Meeting Abstract
Extreme trait elaboration and showiness are the reason Darwin recognized sexual selection as distinct from natural selection. Extreme trait sizes, and disproportionately large traits in large individuals, were thought to be part of this pattern. However, traits under sexual selection vary widely in how they scale on body size (in allometry), from positive (as just mentioned) to negative allometry (with large individuals having disproportionately small traits). To explain this variation, we note that size per se has different consequences in different types of sexual competition. In interactions involving weapons and threats, disproportionately large traits in large individuals are favored due to the combination of two reasons: larger individuals usually win in physical fights; and threats are used by smaller individuals to avoid unwinnable fights with larger individuals. In addition, the reproductive payoffs from threats are often higher for large males, and larger males need greater differences in trait sizes to successfully threaten other large males due to Weber’s Law. Positive allometry is thus predicted for structures (including many weapons) used in threats. Courtship signals, in contrast, convey many types of information to the female that are not necessarily related to the male’s size, so positive allometry is not predicted. A comparative study of the allometry of male traits with “pure” threat vs “pure” courtship signal functions yielded strong support for these predictions, throwing light on the range of allometries that have evolved in sexual traits.
