Rensch's Rule Is Not Supported by a Mammals-Wide Analysis

AbstractRensch's rule posits that sexual size dimorphism increases with overall body size among animal species with larger males and that it decreases with body size among species with larger females. The rule, originally based on patterns observed in a limited dataset rather than theory, has attracted much attention in the sexual selection literature. However, evidence for the rule has been equivocal. We test Rensch's rule with a recently published dataset on sexual size dimorphism in mammals using linear regressions with phylogenetic controls. We find that neither male-biased nor female-biased dimorphic species conform to Rensch's rule across mammals. When the analysis is restricted to within-family comparisons, as Rensch originally intended, the rule applies only to three of the 21 mammalian groups tested. We find very limited support for the "rule" in mammals and suggest that it is unlikely to be the general phenomenon that Rensch proposed.