The Age Composition and Postmetamorphic Growth Characteristics of the Moor Frog (Rana arvalis) from Habitats with a Short Activity Season

The between-population and sexual differences in demographic and postmetamorphic growth characteristics were studied in Rana arvalis from several habitats of Khanty-Mansiiskii Autonomous Okrug–Yugra (KhMAO) with a relatively short (about 3.5 months) activity season. Skeletochronology was used for age determination. The annual size increments and the rates of these increments were determined on the base of back-calculated body length at each age. In three of four populations, a higher average age in females but not significant sexual differences in the average body length were revealed. In both males and females of all studied KhMAO populations, the rate of the annual size increment between the 1st and 2nd wintering was maximal. The rate of the annual size increment between the 2nd and 3rd wintering was kept relatively high. In comparison with R. arvalis populations of Bryansk, Moscow, and Kirov oblasts with a longer activity season (seven, six, and five months, respectively), frogs from KhMAO populations had a relatively small average body length at each age and low population averages of the body length. At the same time, the character of between-age dynamics in the rate of the size increments of KhMAO populations enabled us to reveal the effects of counter-gradient selection not yet mentioned in the literature. These effects represent the maintenance of a relatively high rate of annual increments up to the 5th wintering in R. arvalis from KhMAO populations with a short activity season.