The running speed of pronghorn is exceeded only by that of cheetahs. In this tightly written account of his 14 year study of North America's fastest mammal, Byers first reveals the reason for such speed. For most of their evolutionary history over the past four million years, pronghorn shared a grassland habitat with many dangerous predators, including fleet hyenas, a large lion, and at least two species of cheetahs. Then, at the end Pleistocene extinctions 10,000 years ago, most mammals of the North American savanna fauna, including all of the truly dangerous predators, disappeared. Pronghorn were fortunate survivors, and inherited a world essentially free of danger. In the ensuing 5,000 generations, they changed little. Byers shows that much of pronghorn social behavior and life history, like anatomy and consequent running speed, are designed for life with those former dangerous predators. Effects of selection imposed by predators lingers today in patterns of maternal behavior, sex allocation, grouping tendencies, competition for social rank, and selection of mates by females. The book concludes with a discussion of why pronghorn should not be considered as a special case. Byers argues that it is reasonable to expect the retention of formerly adaptive traits when selection is relaxed, especially when, as is so for pronghorn and other North American fauna, relaxation began such a short time ago. He suggests reform in favor of a more pluralistic approach to analysis of behavioral adaptation. This book is the most complete account to date of the social behavior and life history of this remarkable North American species. Byers' personal writing style and lucid description of behavior and of field work blend with careful presentation of data to produce an easily read book that will find an enthusiastic readership among evolutionary biologists, wildlife managers, animal behaviorists and anyone interested in the natural lives of animals.