MIDDLE, L.B.; BARNES, B.M.: Overwintering physiology of the wood frog, Rana sylvatica, in Interior Alaksa
The wood frog, Rana sylvatica, is one of the most northerly distributed amphibian species with populations ranging up to 100 miles north of the Arctic Circle. The key to its survival at high latitudes is the ability to survive extracellular freezing in body tissues, an adaptation shared by only six other species of vertebrates. Within their distribution in Alaska, average ambient air temperatures range from -19.4oC to below -30oC from October to April. Laboratory studies concentrating on wood frogs from southern populations in North America have determined a lower lethal body temperature (50% survival) of approximately -8oC and freezing duration survival limits of about one month. In a mild Winter, nine out of nine hibernating wood frogs in Fairbanks survived freezing temperatures averaging -5.7oC and extremes of -18oC for 186 days, far beyond lethal limits determined in the laboratory of southern populations. We explore potential mechanisms of extended freeze tolerance by comparing hepatic glycogen and tissue levels of glucose in captive animals before, during and after freezing. This comparison adds insight to whether wood frogs in Interior Alaska differ genetically in their mechanisms of freeze tolerance from southern populations. Alternatively, slower cooling rates or freeze-thaw cycles may enhance the production and distribution of glucose.
