However, to date there are relatively few studies on male reproductive success in bonobos, and additional research is necessary to confirm that these purported species differences are not merely an artefact of having insufficient data to represent the range of variation present in these species. Possible explanations for this particularly high skew include maternal support of sires, female choice acting to enhance the reproductive success of desirable males, and high group cohesion leading to few opportunities for lower-ranking males to exert alternative strategies such as consortship or sneaky copulation. Intriguingly, however, two recent studies found higher levels of reproductive skew in bonobos than in chimpanzees. In contrast, closely related bonobos ( Pan paniscus) are characterized by lower rates of male aggression and relatively higher female social ranks, which could be expected to lead to lower reproductive skew as compared to chimpanzees. Male reproductive success in chimpanzee communities is therefore skewed, but variably so. In larger communities, alpha males may be able to exert relatively less control over mating opportunities, especially in the presence of multiple simultaneously receptive females. ![]() Nonetheless, subordinate males also sire offspring, particularly through the use of strategies such as opportunistic mating and consortship. Males form a linear dominance hierarchy, with alpha males typically siring a disproportionate share of offspring. Sexually receptive females typically mate with multiple males, and although female chimpanzee estrous cycles are asynchronous, simultaneous sexual receptivity of multiple females commonly occurs. They live in multimale/multifemale “communities” that vary in size from approximately 20 to 200 members. Ĭhimpanzees ( Pan troglodytes) offer a useful opportunity to investigate how demographic and social factors are linked to male reproductive skew. In multimale primate species, reproductive skew typically decreases as a function of increasing numbers of males and receptive females, underscoring an important role for demographic factors in predicting the reproductive success of dominant males. In addition, social factors include the costs and benefits to dominant males of ceding opportunities to maintain group stability, garner support, or gain inclusive fitness benefits by allowing mating opportunities for subordinate males. Demographic factors include the number of rival males and reproductive females as these increase, the ability of the dominant male to control mating opportunities tends to decrease. Models of reproductive skew typically focus on the demographic and social factors relevant to male reproductive success. Nonetheless, even for dominant males high rank does not ensure complete control over reproductive opportunities. In many species, social rank reflects the outcomes of potentially costly competition among males, but these costs are frequently offset for high-ranking males by an increase in reproductive success. Sexual selection theory predicts that males compete for access to a limited number of females. The interaction of small community size, dispersal limitations, and male reproductive strategies like those found here may decrease genetic diversity and increase the risk of concomitant inbreeding in chimpanzee communities under strong anthropogenic pressure. Our findings underscore the role of demographic and social factors in male reproductive success and also suggest that conclusions about species differences may be premature. ![]() The skew exceeds that reported in other studies of chimpanzees as well as closely related bonobos ( Pan paniscus). In a small community of eastern chimpanzees inhabiting a human-dominated landscape in Bulindi, Uganda, we found extraordinarily high levels of alpha male reproductive success over a 5-year period (7/8 offspring = 88%), despite the presence of multiple subordinate males. Alpha male chimpanzees ( Pan troglodytes) often sire a disproportionate, though somewhat variable, percentage of offspring compared to other males. ![]() In multimale primate species, reproductive skew tends to decrease with increasing numbers of males and sexually receptive females. Social rank is positively correlated with reproductive success in numerous species, albeit demographic factors often influence those patterns.
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