DO THE Imates HAVE IT?1

Abstract

One of Darwin's great insights, sexual selection as an explanation for sexual differences, simmered for almost 100 years before blossoming into an active area of inquiry in the late 20th century. A major review of theory and evidence came with Malte Andersson's (1994) Sexual Selection. Mating Systems and Strategies is in the same Behavior and Ecology series as the earlier volume. However, its purpose is not to review but instead to establish a more rigorous approach to sexual selection research. Shuster and Wade describe a method to quantify the opportunity for sexual selection, a process that is usually more intense on males and is strongly influenced by the spatial and temporal distribution of mates. The authors also present a very critical examination of current behavioral-ecological concepts that apply to sexual selection. A major chapter reclassifies mating systems in animals, described with regard to the typical number of matings per male or female (e.g., monogamy). Shuster and Wade's proposal to measure sexual selection begins with the simple observation that males vary more than females in their number of mates; some males copulate often but others remain chaste. A statistical measure of this variation, Imates' forms the book's foundation. Why bother and other criticisms of earlier versions of this methodology (Bradbury and Andersson 1987) are tackled in Chapter 1. Shuster and Wade suggest that measuring sexual selection is the way to address an apparent paradox: What sort of processes, operating in only one sex, could possibly produce what appears to be one of the most rapidly evolving types of traits-the remarkably large differences in male ornamentation and sexual displays observed in closely related species? Using the degree of sexual dimorphism in ornaments, researchers should be able to predict the strength of sexual selection and the form of mating system for any given animal species. The dimensionless nature of Imates should fit the bill perfectly for making comparisons between species. The Imates approach may well turn out to be helpful and practical (more on this below) but Shuster and Wade do little in the way of promoting the merits of their method over other published schemes for measuring sexual selection (a tendency to disregard alternative viewpoints crops up throughout the book, including the sections that are critical of theory). There is no direct comparison of methods, of the sort for example, conducted by Fairbairn and Wilby (2001), who