WILD HORSES: SOCIAL COMPETITION AND POPULATION SIZE

Abstract

This book, the latest contribution in the University of Chicago Press Wildlife Behavior and Ecology Series, is, like many of its predecessors, a detailed accounting of a long-term intensive field study of a single species. How do these and similar contributions differ from natural history monographs of previous eras, and what criteria should be used to evaluate the relative merits of today's contributions? Current monographs on the behavior and ecology of single species differ from their forerunners in that they generally address specific issues that are tied to modern theory rather than recount natural history observations in an unfocused way. It follows that these studies should be judged on the importance of the issues addressed and the manner in which the investigation was conceived, executed, analyzed and presented. Wild Horses is entertaining, valuable, and thought-provoking reading, because of the attention given by Berger to each of these factors. Berger's decision to investigate horses was a judicious one because of many important contrasts between the biology of horses and other mammals. For example, although horses are polygynous and males defend year-round harems from rival males, they do not exhibit sexual dimorphism in body size and weaponry. Most group-living ungulates defend their harems only seasonally, and in several mammalian taxa (such as ungulates and primates) there is a positive relationship between the presence of weaponry (enlarged teeth, horns, or antlers) and the degree of polygyny. Another significant comparison concerns dietary preferences. While most ungulates have four-part ruminant stomachs, horses (and other equids) possess monosacculated stomachs and are comparatively nonselective (roughage) feeders, with faster rates of food passage and higher rates of food intake than ruminants. The relationship between monomorphism and digestive efficiency, coupled with variation in habitat quality, sets the stage for many of the topics discussed by Berger. The central theme of the book is clearly stated on the first page: if individuals that are likely to reproduce do not do so because of the behavior of others, then social limitations are imposed on the size of the population. Thus, Berger addresses the venerable problem of population regulation, but through a detailed analysis of the social interactions of individuals. That phenotypically variable behavior patterns are key components in behavioral ecology is receiving increasing recognition, and the data presented here further attest to the importance of this perspective. For Berger,