Allometry, sexual dimorphism, and Rensch’s rule in pygmy and marbled newts

Allometry, the relationship between body size and the size of other body parts, explains a significant portion of morphological variation across biological levels, at the individual level, within and between species. We used external morphology measurements of 6 Triturus (sub)species, focussing on the T. marmoratus species group, to explore allometric parameters within and between taxa. We tested for allometry of sexual size dimorphism in body, head, and limb dimensions and examined whether intraspecific allometry directed evolutionary allometry, as described by Rensch's rule. Our findings indicated that female-biased trunk and head dimensions exhibited positive allometry, whereas male-biased limb dimensions showed isometric relationships or weak correlations with body size. Morphological divergences between sexes occurred along common allometric slopes, most often through changes in the intercepts. Among taxon, comparisons revealed that (sub)species diverged in the direction of the allometric slopes. In line with Rensch's rule, sexual size dimorphism in female-biased traits significantly decreased as overall body size increased. However, the observed intraspecific allometric parameters deviated from theoretical expectations because the steepest allometric slopes for female-biased traits were recorded in the larger species. Our results contribute to understanding the dynamics of allometric relationships and sexual dimorphism in amphibians and provide a robust baseline for future comparative analyses.