Patterns of fluctuating asymmetry in weapons: evidence for reliable signalling of quality in beetle horns and bird spurs

Abstract

Fluctuating asymmetry is an epigenetic measure of the ability of individuals to undergo identical development of a usually bilaterally symmetrical character on both sides of the body. Horns of beetles and spurs of birds are elaborate structures usually restricted to or enlarged in individuals of the limited sex and frequently used in intrasexual combat. Individuals with large horns usually win fights against individuals with small horns which suggests that horn size may reliably signal male fighting ability. A comparative study of the patterns of fluctuating asymmetry in horns of beetles and spurs of birds showed that the degree of asymmetry in these secondary sex traits was considerably larger than in elytra and wing length, which are apparently not subjected to sexual selection. There was a negative relationship between the degree of asymmetry and size of the secondary sex trait while the relationship for elytra and wing length was flat or U-shaped. This demonstrates that horn and spur size may reliably reflect male quality as determined by the degree of fluctuating asymmetry.