Abstract
Males in a number of group-living species fight in intergroup conflicts to defend access to food resources, a seemingly paradoxical behaviour, given that this resource does not usually limit male fitness directly. We investigated the mechanism(s) driving apparent male food defence in wild vervet monkeys (Chlorocebus aethiops pygerythrus) by testing the effect that female resource access, and female audience size and activity had on the response of focal males during simulated intergroup encounters. Males do not appear to defend food to increase the reproductive success of female group members because their response was not influenced by the presence of provisioning boxes that only females could access. Female audience size was also unimportant, suggesting males do not participate in intergroup encounters to advertise their quality to potential mates. However, focal males almost always followed/supported female group members who initiated an approach towards simulated intruders, supporting that male participation largely functions to gain status as a cooperative group member, and that apparent male food defence in this species arises as a by-product of intersexual cooperation. Our study highlights that considering audience composition and activity can reveal the presence of social incentives and illuminate the evolutionary mechanism(s) promoting joint action in intergroup aggression.
Highlights
If males defend food resources to increase female reproductive success, males should exhibit the strongest response to simulated intruders when females had access to high-quality resources, and their response should be independent of the composition and/or response of audience members
If males participate in intergroup conflicts to gain social prestige, the size of the female audience present to observe the honest and costly signal should have the strongest influence on the response of males
If apparent food defence is a by-product of intersexual cooperation, the response of males should be most influenced by the presence of a cooperative partner
Summary
Male food defence possibly serves to increase female reproductive success, to gain social prestige, to build status as a cooperative group member, or males may be acting as ‘Hired Guns’ All these functions are possible in vervet monkeys and the four mechanisms are not mutually exclusive. Apparent male food defence could be a by-product, produced as males signal their genetic quality to female group members This correlation could mean that apparent male food defence is a by-product of mechanisms that promote intersexual cooperation, for example, that males trade support in intergroup conflicts for sex. To elucidate which mechanism(s) underlie apparent male food defence in a wild population of vervet monkeys, we took an experimental approach in which we used playbacks to simulate the presence of a neighbouring group nearby when females had, or did not have, access to provisioning boxes (i.e. a high-quality food resource). If a female group member instigates an approach towards the simulated intruders, focal males should follow and support them
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