Sexual dimorphism in toothed whales (Odontoceti) follows Rensch’s rule

Rensch’s rule entails that male-biased sexual size dimorphism scales allometrically with body size and is assumed to generally apply to polygynous taxa. However, so far only few mammalian groups have been shown to conform to it. Toothed whales (Odontoceti) not only span a substantial range of body sizes, but are commonly presumed to be predominantly polygynous, thus representing a promising candidate group to test for Rensch’s rule. Here we compiled a dataset of sex-specific body lengths in 57 species of toothed whales and demonstrate that sexual size dimorphism in this group does indeed follow Rensch’s rule. When focusing on selected subgroups of toothed whales, conformity to the rule was prominent among lineages of the speciose superfamily Delphinoidea, while it was not found in the beaked whales of the family Ziphiidae. These results support the assumption that polygynous and polygynandrous mating systems and marked precopulatory intrasexual competition between males are common among toothed whales. However, female-biased sexual size dimorphism as well as monomorphism occur at notable frequencies as well, suggesting that reproductive strategies are nevertheless variable among these marine mammals.