In the past twenty years there have been many new developments in the study of animal behaviour: for example, more sophisticated methods of neurophysiology; more precise techniques for assessing hormonal levels; more accurate methods for studying animals in the wild; and, on the functional side, the growth of behavioural ecology with its use of optimality theory and game theory. In addition, there has been a burgeoning number of studies on a wide range of species. The study of aggression has benefited greatly from these develop- ments; this is reflected in the appearance of a number of...