Middle, L.B.*; Barnes, B.M: Overwintering strategies of the wood frog, Rana sylvatica, in interior Alaska
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 of body tissues, an adaptation shared by only six species of vertebrates. Laboratory studies of limits to cold tolerance of R. sylvatica have concentrated on individuals from southern populations from Ohio, USA and Ottawa, Canada. These studies determined a lower lethal body temperature (50% survival) of -8oC and limits to survival duration of freezing of ca. one month. Within the range of their distribution in Alaska, average ambient air temperatures range from -19.4oC to below -30oC from October to April, and soil-snow interface temperatures are regularly below -10oC, 2oC below the lower lethal temperature. Wood frogs overwinter in terrestrial habitats with individuals found within the leaf layer 3-6 cm below the surface. Wood frogs in Alaska may survive prolonged freezing and minimum temperatures below lower lethal temperatures known from southern populations. Alternatively, Wood frogs in Alaska may share limits to freeze tolerance with southern populations and survive at high latitudes by finding hibernacula that are protected from extreme temperatures. We explore potential mechanisms of extended freeze tolerance by determining hepatic glycogen and tissue levels of glucose in captive animals before, during and after freezing to these new limits. This comparison adds insight as to whether Wood frogs in Interior Alaska differ physiologically in their mechanisms of freeze tolerance from southern populations. In addition, we present temperatures experienced by naturally hibernating Wood frogs in Interior Alaska.