Living in close association with conspecifics provides benefits through group augmentation, but may also be accompanied by disadvantages such as within-group competition over resources or mating partners (Pulliam & Caraco 1984
). In mammals, a male's reproductive success is mainly limited by access to mates (Clutton-Brock & Parker 1992
), which in relation to group-living means that males should try to exclude rivals from groups of females. It has therefore been predicted, that where females form groups that are small enough to be defended by a single male, one-male groups should be the predominant form of social organization (Clutton-Brock 1989
). However, this prediction overlooks the possibility that males could also benefit from associating with other males (Kappeler 1999
), e.g. through coalition formation and joint defence of territories. Where such possible benefits come into play, selection could have favoured increased tolerance among males and, hence the evolution of multi-male groups.
Using long-term demographic data on redfronted lemurs (Eulemur fulvus rufus
), we aim to estimate the costs and benefits of multi-male associations from a dominant male's perspective. According to theory, owing to small female group sizes (one to three adult females), redfronted lemur groups could potentially be monopolized by a single male, yet the number of adult males equals or exceeds the number of females (Kappeler & Port 2008
). Among the males, one male is clearly dominant over all others. Subordinates can either be immigrants or natal males, which delayed dispersal beyond sexual maturity. Even though subordinates can get evicted if group size grows extraordinarily large, immigrants often join groups without overt signs of aggression from resident males. Besides that, groups are occasionally subject to forcible takeovers, as a consequence of which resident males are evicted by coalitions of intruders. To counteract takeovers, preliminary evidence suggests that males benefit from jointly defending their territory (Ostner & Kappeler 2004
). On the other hand, grouping together with other males is also costly because of reproductive competition (Kappeler & Port 2008
Here, we ask whether the benefit of joint territory defence outweighs the costs of reproductive competition. We apply a simple optimality approach to define the costs and benefits of multi-male associations for dominant males and use demographic data from a population of redfronted lemurs to estimate the parameters of our model.