Are there real sexual morphometric differences in yellow-bellied toads (Bombina spp.; Bombinatoridae)?

Abstract

In this study, we explored the level and pattern of sexual size dimorphism and sexual shape dimorphism in two closely related Bombina species that have low levels of sexual dimorphism in body size and shape. We applied an experimental protocol to explore sexual variations in morphological traits, including a preliminary evaluation of the measurement error. Mean measurement error (MME) and measurement error index (MEI) were estimated on each of the eleven morphometric variables to exclude any possible subjective factor in measuring and to perform, for the first time, an objective functional and statistical evaluation of sexual size differences in the two species. Even if statistically significant, each difference that lies below the level of uncertainty of the measure could not be reliable. Therefore, statistically significant differences in head shape have been rejected, due to an average difference between males and females smaller than the possible MME. We detected significantly longer distal segments of the hind limbs in males, which could account for their use in mating behaviour (e.g. scramble competition, water-wave communication). However, major and more reliable evidences of sexual dimorphism have been found on forelimb measures (MEI > 1), in particular humerus length and amplexus, which are significantly larger in males than in females. These results indicate a mating related sexual dimorphism, when larger and stronger forelimbs can give an advantage during coupling as well as during male-male fighting. The mean measurement error values and formulas provided in this work could be applied to future morphometric studies on Bombina species.