Sperm Competition, Sperm Depletion, Paternal Care, and Relative Testis Size in Birds

Abstract

Testis mass in birds scales allometrically to body mass, the exponent being significantly smaller than unity. Two hypotheses were formulated in order to account for variation in relative testis size: (1) the sperm competition hypothesis, according to which males of taxa with a high intensity of sperm competition have larger testes than males of other taxa; and (2) the sperm depletion hypothesis, which proposes that taxa in which males copulate often also have large testes. Variation in testis mass after controlling for the effects of body mass and phylogeny was investigated in relation to copulation frequency per female (as a measure of the intensity of sperm competition) and per male (as a measure of the intensity of sperm depletion). Variation in relative testis mass was positively related to variation in copulation frequency per female, and the variation was significant; the sperm competition hypothesis was therefore supported by data. The sperm depletion hypothesis could not account for variation in testis size. Variation in relative testis mass was positively correlated with variation in social dispersion and was negatively correlated with variation in mate-guarding behavior. The extent of paternal care was related to certainty of paternity as estimated from the presence of mate guarding, but not from relative testis mass.