Females Have Larger Ratio of Second‐to‐Fourth Digits Than Males in Four Species of Salamandridae, Caudata

Abstract

Digit ratio (2D:4D) denotes the relative length of the second and fourth digits. It is considered to be a suitable biomarker of the in utero balance of fetal sex hormones, which affect early development of individua ls behavioral and morphological characteristics. In recent decades, digit ratio attracted a great attention in biology and psychology. However, for unmasking the biological basis of the phenomenon, extensive studies on nonhuman animals are necessary. Despite it was hypothesized that digit ratio is well conserved in all Tetrapoda, and there exist studies on mammals, birds, and reptiles, there are only two such study on anuran amphibians. Therefore, the aim of this study is to investigate the 2D:4D in the most basal salamanders, Caudata. We have studied digit ratio in four species of newts: Triturus cristatus, Mesotriton alpestris, Lissotriton montandoni, and Lissotriton vulgaris, using museum collection. We used computerized measuring of each limbs photos. We have found out that, in M. alpestris, females 2D:4D of all four limbs were significantly larger than in males. In L. montandoni and L. vulgaris, only 2D:4D of rear limbs significantly differed, in females being larger. In T. cristatus, digit ratios of males and females did not statistically differ. Thus, the results confirmed our hypothesis that at least in M. alpestris, L. montandoni, and L. vulgaris, females seem to have larger 2D:4D comparing to males, the pattern known from most mammals and opposite to birds, reptiles and anuran amphibians.

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

“…Gray columns—nonsignificant sexually dimorphic pattern of digit ratios, light green—mammalian‐like sexually dimorphic pattern of digit ratios, dark green—avian and reptilian‐like sexually dimorphic pattern of digit ratios, nonhatched columns—front limbs, hatched columns—back limbs. Literary sources: (1) Voracek and Pietschnig, ; (2) Gooderham and Schulte‐Hostedde, ; (3) Zhao et al, ; (4) Roney et al, ; (5) Lombardo et al, ; (6) Forstmeier, ; (7) Dreiss et al, ; (8) Lombardo and Thorpe, ; (9) Direnzo and Stynoski, ; (10) Kaczmarski et al, .…”

“…Our understanding of the evolutionary and developmental processes that underpin digit ratios grows with each new species. Species studied to date include primates (Roney et al, ; McIntyre et al, ; Nelson and Shultz, ), laboratory mice ( Mus musculus ) (Brown et al, ), field voles ( Microtus agrestis ) (Lilley et al, ), wood mice ( Apodemus sylvaticus ) (Leoni et al, ), David's rock squirrels ( Sciurotamias davidianus ) (Lilley et al, ; Zhao et al, ), red squirrels ( Tamiasciurus hudsonicus ) (Gooderham and Schulte‐Hostedde, ), house sparrows ( Passer domesticus ) (Navarro et al, ), ring‐necked pheasants ( Phasianus colchicus ) (Romano et al, ), tree swallows ( Tachycineta bicolor ), budgerigars ( Melopsittacus undulates ), chickens ( Gallus domesticus ) (Lombardo et al, ), zebra finches ( Taeniopygia guttata ) (Burley and Foster, ; Forstmeier, 2005, ), dark‐eyed juncos ( Junco hyemalis ) (Cain et al, ), collared flycatchers ( Ficedula albicollis ) (Garamszegi et al, ), pied flycatchers ( Ficedula hypoleuca ) (Ruuskanen et al, ), barn swallows ( Hirundo rustica ) (Dreiss et al, ), common wall lizards ( Podarcis muralis ) (Rubolini et al, ), humble anoles ( Anolis humilis ) (Direnzo and Stynoski, ), green anoles ( Anolis carolinensis ) (Chang et al, ; Lombardo and Thorpe, ), border anoles ( Anolis limifrons ), tree skinks ( Mabuya planifrons ) ( Rubolini et al, ), painted dragon lizards ( Ctenophorus pictus ) (Tobler et al, ), species from the family Iguanidae (Gomes and Kohlsdorf, ), Bransford's robber frogs ( Craugastor bransfordii ) (Direnzo and Stynoski, ), strawberry poison dart frogs ( Oophaga pumilio ) (Chang, ), great creasted newts ( Triturus cristatus ), alpine newts ( Mesotriton alpestris ), Carpathian newts ( Lissotriton montandoni ), and smooth newts ( Lissotriton vulgaris ) (Kaczmarski et al, ). As might be expected with such a diverse array of taxa, sexually dimorphic patterns in digit ratios have been shown to vary across species (Fig.…”

“…Direnzo and Stynoski () also detected higher 2D:4D ratio on the front limbs in males of two frog species, strawberry poison frog and Bransford's robber frog. In a recent study of newts (order, Caudata, Salamandridae), a mammalian‐like pattern (i.e., males with lower 2D:4D ratio than females) was detected in three species, Alpine newts ( Ichthyosaura alpestris ) (all four limbs), Carpathian newts ( Lissotriton montandoni ) (rear limbs only), and smooth newts ( Lissotriton vulgaris ) (rear limbs only) (Kaczmarski et al, ). However, no sexual differences were found in northern crested newts ( Triturus cristatus ) (Kaczmarski et al, ).…”

“…In a recent study of newts (order, Caudata, Salamandridae), a mammalian‐like pattern (i.e., males with lower 2D:4D ratio than females) was detected in three species, Alpine newts ( Ichthyosaura alpestris ) (all four limbs), Carpathian newts ( Lissotriton montandoni ) (rear limbs only), and smooth newts ( Lissotriton vulgaris ) (rear limbs only) (Kaczmarski et al, ). However, no sexual differences were found in northern crested newts ( Triturus cristatus ) (Kaczmarski et al, ). Although general patterns of sexual dimorphism can be detected at higher taxonomic levels (Class, Order), patterns are often not clear‐cut at lower levels and variability across some taxa has been difficult to explain.…”

“…Evidence is gradually accumulating to suggest that sexually dimorphic patterns across species in different taxonomic classes may not only be associated with hormonally sensitive HOX genes, but could also be linked to differences in genetic sex determination (GSD) system (Chang et al, ; Lombardo et al, ; Direnzo and Stynoski, ; Kaczmarski et al, ). In mammals, males are the heterogametic sex (XY), while in birds females are heterogametic (ZW) (Graves, ; Lombardo et al, ).…”