DIRECTIONAL SELECTION ON DIRECTIONAL ASYMMETRY - TESTES SIZE AND SECONDARY SEXUAL CHARACTERS IN BIRDS

A standard example of directional asymmetry with one side of the body having a larger character value than the other is testes size in many vertebrates. The relation between directional asymmetry in testes size and the expression of secondary sexual characters (a measure of pheno typic quality) in two bird species, the barn swallow, Hirundo rustica, and the house sparrow, Passer domesticus, was used to test whether di rectional asymmetry was subject to directional selection. Testes size, which was positively related to the size of the secondary sexual char acter, demonstrated directional asymmetry, with the left testis genera lly being larger than the right testis. The amount of directional asym metry was positively related to the expression of the secondary sexual character, suggesting that males with the largest degree of direction al asymmetry were at a selective advantage, since males with the most extravagant secondary sexual characters have the highest mating succes s. These relations were not confounded by variables such as body mass, body size and age. Males in poor condition (with small secondary sexu al characters) may be unable to develop large degrees of directional a symmetry, and deviations from directional asymmetry may therefore be v iewed as a measure of developmental homeostasis.