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Abstract

Body size is a key determinant of male fighting ability and reproductive success in many animal species, but relationships between these variables have only rarely been examined in group-living animals in which body size often correlates with dominance rank. We examined the relationships between body size (crest height, back breadth and body length), dominance rank, alpha male tenure length, number of adult females and patterns of aggression in 26 wild adult male mountain gorillas, Gorilla beringei beringei, living in multimale groups. A composite measure combining crest height and back breadth (variables were highly correlated and combined into a crest–back score), but not body length, significantly correlated with dominance rank, alpha male tenure length and number of adult females per group. The alpha male had the largest crest–back score in six of the seven groups, and in the majority of dyads the male with the higher crest–back score was higher ranking. The frequency (and intensity on mating days) of aggressive contests was higher between males close in rank. Additionally, aggression occurred more frequently when the initiator was larger than the recipient. Our results suggest that factors other than body size are likely to influence dominance rank, but large size helps males attain and retain high dominance rank, probably leading to greater reproductive success. Further studies on how the timing and intensity of male–male competition influences life history trade-offs between investment in secondary sexual characteristics, body condition and survival may explain variance in lifetime reproductive success within and between species.