Sexual Dimorphism in the Ontogeny of the Black Forest–Steppe Adder (Pelias berus nikolskii (Vedmederja, Grubant et Rudaeva 1986), Serpentes, Viperidae) from Voronezh Oblast, Based on Traditional Measurements

Males and females of the black forest–steppe adder (Pelias berus nikolskii) inhabiting Voronezh oblast, apart from differences in tail length, show external similarities. Individuals have a black body; females may have red–brown spots on their heads. We studied the differences in the ontogeny of 118 males and females caught between 2008 and 2017 in seven locations in Voronezh oblast. Seven traditional body measurements and one calculated trait of the overall head size were used for the analysis. Ontogenetic allometry was examined in both sexes. Allometric trajectories were constructed using reduced major axis (RMA, model II) regression. Bootstrap the residuals were used to test for the significance of the angles between the trajectories for the different sexes. A comparison of snake sizes at birth was conducted using a permutation test. To calculate both the sexual dimorphism index (SDI) and the allometry line of sexual size dimorphism (SSD), the residues obtained from the calculations of ontogenetic allometry trajectories were used, to which the predicted measurement values were added. The head size in relation to the body length was found to decrease with age. At birth, females were larger than males in the following traits: relative width of the head at eye level, relative length of the pileus, relative muzzle length, and overall head size. In the adult state, higher values of these traits were found in males. Males have longer tails. Differences in tail length appear at birth and persist throughout growth. Sex differences in growth changes in the traits studied, with the exception of the differences in head length, greatest width of the head, and tail length, are non-allometric in nature and, in addition to prenatal divergence, are caused by a greater growth rate in males. The growth of the tail in both sexes appears to be isometric. Rensch’s rule predicts that SSD will be positively correlated with size in taxa in which males are the larger sex, but negatively correlated with size in taxa in which the larger sex is the female. A study of SSD allometry showed that only width of the head at eye level, length of the pileus, muzzle length, and overall head size are consistent with this rule. It is therefore assumed that the larger head in males could have been formed under the action of sexual selection. This may be due to the need for them to participate in combat for access to females during the mating season.