Directional selection on directional asymmetry: testes size and secondary sexual characters in birds

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Abstract
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 phenotypic quality) in two bird species, the barn swallow, Hirundo rustica, and the house sparrow, Passer domesticus, was used to test whether directional asymmetry was subject to directional selection. Testes size, which was positively related to the size of the secondary sexual character, demonstrated directional asymmetry, with the left testis generally being larger than the right testis. The amount of directional asymmetry was positively related to the expression of the secondary sexual character, suggesting that males with the largest degree of directional asymmetry were at a selective advantage, since males with the most extravagant secondary sexual characters have the highest mating success. These relations were not confounded by variables such as body mass, body size and age. Males in poor condition (with small secondary sexual characters) may be unable to develop large degrees of directional asymmetry, and deviations from directional asymmetry may therefore be viewed as a measure of developmental homeostasis.