Plasticity of reproductive behaviour in wood bison bulls: on risks and opportunities

Abstract

Adolescent animals are expected to take advantage of mating opportunities, but should expend less reproductive effort than older individuals. We observed aggressive behaviour of free-ranging male wood bison and compared it to behaviour observed in two captive populations, in one of which mature bulls were removed. During the peak of the rut, old free-ranging bulls initiated a higher proportion of their aggressive interactions and engaged in fights more often than younger bulls. During the late-rut, older bulls decreased their involvement in fighting and they interacted relatively more often with younger bulls than during the peak of the rut. However, younger bulls did not fight more than during the peak rut. In captivity, subadults were involved more often in aggressive interactions when mature bulls were removed, but they decreased their rate of aggression earlier than subadults in the presence of mature bulls. However, the involvement in fighting among subadults was not affected by the presence of mature bulls: subadults in both populations fought less than mature bulls. We conclude that the willingness of bulls to risk injury in fights increases with age and is unaffected by the social environment. This finding suggests an increase in reproductive effort with age. However, younger bulls take advantage of mating opportunities by adapting those behaviours to the social environment, which do not compromise future reproduction. Adolescent bulls may therefore adopt alternative mating strategies which reflect the age structure of the population.