Male coercion and the costs of promiscuous mating for female chimpanzees. Proc R Soc Lond B

Department of Anthropology, Boston University, Boston, MA 02215, USA.
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 05/2007; 274(1612):1009-14. DOI: 10.1098/rspb.2006.0206
Source: PubMed


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.

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    • "Besides dominance rank and age, additional male attributes, such as aggressiveness, have been shown to influence male reproduction [Packer, 1979b]. For example, in chimpanzees, aggressive males who charge at females more frequently and use contact aggression toward these females also mate more frequently [Muller et al., 2007] and sire more offspring [Feldblum et al., 2013], especially when forming coalitions [Gilby et al., 2013]. More generally, most of the aggression received by adult female primates from males is connected to receptivity and interpreted as sexual coercion [Muller et al., 2009], which means it increases the likelihood that a female, when fertile, mates with the coercive male [Smuts & Smuts, 1993]. "
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    ABSTRACT: In species with strong male-male competition, access to females in multimale-multifemale groups is usually regulated via a dominance hierarchy. The highest ranking (alpha) male often has priority of access and sires most offspring. The alpha male can change in three basic ways: (i) a recent immigrant or a resident challenges and becomes the new alpha; (ii) formation of a new group; (iii) succession—becoming alpha after higher ranking males have left. When, in a given primate population, the alpha male changes in different ways, two questions arise: (a) which is the most successful tactic and (b) do male attributes, such as age, aggressiveness or propensity to commit infanticide, affect the outcome? We examined these questions in the seasonally breeding Nepal gray langurs (Semnopithecus schistaceus) at Ramnagar, where new alpha males were either recent immigrants or residents. Success was measured as alpha tenure, residency duration, and the number of offspring sired (paternity exclusion based on DNA analysis, 28 infants). We documented 12 alpha-male tenures across two multimale-multifemale groups between 1991 and 1997. The predominant mode of change was the immigrant tactic. Age had no effect perhaps because alpha males were among the youngest adult males in their group. As expected, infanticidal males performed similarly to non-infanticidal ones. Alpha tenure was highly variable and longer for immigrant alphas and hyper-aggressive ones. However, none of the tactics or attributes examined resulted in significantly longer residencies or more offspring, likely because of the timing of immigrations and stochastic effects (i.e., the number of conceptions occurring per alpha tenure). The influence of female mate choice on male reproductive success requires further investigation. Furthermore, it remains to be examined why resident alpha males—with their presumed better knowledge of their opponents —performed so poorly. Am. J. Primatol.
    American Journal of Primatology 06/2015; DOI:10.1002/ajp.22437 · 2.44 Impact Factor
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    • "The second question—why females should compromise their mating strategy for grooming—is perhaps more intriguing . Female promiscuity is thought to function as a counterstrategy to infanticidal behaviour by males, by providing all potential fathers with a nonzero probability of paternity and so creating 'paternity confusion' (Wrangham 1993, 2002; van Schaik 2000; Muller et al. 2007; Watts 2007); allowing males to establish mating biases decreases this confusion and so, at least in principle, increases infanticide risk. Whilst a coercive male mating strategy imposes costs on females to force such a compromise, a sex-for-grooming trade suggests an exchange of benefits. "
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    ABSTRACT: Across taxa, males employ a variety of mating strategies, including sexual coercion and the provision, or trading, of resources. Biological market theory (BMT) predicts that trading of commodities for mating opportunities should exist only when males cannot monopolize access to females and/or obtain mating by force, in situations where power differentials between males are low; both coercion and trading have been reported for chimpanzees (Pan troglodytes). Here, we investigate whether the choice of strategy depends on the variation in male power differentials, using data from two wild communities of East African chimpanzees (Pan troglodytes schweinfurthii): the structurally despotic Sonso community (Budongo, Uganda) and the structurally egalitarian M-group (Mahale, Tanzania). We found evidence of sexual coercion by male Sonso chimpanzees, and of trading—of grooming for mating—by M-group males; females traded sex for neither meat nor protection from male aggression. Our results suggest that the despotism–egalitarian axis influences strategy choice: male chimpanzees appear to pursue sexual coercion when power differentials are large and trading when power differentials are small and coercion consequently ineffective. Our findings demonstrate that trading and coercive strategies are not restricted to particular chimpanzee subspecies; instead, their occurrence is consistent with BMT predictions. Our study raises interesting, and as yet unanswered, questions regarding female chimpanzees’ willingness to trade sex for grooming, if doing so represents a compromise to their fundamentally promiscuous mating strategy. It highlights the importance of within-species cross-group comparisons and the need for further study of the relationship between mating strategy and dominance steepness.
    Behavioral Ecology and Sociobiology 04/2015; 69(6). DOI:10.1007/s00265-015-1917-x · 2.35 Impact Factor
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    • "Bonobos (Pan paniscus) provide a unique opportunity for examining the function of sexual swelling as they have very prolonged periods of maximal swelling (Thompson-Handler et al., 1984; Furuichi, 1987; Dahl et al., 1991; Kano, 1992) and also females that achieve high social status in a male philopatric society (Furuichi, 1997, 2011; Surbeck et al., 2011). In many primate species sexual swelling reaches its maximal size near ovulation (Wildt et al., 1977; Dahl et al., 1991; Emery & Whitten, 2003; Young et al., 2013) and affects male behaviours related to mating, such as consortship (Tokuda, 1961; Hall & de Vore, 1965; Hill, 1987) or coercive mate guarding (Smuts & Smuts, 1993; Muller et al., 2007). However, bonobos seem to deviate from this general trend, since the timing of their ovulation and onset of maximal swelling are highly variable (Furuichi, 1987; Heistermann et al., 1996; van Schaik et al., 2000; Reichert et al., 2002, but also see Surbeck et al., 2011) even compared with those of chimpanzees (Deschner et al., 2003). "
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    ABSTRACT: Perineal sexual skin swelling in relation to menstrual cycle occurs in a variety of primate taxa. However, sexual swelling with exaggerated size and colour is found only in some Old World monkeys and the two Pan species. Although several hypotheses have been proposed (e.g., reliable indicator hypothesis and graded signal hypothesis), it seems unlikely that a single explanation can account for the significance of the sexual swelling in all of these species. Bonobos (Pan paniscus) provide an excellent opportunity for studying sexual swelling since they have the most prolonged maximal swelling periods among primates. In this study we propose a new hypothesis that sexual swelling in female bonobos increases their attractiveness to other females and thereby facilitates affiliative social interaction with them. We found that free-ranging female bonobos with maximal sexual swelling engaged in affiliative social interactions with other females, including genito-genital rubbing, staying in close proximity and grooming, more frequently than females without maximal swelling. These tendencies suggest that females with maximal swelling were attractive to other females. The results also suggest that the benefits of maximal swelling might vary among females depending on their life-history stage. In particular, young females may get more benefits from prolonged maximal swelling through increased grooming reciprocity and staying in close proximity to other females. Thus our study supported the hypothesis that one function of prolonged maximal swelling in bonobos is to increase attractiveness to other females, thereby enhancing affiliative relationships between females in a male-philopatric social system.
    Behaviour 02/2015; 152(3-4):285-311. DOI:10.1163/1568539X-00003212 · 1.23 Impact Factor
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