Dynamics of Intersexual Dominance and Adult Sex- Ratio in Wild Vervet Monkeys.

Charlotte Korinna Hemelrijk,Gerrit Gort,Jennifer Botting,Erica Van De Waal,Matthias Wubs

doi:10.3389/fpsyg.2020.00839

Charlotte Korinna Hemelrijk, Gerrit Gort + Show 3 more

Open Access

https://doi.org/10.3389/fpsyg.2020.00839

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Abstract

Intersexual dominance relations are important for female mammals, because of their consequences for accessing food and for the degree of sexual control females experience from males. Female mammals are usually considered to rank below males in the dominance hierarchy, because of their typical physical inferiority. Yet, in some groups or species, females are nonetheless dominant over some males (partial female dominance). Intersexual dominance, therefore, also depends on traits other than sexual dimorphism, such as social support, social exchange, group adult sex-ratio, and the widespread self-reinforcing effects of winning and losing fights, the “winner-loser effect.” The importance of sex-ratio and the winner-loser effect remains poorly understood. A theoretical model, DomWorld, predicts that in groups with a higher proportion of males, females are dominant over more males when aggression is fierce (not mild). The model is based on a small number of general processes in mammals, such as grouping, aggression, the winner-loser effect, the initially greater fighting capacity of males than females, and sex ratio. We expect its predictions to be general and suggest they be examined in a great number of species and taxa. Here, we test these predictions in four groups of wild vervet monkeys (Chlorocebus pygerythrus) in Mawana game reserve in Africa, using 7 years of data. We confirm that a higher proportion of males in the group is associated with greater dominance of females over males; a result that remains when combining these data with those of two other sites (Amboseli and Samara). We additionally confirm that in groups with a higher fraction of males there is a relatively higher (a) proportion of fights of males with other males, and (b) proportion of fights won by females against males from the fights of females with any adults. We reject alternative hypotheses that more dominance of females over males could be attributed to females receiving more coalitions from males, or females receiving lowered male aggression in exchange for sexual access (the docile male hypothesis). We conclude that female dominance relative to males is dynamic and that future empirical studies of inter-sexual dominance will benefit by considering the adult sex-ratio of groups.

Highlights

In many group-living animals there is a dominance hierarchy and dominant individuals usually have priority of access to resources (Drews, 1993)
In terms of dominance between the sexes, females may benefit from dominating males for several reasons, for instance by: (A) suffering less sexual coercion (Smuts and Smuts, 1993; Muller and Wrangham, 2009; Surbeck and Hohmann, 2013; Palombit, 2014), (B) having more freedom in choosing mates (Soltis, 1999; Muller and Wrangham, 2009; but see Rosenblum and Nadler, 1971), (C) being able to protect their infants better against harassment by males (Smuts and Smuts, 1993; Muller and Wrangham, 2009). (D) having more opportunity to lead group movement, which may result in feeding priority (Waeber and Hemelrijk, 2003; Overdorff et al, 2005; Van Belle et al, 2013)
Female dominance over males happens in the model because the large impact of the winner-loser effect causes some males and females to lose much of their fighting ability and others to gain a lot

Summary

Introduction

In many group-living animals there is a dominance hierarchy and dominant individuals usually have priority of access to resources (Drews, 1993). If body size and weaponry (so-called prior attributes) alone contributed to an individual’s position in the hierarchy, in those species where each adult male is larger than each adult female in the group (as in many mammalian species), all males should always be dominant over all females (the socalled prior attribute hypothesis, Chase et al, 2002). This is not always the case and smaller females are sometimes observed to beat larger males, so-called partial female dominance (Smuts, 1987). Tonkean macaques (M. tonkeana) are an example of a primate species which exhibit mild aggression during conflicts (such as staring), and the outcomes of their fights have only a small impact (Thierry et al, 2008)

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