Causes of assortative mating in arthropods

Abstract

Assortative mating by size, one of the most common mating patterns in natural populations, may be explained by three hypotheses: (1) mate choice, whereby large males, large females or large individuals of both sexes choose large mates because they benefit reproductively and are differentially capable of exercising choice; (2) mate availability, whereby large females or large individuals of both sexes are differentially available for mating because of size-differential variation in mating probability or timing that is unrelated to mate choice; and (3) mating constraints, whereby relative size differences between males and females cause physical or energetic difficulties with courtship, pairing or mating. Analysis of the causes of assortative mating in well-studied arthropod species provides some support for each hypothesis in one or more species. However, comparative analysis of many species indicates that assortative mating is most closely associated with the pattern of large size of paired males and females relative to unpaired males and females and the occurrence of long pairing durations and male fighting. These comparative data therefore implicate two hypotheses for the causes of assortative mating in most arthropods: male choice of large females combined with male-male competition, and increased availability of large females combined with male-male competition. The female availability hypothesis may be more plausible because male choice and male-male competition are apparently antagonistic processes whose effects usually do not coincide. Distinguishing between hypotheses based on mate choice, mate availability and mating constraints is crucial to understanding the roles of natural selection and sexual selection in causing assortative mating.