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

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Amphibian morphology

Background:

• Allometry, the relationship between body size and other body parts, is a key driver of morphological variation.
• Sexual size dimorphism (SSD) is common in many species, but its allometric basis is not fully understood.
• Rensch's rule predicts how SSD changes with body size, but empirical evidence varies.

Purpose of the Study:

• To investigate allometric parameters within and between Triturus (sub)species, focusing on the T. marmoratus species group.
• To test for allometry of sexual size dimorphism in body, head, and limb dimensions.
• To examine if intraspecific allometry influences evolutionary allometry according to Rensch's rule.

Main Methods:

• External morphology measurements of six Triturus (sub)species.
• Analysis of allometry in sexual size dimorphism for body, head, and limb dimensions.
• Comparison of intraspecific allometric slopes with interspecific evolutionary allometry.

Main Results:

• Female-biased trunk and head dimensions showed positive allometry; male-biased limb dimensions showed isometric relationships.
• Morphological divergence between sexes occurred along common allometric slopes, primarily through changes in intercepts.
• While SSD in female-biased traits decreased with body size (supporting Rensch's rule), steeper allometric slopes were observed in larger species, deviating from theoretical expectations.

Conclusions:

• Intraspecific allometry in Triturus amphibians provides insights into the evolution of sexual dimorphism.
• Deviations from Rensch's rule highlight the complexity of allometric scaling and its influence on evolutionary trajectories.
• This study establishes a baseline for future comparative analyses of allometry and sexual dimorphism in amphibians.