In most animals, from bees to bulls, mice to men, it is possible to see at a glance whether an individual is male or female. How and why these differences in appearance and behavior developed and the nature and extent of the differences between males and females is a complex subject. This book reviews the latest molecular, genetic, hormonal, anatomical, and behavioral data in a wide range of species in a series of lively and highly readable articles from the world's leading experts in this field. Unashamedly Darwinian, this book brings sexual selection up to date and discusses not only a dazzling array of differences between the sexes, but probes the mechanisms by which they are produced and the adaptive significance of the differences themselves. It should have a wide appeal, especially to undergraduates and graduates in the biological and medical sciences, and should help to bridge the gap between those who study genes and molecules in the laboratory and those who study the behavior of animals in the wild.