Rensch's rule and sexual dimorphism in salamanders: patterns and potential processes

Body size is influenced by the interaction of multiple forces, whose effects can determine the occurrence of sexual size dimorphism (SSD). Rensch's rule is the increase of SSD with body size in taxa where males are the largest sex, and the opposite pattern in female-biased SSD taxa. This pattern was detected in many animal groups, but contrasting results were also highlighted. This study evaluated the existence of Rensch's patterns for body size and for the number of caudal vertebrae in salamandrid caudate amphibians. Furthermore, we tested the support of alternative hypotheses on processes that may determine allometric patterns: sexual selection, fecundity selection and constraining selection by performing separate analyses on species with male- and female-biased SSD. We used the literature and original data to gather information on body size and number of caudal vertebrae in 52 species of salamandrids over four continents. We then tested the support of the three hypotheses using a phylogenetic approach. Rensch's rule was valid for body size in salamanders only for species with male-biased dimorphism. No allometric relationships were detected by analyses on all the species, or by analyses on female-biased SSD species. Analyses performed on the number of caudal vertebrae showed no significant patterns. Our study supports the role of sexual selection in promoting positive allometry for body size in male-biased SSD species, whereas the alternative hypotheses were not supported by our data. These results highlight the importance of distinguishing male- and female-biased species as different evolutionary pressures and constraints may be at the basis of evolution of SSD in these groups.