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Week 5 Reading B - Proc R Soc B doi:10.1098/rspb.2006.0206...

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Male coercion and the costs of promiscuous mating for female chimpanzees Martin N. Muller 1, * , Sonya M. Kahlenberg 2 , Melissa Emery Thompson 2 and Richard W. Wrangham 2 1 Department of Anthropology, Boston University, Boston, MA 02215, USA 2 Department of Anthropology, Harvard University, Cambridge, MA 02138, USA For reasons that are not yet clear, male aggression against females occurs frequently among primates with promiscuous mating systems. Here, we test the sexual coercion hypothesis that male aggression functions to constrain female mate choice. We use 10 years of behavioural and endocrine data from a community of wild chimpanzees ( Pan troglodytes schweinfurthii ) to show that sexual coercion is the probable primary function of male aggression against females. Specifically, we show that male aggression is targeted towards the most fecund females, is associated with high male mating success and is costly for the victims. Such aggression can be viewed as a counter-strategy to female attempts at paternity confusion, and a cost of multi-male mating. Keywords: chimpanzee; sexual coercion; intersexual aggression; promiscuous mating; sexual selection; stress physiology 1. INTRODUCTION Females in many mammalian species mate promiscuously, actively soliciting copulations from multiple partners ( Dixson 1998 ; Wolff & Macdonald 2004 ). Primates represent a particularly interesting group in this regard because, in many Old World species, females display clear anatomical and physiological adaptations that promote multi-male mating (i.e. sexual swellings; Hrdy 1981 ; Nunn 1999 ; Zinner et al . 2004 ). In these species, male aggression against females is a common occurrence ( van Schaik et al . 2004 ). However, this behaviour has not yet been fully explained. Hypotheses include male aggression towards females being an incidental outcome of male–male compe- tition, or the result of intersexual dominance or feeding competition. The predominant hypothesis, however, is that male aggression towards females represents sexual coercion, making femalesmore likely to mate withsomemalesand less likely to mate with others ( Smuts & Smuts 1993 ). Here, we test the sexual coercion hypothesis. Clutton-Brock & Parker (1995) identified three types of sexual coercion: forced copulation, harassment and intimidation. These strategies are differentiated primarily by the temporal proximity of their effects. Forced copulation involves violent restraint, resulting in immedi- ate mating. Harassment involves repeated attempts to copulate that impose costs on females, inducing eventual female submission. Intimidation involves physical punish- ment of female refusals to mate, increasing the likelihood of submission in the future. All of these strategies are expected to involve non-preferred males, as they presume female resistance.
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