Universite de Sherbrooke, De Âpartement de biologie, Sherbrooke, Que Âbec Group Choice by Subadult Bighorn Rams: Trade-os between Foraging Eciency and Predator Avoidance Kathreen E. Ruckstuhl & Marco Festa-Bianchet Ruckstuhl, K. E. & Festa-Bianchet, M. 2001: Group choice by subadult bighorn rams: trade-os between foraging eciency and predator avoidance. Ethology 107, 161Ð172. Abstract In addition to sexual segregation, many social ungulates show varying degrees of age segregation, especially among males. We investigated factors aecting group choice by subadult male bighorn sheep, using census data collected between 1982 and 1998 in a marked population. We examined whether group composition varied with population size and structure. Changes in total population size were correlated with the number of yearling males and yearling 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 nonsigni®cant trend in the same direction. Typical group sizes of bachelor groups and the occurrence of mixed or bachelor groups were not aected by population size. When there were few subadult males in the population, groups of subadult males were less frequent than in years with many subadult males in the population, 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 aected by its body mass, but also by the availability of enough potential group mates to provide sucient predator-detection eciency. Corresponding author: K. E. Ruckstuhl, Department of Zoology, LARG, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK. E-mail: kruckstuhl@hotmail.com Introduction Many animals form temporary groups when migrating, breeding or searching for food, while others live in ¯uid or stable groups all year round (Alexander 1974; Wrangham & Rubenstein 1986; Avery 1994; Christal et al. 1998). Predation risk is thought to be one of the selective pressures leading to Ethology 107, 161Ð172 (2001) Ó 2001 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179±1613 U. S. Copyright Clearance Center Code Statement: 0179-1613/2001/1072±0161$15.00/0