Mixed-sex group formation by bighorn sheep in winter: trading costs of synchrony for benefits of group living

Animals commonly form groups and most sexually dimorphic ungulates form sex-specific groups outside the breeding season. Differences in diet, in vulnerability to predation, and in behaviour between the sexes are hypothesized to drive sexual segregation; however, low-density conditions sometimes favour aggregation. This may occur because larger groups provide greater antipredator benefits that overcome the cost of association between the sexes. In this study we sought to determine whether bighorn rams and ewes (Ovis canadensis) aggregate during winter as an antipredator strategy and whether they alter their behaviour to maintain mixed-sex groups. We compared the occurrence of mixed-sex groups during winter against the population size and sex ratio, and assessed vigilance as an indication of predation risk. We also analysed activity budgets to determine whether group cohesion required departure from sex-specific time budgets. We observed a low proportion of mixed-sex groups in all years of our study except one, in which the population size was very small and the proportion of females was the smallest on record. Mixed-sex groups were larger than single-sex groups, and vigilance was lower in larger groups, indicating that mixed-sex groups provided greater antipredator benefits. However, there was evidence of the oddity effect for older females, which displayed greater vigilance in groups composed primarily of males. Females modified their activity budgets in mixed-sex groups by spending less time foraging and travelling and more time bedding, providing support for the activity budget hypothesis. The costs of mixed-sex group formation limit the circumstances under which aggregation of the sexes is favourable. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.