Male behaviour and the evolutionary ecology of chacma baboons

Although males in East African subspecies of the savanna baboon are known to form coalitions with one another In the pursuit of reproductive goals, such behaviour, which Is indicative of social complexity, has never reliably been recorded from the southern African chacma baboon. We use data from 23 populations and three baboon subspecies to test the argument that harsher environmental conditions below the Tropic of Capricorn reduce the number of males in the average chacma troop by setting limits on group size and by skewing the sex ratio in favour of females. With fewer males, coalitions are unlikely to emerge. Our data confirm this argument and we extrapolate from them to propose that such conditions, acting during the emergence of chacmas, have prevented selection for a behavioural phenotype capable of coalition formation.