Bergmann-Rensch Continuum Under Shell: Gender-Specific Trend in Response to Latitudinal Gradient

Two macroecological and evolutionary rules are strongly related to the body size of organisms: Bergmann’s and Rensch’s rules. Bergmann’s rule states that organisms are larger in colder regions (high latitudes). Rensch’s rule states that sexual size dimorphism increases when males are larger. Organisms with widespread distribution and resource-mediated growth—such as hermit crabs and their gastropod shells—become excellent models for investigating these patterns. This study is the first to address macroecological and evolutionary patterns in body size among populations and also among sex of the three species of hermit crabs of the genus Clibanarius: C. antillensis, C. sclopetarius and C. symmetricus throughout their distribution. This research included systematic review of data from specialized literature along with primary data by traditional morphometrics of intersexual and populational average body size of the three different species. Regression models were designed to assess the rules separately and altogether. We have observed that the three species of Clibanarius showed interpopulational clines consistent with Bergmann’s rule. Surprisingly, our findings showed a gender-specific divergence from females as a response to latitudinal gradients, suggesting that latitude (as a proxy) increased the interpopulational body effect only in females. We suggest that phenotypic plasticity due to decreased selective pressure with higher latitude (decreased temperature) and greater productivity may affect the bias of these rules. Our data also suggest that female body size variation is modulated by the selection of fecundity in body size.