Group choice by subadult bighorn rams: Trade-offs between foraging efficiency and predator avoidance

In addition to sexual segregation, many social ungulates show varying degre es of age segregation, especially among males. We investigated factors affe cting group choice by subadult male bighorn sheep, using census data collec ted between 1982 and 1998 in a marked population. We examined whether group composition varied with population size and structure. Changes in total po pulation size were correlated with the number of yearling males and yearlin g females, but not with the size of other sex-age classes. In years of high population size, female groups were larger than in years of low population size, while mixed sex-age and subadult groups showed a nonsignificant tren d in the same direction. Typical group sizes of bachelor groups and the occ urrence of mixed or bachelor groups were not affected by population size. W hen there were few subadult males in the population, groups of subadult mal es were less frequent than in years with many subadult males in the populat ion, but the typical group size did not change. Subadult males were rarely seen in peer groups, and switched from female groups in spring to bachelor groups in autumn. An individual's choice of group type is affected by its b ody mass, but also by the availability of enough potential group mates to p rovide sufficient predator-detection efficiency.