Growth Layers and Its Complex Structure in a Common Species Under Uncommon Conditions: Pelophylax ridibundus in the Talysh Mountains

In the Talysh Mountains, the marsh frog, Pelophylax ridibundus (Pallas, 1771), increases its distribution due to human activity. In the mountain forest belt, frogs inhabit flowing ponds with cold spring water. These reservoirs are characterized by a stable temperature regime: the water in them is kept at the level of 10 – 12°C in winter and does not fall below 6°C, and in summer does not rise above 18°C. Probably, the cold flow water of ponds in the mountain-forest belt of Talysh can cause some features of growth and maturation for P. ridibundus. The aim of our work was to study the features of the structure of growth layers including the variation in degree of expression of lines of arrested growth (LAGs) of the marsh frogs in ponds with cold flow water. Frogs were collected in the upper part of the Tangeru River gorge in Sym village of Astara District of Azerbaijan (480 m a.s.l.) in August 2018. In total, we studied skeletochronologically 8 females and 9 males. In contrast to the results of the tubular bones studies in the marsh frog presented in earlier works, the studied individuals are characterized by a more complex and diverse structure in the cross section of shin bones. All the revealed diversity of this structure can be divided into three groups. To the first group are relatively rare cases in which there are growth layers with wintering LAGs and additional growth layers but without additional LAGs. The second group includes more frequent cases in which there are growth layers not only with wintering LAGs but with additional LAGs differing discretely from wintering LAGs. The third group includes the rarest cases where there are growth layers with wintering LAGs and with additional LAGs without discrete difference from wintering LAGs. The revealed high variable structure of growth layers and distinctiveness of LAGs in P. ridibundus can be explained by unusual temperature regime in habitat of studied population and the existence of several activity period during a year. The most continuous period of low temperatures (February and March) corresponds to formation hibernation LAGs and dark rings in growth layers. Besides, period from early December to late January may be so cold that frog growth retard sufficiently or stop completely. The formation of more dark parts of growth layers or even several additional LAGs can correspond to this period.