No Sexual Dimorphism Detected in Digit Ratios of the Fire Salamander (Salamandra salamandra)

Abstract

It has been proposed that digit ratio may be used as a biomarker of early developmental effects. Specifically, the second-to-fourth digit ratio (2D:4D) has been linked to the effects of sex hormones and their receptor genes, but other digit ratios have also been investigated. Across taxa, patterns of sexual dimorphism in digit ratios are ambiguous and a scarcity of studies in basal tetrapods makes it difficult to understand how ratios have evolved. Here, we focus on examining sex differences in digit ratios (2D:3D, 2D:4D, and 3D:4D) in a common amphibian, the fire salamander (Salamandra salamandra). We used graphic software to measure soft tissue digit length and digit bone length from X-rays. We found a nonsignificant tendency in males to have a lower 2D:3D than females; however, no sexual differences were detected in the other ratios. We discuss our results in the context of other studies of digit ratios, and how sex determination systems, as well as other factors, might impact patterns of sexual dimorphism, particularly in reptiles and in amphibians. Our findings suggest that caution is needed when using digit ratios as a potential indicator of prenatal hormonal effects in amphibians and highlight the need for more comparative studies to elucidate the evolutionary and genetic mechanisms implicated in sexually dimorphic patterns across taxonomic groups.

Mixed Sex Effects on the Second‐to‐Fourth Digit Ratio of Túngara Frogs (Engystomops pustulosus) and Cane Toads (Rhinella marina)

“…Our sample size represents an exhaustive search for adults for about 3 months that resulted in frequent recaptures suggesting we had sampled the majority of active adults. Although our sample size for cane toads is much smaller than our t ungara frog sample, it is similar to those of other studies examining sexual differences digit ratios (Balogov a et al, 2015). Adults were recognized as individuals larger than 9 cm following Zug and Zug (1979).…”

“…In two species of anurans, males exhibit a larger digit ratio than females (Chang, 2008, Direnzo andStynoski, 2012), while there is no difference between the sexes in a third species (Germano et al, 2011). Two recent studies on five species of caudates found convincing evidence that females had larger digit ratios than males in one species, but found inconsistent or no sex-biased pattern in the digit ratios in four other species (Balogov a et al, 2015;Kaczmarski et al, 2015).…”

Flesh and bone: An integrative approach towards sexual size dimorphism of a terrestrial salamander (genus Salamandrina)

“…Hands and feet, respectively, were placed dorsal side up on an ocular micrometer and enclosed by a second glass slide to obtain straightened digits without compression (Balogová et al, ). Digital photographs were taken with a Canon EOS 550D DSLR with an attached EF‐S 60 mm Macro lens from a fixed distance.…”

A non-invasive geometric morphometrics method for exploring variation in dorsal head shape in urodeles: sexual dimorphism and geographic variation inSalamandra salamandra

“…In spite of the high morphological diversity observed at the intraspecific level, there is a limited number of morphological studies on this species, most of which are focused on coloration (Bosch and López‐Bueis, ; Pasmans and Keller, ; Balogová and Uhrin, ; Beukema et al, ). Some studies have also explored sexual dimorphism in size (Degani, ; Cordero‐Rivera et al, ; Balogová et al, ) and shape (Labus et al, ), showing that, as in many other urodeles, females are the larger sex, with relatively larger heads, interlimb distances and parotid glands while males have longer limbs and feet (Labus et al, ; Reinhard et al, ). However, although snout shape has been considered as a key trait to identify sexual dimorphism (Labus et al, ) and to distinguish between subspecies in S. salamandra (Bas and Gasser, ; Velo‐Antón and Buckley, ) head shape variation has been poorly studied.…”