Sexual size dimorphism and the strength of sexual selection in mammals and birds

Background: Sexual selection has been used as a proximate explanation for sexual size dimorphism (SSD) in many taxa, but the relationship between SSD and a quantitative measure of the strength of sexual selection has yet to be tested. Hypothesis: The strength of sexual selection is positively correlated with SSD in birds and mammals. Methods: We used two measures of SSD: the SSD ratio and the size dimorphism index (SDI). The SSD ratio is log(male body mass/female body mass). SDI is the ratio of the larger sex to the smaller sex and minus 1; the resulting value is then made negative for male-biased dimorphism and positive for female-biased dimorphism. Our measure of the strength of sexual selection is the standardized variance in male fitness (I) calculated as the square of the coefficient of variation in male mating or reproductive success, which reflects the maximum potential strength of sexual selection. We carried out a phylogenetic generalized least squares regression (PGLM) of SSD ratio or SDI on I. Results: There was no consistent relationship between SSD and I, as they were strongly positively related in mammals, but not in birds. This may relate to the form of competition (contests vs. display) that is found most often in mammals and birds.