Dissertations, Theses, and Capstone Projects

Date of Degree

6-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Anthropology

Advisor

Larissa Swedell

Committee Members

Shahrina Chowdhury

Thomas Plummer

W. Scott McGraw

Subject Categories

Biological and Physical Anthropology

Keywords

hamadryas baboons (Papio hamadryas), male aggression, female compliance, morphology, reproductive strategies, reproductive success

Abstract

Hamadryas baboons maintain a multilevel social system in which the most basal social grouping, the one-male unit (OMU), is formed by one or more strong social bonds, or pair bonds, between the “leader” male and each female. These pair bonds are maintained via both affiliative and agonistic behavioral dynamics, i.e., reciprocal grooming, aggressive male coercion, and female compliance. While the coercive male likely benefits from greater access to females, the compliant female may benefit from being with a male that is more defensive. The presence of “follower” males, who attach themselves transiently to OMUs, also plays a role in preserving unit cohesion. In addition to the behavioral mechanisms maintaining the OMU, males have striking morphological features – large manes and red paracallosal skin – that likely also aid them in the maintenance of their OMUs, either via their role in attracting females or their function as a signal of vigor to other potential challenger males. The formation of these OMUs persists across both wild and captive settings. This persistence, in combination with the presumed fitness benefits accrued to males and females in this system, suggests that selection has preserved behavioral and morphological elements associated with the establishment and maintenance of the coercive pair bond and the OMUs within which they exist.

This dissertation tests the hypothesis that selection has acted to preserve traits associated with the coercive hamadryas pair bond due to their adaptive value. To test this hypothesis, I used behavioral and morphological data to (a) assess phenotypic variation and (b) estimate fitness outcomes of these traits. The study populations consist of a habituated band of wild baboons inhabiting the surrounding areas of the Filoha field site in Awash National Park, Ethiopia, and two zoo colonies – the San Diego Zoo and the Oakland Zoo – in California, U.S.A. I assessed variation in both captive and wild baboons and estimated fitness outcomes solely in wild baboons.

I quantified behavioral variation in the context of takeovers (events – often involving agonism – in which a male acquires a female from another male’s unit) and outside of takeovers. To assess male aggression in the context of takeovers, I used long-term observational data from the wild and, to assess male aggression outside of takeovers, I conducted focal follows in both the wild, using pre-recorded videos, and zoos, using in-person observations. In focal follows in both the wild and zoos, I also assessed female responses to aggression (compliance and fear) and reciprocal grooming. During takeovers, I assessed each leader male’s use of aggression and the intensity of aggression directed toward females and other males. Outside of takeovers, I assessed counts, rates, intensities, and escalations of aggression, as well as counts and intensities of female responses to coercive aggression. I recorded the number and duration of grooming bouts between the leader male and his females, both given and received.

To assess the size of male manes and paracallosal skin, I used photogrammetry to collect linear morphometric data from digital photographs of zoo baboons and video still frames of wild baboons. Due to the lack of a calibrator in the Filoha videos, I collected relative rather than absolute linear morphometric data in both the wild and zoos. I also collected absolute morphometric data in zoo baboons, which allowed me to validate the use of relative sizes in predicting absolute sizes.

Findings from this study provide evidence of the fitness benefits of these behavioral and morphological traits to leader males. In Chapter 3, I found that males varied in their use of aggression and appear to benefit from a strategy of selectively using aggression in the context of takeovers (acquisitions of females from other OMUs), limiting it in certain contexts (e.g., toward other males) and increasing it in others (e.g., toward females in different higher-level groups, “bands”). In Chapter 4, I found that leader males varied morphologically and those with larger paracallosal skin, but not larger manes, had higher fitness (all metrics: number of births, females, and followers). I also found that zoo males had larger manes and reduced paracallosal skin compared to wild males. In Chapter 5, I found that leader males varied in their use of aggression to coerce and condition females in their unit and that a male’s frequency and escalation of aggression were positively related to his numbers of offspring and females. I also found that male aggression rate was positively correlated with the number of female fear responses in both the wild and in zoos, as well as the number of compliant responses in the wild. In captivity but not in the wild, the intensities of male aggression were positively correlated with the intensities of female compliance.

This dissertation provides insight into the evolutionary traits and mechanisms that maintain the coercive hamadryas pair bond. These findings suggest that hamadryas leader males are flexible in their use of aggression and use it at times when it is most advantageous while limiting it in contexts in which it is potentially detrimental. While males that limit their aggression in the context of takeovers are conferred fitness benefits, the use of aggression outside of takeovers is an effective strategy in which males coerce and condition females and in turn derive fitness benefits. In addition, the finding that male paracallosal skin size is positively related to male fitness suggests that this trait is under selection because it aids males in their acquisition of females. With this finding, this study becomes one of the first in primatology, and the first in hamadryas baboons, to provide evidence of sexual selection of an exaggerated morphological trait. Trait comparisons between captive and wild settings also provide insight into the ways in which captivity influences behavioral dynamics and morphological traits, e.g., via reduced selective pressures and ecological effects on relatively plastic phenotypes. Together, my findings document variation in traits that maintain the coercive hamadryas pair bond in both the wild and in captivity and suggest that these traits are or have been under selection due in part to the fitness benefits conferred to males. Overall, this dissertation provides insight into the evolution of coercive male-female bonds in hamadryas baboons and humans as well as those presumed in hominins.

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