ABSTRACT Aim Body size is of paramount importance to the survival of all organisms because of the intimate associations between size, physiological processes and ecology. Sexual size dimorphism (SSD: differences in size between the sexes) is a direct consequence of these ecomorphological relationships, arising due to sex‐based differences in selection on body size. Latitudinal variation in SSD—equivalent to sex‐specific manifestations of Bergmann's, Rensch's and other temperature‐size rules—has been recorded in many terrestrial species. Yet, the underlying proximate and ultimate causes of this geographical variation remain poorly understood and have been quantified in just a handful of marine taxa. Here, we assessed latitudinal variation in SSD among three distinct epaulette shark ( Hemiscyllium ocellatum ) populations across the Great Barrier Reef to better understand factors influencing geographical body size variation in a marine context. Location The Great Barrier Reef, Queensland, Australia. Taxon The epaulette shark ( Hemiscyllium ocellatum ) (Bonnaterre, 1788). Methods We measured the length and mass of 183 adult and subadult H. ocellatum individuals from three populations along the Great Barrier Reef (GBR), Australia. Using these data, we also calculated Fulton's K, a measure of overall condition. We then fit ANOVA and GLM models comparing the latitudinal distribution of each body size metric between the sexes. Results and Main Conclusion Both length and mass decrease with increasing latitude, directly contradicting the temperature–size rule. The slope of this cline varies significantly among the sexes, with males responding more strongly to latitudinal changes. This results in male‐biased SSD in lower‐latitude populations becoming statistically non‐significant at higher latitudes. We suggest that these results are indicative of a latitudinal gradient in resource availability and sex‐specific resource allocation.
Gayford et al. (Wed,) studied this question.