This edition has been completely rewritten and reorganized to include discoveries in the field and over 100 new illustrations. The author analyzes all aspects of the subject, stressing the utility of evolutionary theory in unifying different behavioural disciplines. However, the book treats both the ranging proximate mechanisms and the evolutionary, or ultimate causes of behaviour. The first chapter introduces the distinction between the two, and the rest of the text is organized into two main sections devoted to each. Examples are drawn from studies of invertibrates and vertibrates, and are supported by more than 1300 reference citations, many to recent articles. The book also emphasizes the tentative nature of scientific conclusions and identifies controversial and unresolved issues.