ADAPTIVE STRATEGIES OF BROWN FROGS (AMPHIBIA, ANURA, RANA) IN RELATION TO WINTER TEMPERATURES IN THE NORTHERN PALAEARCTIC

摘要

Adaptive strategies of the common frog, Siberian wood frog and Dybovsky's frog (Rana temporaria, R. amurensis, R. dybowskii) in relation to winter temperature was studied. They all tolerate only small negative temperatures in a state of supercooling and die during the next freezing. A threshold of long-term (10 days) tolerated temperatures appears to be similar in all species: only -1.5 degrees C. Solely R. amurensis survive without loss a short period (3 days) at -2.5 degrees C, whereas 50-60% individuals of the other two species perish. But a 10-day long exposure to that temperature is fatal to 90% of sampled Siberian wood frogs as well. The low cold-hardiness of the species in question shows that none of them (including R. amurensis which penetrates most deeply into the Subarctic and Arctic) can overwinter beyond water bodies in areas with grounds that freeze solid. Egg cold-hardiness of all of these species varies little and is low. The clutches that remain freezing outside water for several hours even at negative temperatures close to zero lose 100% embryos. Clutches submerged in water can be completely ice-included, but since even small (about 200 mL) clutches fail to freeze solid at -3 degrees C in two days, some embryos retain viability. The uniform cold-hardiness values obtained for the species under study reflect similar overwintering conditions in water bodies in cold regions, but fail to explain the differences in frog distribution ranges. All frog species that require overwintering in water can be concluded to have low cold-hardiness, whereas those that hibernate on land are cold-resistant to varying degrees.