Sharks Violate Rensch's Rule for Sexual Size Dimorphism.

Systematic trends in body size variation exist in a multitude of vertebrate radiations, however their underlying ecological and evolutionary causes remain poorly understood. Rensch's rule describes one such trend-in which the scaling of sexual size dimorphism (SSD) depends on which sex is larger. Where SSD is male-biased, SSD should scale hyperallometrically, as opposed to hypoallometrically where SSD is female-biased. The evidence for Rensch's rule is mixed, and comes from a small subset of total vertebrate diversity. We conducted the first empirical test of Rensch's rule in sharks, seeking to confirm or refute a long-hypothesied trend. We find that sharks violate Rensch's rule, as the magnitude of SSD increases with body size despite sharks predominantly exhibiting female-biased SSD. This adds to a growing literature of vertebrate clades that appear not to follow Rensch's rule, suggesting the absence of a single, conserved scaling trend for SSD amongst vertebrates. It is likely that selection associated with fecundity results in the "inverse Rensch's rule" observed in sharks, although additional studies will be required to fully reveal the factors underlying SSD variation in this clade.