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Adaptive Strategies of Frogs to Wintering Temperatures on the Holarctic North Margins

   First of all, the North is interesting for biologists because of its extreme conditions, which restrict penetration of many organisms, and, due to this fact, the species diversity is known to be not numerous there. It also refers to terrestrial cold-blooded animals, whose life totally depends on external temperature, as, for example, amphibian. Nevertheless, some of amphibians “burst” into the Subarctic and even the Arctic.

   Schematic map of the area in the Holarctic inhabited by brown frogs (filled with dots): common frog, moor frog, Siberian wood frog, Dybovsky’s  frog and wood frog.

   Among the tailed amphibians only the Siberian Salamander (Salamandrella keyserlingii), tolerating temperatures down to -55⁰C, colonized almost all North Asia and some part of the European territory. At the same time, frog areals compose northern border of the Holarctic. It is formed by the wood frog (Rana (Lithobates) sylvaticus) in North America, the common frog (Rana temporaria), the moor frog (R. arvalis), the Siberian wood frog (R. amurensis) and the Dybovsky’s frog (R. dybowskii) in Eurasia. Studying of their adaptive strategies in the North, which is absolutely extrinsic landscape area for this class, is one of the research spheres of the laboratory of Biocenology in our Institute.

   The mentioned amphibians have no universal strategy in relation to methods of low temperatures surviving. Each species could escape them going into water during severe time periods. However, the majority of species, except for the Siberian Salamander and the North American wood frog, wintering ashore only, are known to hibernate both in water and on the shores. Studying of the amphibian hibernation conditions outside water, especially in cold regions, is rather time-consuming. It’s for a reason that literature supplies a lot of data about findings of the hibernating animals in the anthropogenic conditions (wells, cellars, vaults, basement of haystack, etc.), whereas the descriptions of such findings in nature are rare. That is why the laboratory staff took a different approach to the problem: they studied the most possible cold hardiness of only three frog species mentioned above (the common, Siberian wood and Dybovsky’s frogs). That approach allowed making a conclusion about the ability or inability to tolerate low negative temperatures, i.e. to winter both ashore and in water or just in water. By the start of the experiment two species of frogs from the most northern ones had been known to survive freezing -16⁰C – R. (L.) sylvaticus and -10⁰C – R. arvalis and winter ashore.

   The common, Siberian wood and Dybovsky’s frogs appeared to survive only small negative temperatures and die during freezing. The Siberian wood frog is an inhabitant of the extreme winter regions of North Asia, penetrating along the valleys of the Yana, Omolon and Lena to 69-71⁰ NL. It can tolerate temperature only above -1.5⁰C for a long time (10 days), i.e. it’s cold hardiness corresponds to that of more “southern” species (the common and Dybovsky’s frogs). However, unlike the two latter, R. amurensis is active under  -1…-1.5⁰C. It can move even without disturbance. Only R. amurensis survived without loss (50-60% individuals of other species died) under -2.5⁰C during 3 days, but a 10-day period under that temperature was fatal almost for all exemplars of the Siberian wood frog.

   Low cold hardiness of the studied frogs evidences their inability (including R. amurensis , penetrating deep into the Subarctic and the Arctic of Asia) to survive winter outside the water basins. All of them use “passive adaptive way” (according to Chernov, 1984) during colonization of the northern territories. The ability to survive under small negative temperatures (down to -2…-2.5 ⁰C) and tolerate incomplete and short freezing is “helpful” for the common, Siberian wood and Dybovsky’s frogs living in the regions with mild winter (characterized by a short and weak cold spell). They freeze and die inevitably under the long freezing or lower temperatures.

   Though all the three studied species of the frogs turned out to have almost the same and low cold hardiness which forces them to winter in water in the northern regions, their areals are crucially different. It is evident, that the character of geographic distribution of the common, Siberian wood and Dybovsky’s frogs is not connected with cryoresistance and is apparently determined by a number of factors.

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Details of the study one can find in Adaptive strategies of the brown frogs (Amphibia, Anura, Rana) in relation to winter temperatures in the north Palaearctic (Berman D.I., Bulakhova N.A., Meshcheryakova E.N.// Zoologichesky zhurnal. 2017. V. 96, №11. P. 1392-1403).

 

The material is represented by N.A. Bulakhova