Assessing Rensch’s rule in a newt: Roles of primary productivity and conspecific density in interpopulation variation of sexual size dimorphism

Abstract

[Aim]: Major efforts have been devoted to understanding the geographic pattern of sexual size dimorphism (SSD). Rensch's rule posits that SSD increases with body size in male-biased SSD species and decreases with body size in female-biased SSD species. This pattern, and its inverse, have been mainly explored at the interspecific level, whereas research at the intraspecific level has been largely neglected. Here, we test whether the allometric pattern of SSD in an urodele amphibian conforms to Rensch's rule and evaluate the relative role of four potential mechanisms: sexual selection, fecundity selection, density-dependent resource availability and differential plasticity.

[Location]: Iberian Peninsula.

[Taxon]: Lissotriton boscai (Amphibia: Caudata: Salamandridae).

[Methods]: We used original (field-based) and published data on body size, courtship behaviour and fecundity parameters and a suite of climatic, ecological and genetic (mitochondrial and nuclear markers) predictors to assess these hypotheses.

[Results]: The results showed that SSD increased with increasing female mean body size, supporting the inverse Rensch's rule pattern. Primary productivity-related variables and female density were among the most relevant ecological predictors of SSD after accounting for genetic structure and capture date.

[Main conclusions]: This study reveals that the interplay between the density-dependent resource availability hypothesis and the differential plasticity hypothesis explains the inverse Rensch's rule. We discuss how combining biogeographical and experimental approaches can provide alternative interpretations to the classical sexual and fecundity selection hypotheses on the interpopulation variation in SSD.