Thus, it becomes important to understand how variance in male reproductive success in group-living animals depends upon the complex interplay of ecological, demographic, social, and life history factors (Pereira et al. 2002 West and Packer 2002 Bro-Jorgensen and Durant 2003). Alternatively, female fertility, offspring survival, and male longevity may all be greater in larger groups if those groups tend to have higher quality males or occupy better habitats (McElligott et al. Male longevity may be reduced in larger groups if increased mating competition increases the risk of mortality, leading to a potential tradeoff between having a few mates for a long time versus many mates for a short time (Dunbar 1984 Robinson et al. Offspring survivorship may increase with group size due to improved vigilance against predators, or it may decrease due to greater threats from infanticidal outsider males (Dunbar 1984 van Schaik 1989 Crockett and Janson 2000 Steenbeek and van Schaik 2001). Socio-ecological models predict that female fertility may decrease with group size due to greater feeding competition within groups, or it may increase due to improved competitiveness against other groups (e.g., Sterck et al. 1999).Ĭomponents of male reproductive success can be related to each other, particularly among group-living polygynous species. Components of reproductive success can also vary due to factors that are not directly related to reproductive competition, including environmental fluctuations or chance (Sutherland 1985 Coltman et al. Variance in longevity and the length of reproductive careers (tenure length) could arise if males are killed during mating competition, or if mortality increases due to costly dimorphic traits (Dunbar 1984 Clutton-Brock and Isvaran 2007). Variance in offspring survival may arise if more competitive males provide better protection against infanticide by their rivals (Packer et al. Variance in female fertility could arise if more competitive males have preferential access to better resources or higher quality mates (Kelly 2008 Lee et al. The other three components are not typically associated with sexual selection on males, but they can be influenced by reproductive competition in some species. The most apparent manifestation of reproductive competition is often the variance in the number of mates per male, including variance between mating versus non-mating males (Wade and Shuster 2004). Male reproductive success can be decomposed into different components such as the number of mates, the fertility of those mates, offspring survival, and longevity (Brown 1988). 1982 Kappeler 1997) as well as competition to attract mates (intersexual selection: Clutton-Brock and McAuliffe 2009 Jones and Ratterman 2009), and other mechanisms such as coercion (Andersson and Iwasa 1996 Pradhan and Van Schaik 2009). Sexual selection can involve competition to kill, suppress, or exclude rivals (intrasexual selection: Clutton-Brock et al. Sexual selection has been broadly defined as the portion of natural selection that arises from reproductive competition with members of the same sex, or more narrowly defined as the selection that arises from variance in the number of mates per male (Darwin 1871 Wade 1987 Clutton-Brock 2004). High variance in male reproductive success is assumed to provide a potential for sexual selection that can lead to sexual dimorphism (Darwin 1871 Andersson 1994). Thus, this study illustrates how a detailed analysis of the components of male reproductive success can shed light on the interrelated social and ecological aspects that affect it. The lower mortality in larger harems may reflect improved vigilance against predators, and females may cluster around males that enhance offspring survival.

Males with larger harems had lower offspring mortality with no apparent reduction in female fertility or observed tenure length, so the size of harems did not seem to be limited by female feeding competition or by the risk of takeovers and infanticide by outsider males. Access to mates and offspring survival were both major sources of variance in male reproductive success.

Here, we investigate the variance in reproductive success of male western gorillas ( Gorilla gorilla), a highly dimorphic primate with long-term male–female associations, using 12.5 years of data collected at Mbeli Bai in northern Congo. Variance in male reproductive success is expected to be high in sexually dimorphic mammals, even when it is modulated by the costs and benefits of group living.

Do all male gorillas become silverbacks