model). While paternity studies in several wild primates have confirmed a positive relationship between male dominance rank and reproductive success, the proximate mechanisms that affect reproductive skew are not yet understood. The challenge hypothesis, which posits that temporal variation in circulating testosterone concentrations is related to patterns of male aggression in reproductive contexts, combined with the relationship between social rank and patterns of individual stress response measured via cortisol levels, provides a new framework for investigating key components of the mechanisms of male reproductive skew. The PIs will also gather data on relatedness among back howler males to gain further insights into the influence of kinship on levels of cooperative and competitive interactions among males and their reproductive strategies. Female mate choice should influence male reproductive skew through its effects on the dominant male's ability to monopolize fertilizations. The PIs will use fecal progesterone and estradiol profiles to determine the timing of their ovarian cycles, and then map male and female sexual solicitations and rejections onto these cycles to evaluate whether females choose to mate exclusively with dominant males, with multiple males throughout their fertile periods, or with multiple males except during their periovulatory periods, when conceptions are more likely to occur. The study will build on established theories of the evolution of primate sociality positing that male social interactions are a balance between cooperation and competition. The research and analyses will be conducted by a graduate student for her doctoral dissertation research, thereby advancing the educational and training mission of the NSF. Conservation programs for populations of this endangered primate will also benefit because the study will permit a comparison between social and hormonal variables in continuous versus fragmented habitats. The study will enhance local capacity building through the participation of Mexican university students.