Dispersal and Male Reproductive Strategies in Wild Gelada (Theropithecus Gelada)

Abstract

Male reproduction is typically a zero-sum game, where each fertilization by one male comes at the expense of another (Andersson 1994; Darwin 1871). Therefore, in social systems where females can be monopolized, natural selection should favor competitive male reproductive strategies that enhance access to fertile females (Emlen and Oring 1977). In most systems, this is not a singular strategy, but rather comprises many individual decisions facilitated (or constrained) by variation in male quality, development, opportunity, and disposition that subsequently lead to differences in reproductive success.

In this dissertation, I consider two major stages of male life history in a cercopithecine primate, the gelada (Theropithecus gelada): male dispersal “decisions” across development and maturation; and male reproductive strategy “decisions” upon entering a new group as the dominant breeding male. Geladas are an ideal system for this study as juvenile males exhibit individual variation in dispersal “decisions” - starting from the age of dispersal (when), to dispersal patterns (how), and final dispersal destination (where); while dominant adult males exhibit variation in reproductive strategies immediately upon takeover - from killing either all, many, or very few infants on one hand (infanticidal) to not killing any vulnerable infants upon takeover (non-infanticidal).

Using cross-sectional and longitudinal data from Simien Mountains National Park, Ethiopia where long term gelada research (Simien Mountains Gelada Research Project, SMGRP) has been ongoing since 2006, I first investigated how the social environment experienced by males as juveniles leads to variation in male development via androgen production and dispersal decisions. I examined the effect of maternal attributes and group attributes on both these aspects of male development. I found that juvenile males with high-ranking mothers had lower fecal androgen levels compared to males with low-ranking mothers, and that male geladas dispersed earlier in larger groups and when they had more peers in their groups suggesting that maternal resources (via rank) and availability of juvenile partners can alter the pace of male development in geladas.

Second, I investigated factors that drove overall infant mortality rates in our population and immediately following dominant male replacements (takeovers). I found that takeovers drive overall infant mortality in our study population, however, all leader males do not commit infanticide upon takeovers even when the opportunity exists. Higher infant survival following a takeover was particularly attributable to instances when there were already reproductive opportunities present in the unit (in the form of cycling females) and when infants were older at the time of takeover.

Finally, I examined how demographic variables and individual male attributes led to variation in infanticidal behavior during takeovers. I found that the presence of former leaders increased takeover rates and decreased overall leader male tenure length. I found no differences in the attributes of infanticidal vs non-infanticidal leader males (wrt androgens and glucocorticoid profiles). There was also no effect of killing infants in the first year on overall tenure length suggesting that the reproductive payoffs between infanticidal males and non-infanticidal males might be similar. This study contextualizes how variation in male behaviors can emerge and builds a framework to test future predictions on male reproductive strategies in geladas. Future research will aim at connecting individual male dispersal decisions with subsequent strategies as dominant males and measure paternity success using genetic data.