The size and composition of primate groups varies tremendously across species, within species, and within groups over time. Written by leading authorities, this book focuses on the causes and consequences of variation in the number of males per group. This variation lies at the heart of understanding adaptive variation among primate social systems. The volume also provides an extensive overview of variation in group composition across all major primate taxa using up-to-date reviews, case studies, evolutionary theory, and theoretical models. A comparative review of birds and selected other mammals is included. This text will become a favorite with all those interested in the behavioral ecology of primates.