Modelling sexual segregation in ungulates: effects of group size, activity budgets and synchrony

Sexual segregation is common in ungulates and some social mammals but its causes are still poorly understood. We developed an individual-based, spatially explicit simulation model to test whether sexual differences in activity could lead to sexual segregation. In our model, males and females differed only in their propensity to switch from an active to a passive state and vice versa, with males being more reluctant to get up and more ready to lie down than the more active females, or vice versa. The only factor in our model that affected sexual segregation was the sexual difference in the propensity to switch from an active to a passive state and vice versa. As differences in activity budgets increased, the degree of sexual segregation increased. Sexual segregation reached a peak when sexual differences in activity budgets were greatest. This high level of segregation remained, even when animals were programmed to adjust their activity to other animals in their vicinity. Our results verify the logic of the activity budget hypothesis that sexual differences in time spent active (grazing, walking) versus passive (lying, ruminating) can, in principle, result in sexual segregation. However, this does not exclude alternative mechanisms. Our model could also be applied to any social animal foraging in groups. (C) 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.