Meeting Abstract
Ice nucleation of wood frogs (Lithobates sylvaticus) induces production of glucose, a cryoprotectant that is necessary for freeze tolerance. Under laboratory conditions, freezing of wood frogs increases glucose concentrations within heart, liver, and leg muscle tissues by 5-fold or more compared to levels in unfrozen frogs. To induce these cryoprotectant concentrations, the standard laboratory practice is to cool wood frogs at rates of -0.05°C h-1 or slower; whereas, under natural conditions in Alaska wood frogs cool at rates of -0.35 to -1.6°C h-1. Despite rapid cooling rates, glucose concentrations in naturally frozen wood frogs are 12-fold higher in muscle, 10-fold higher in heart, and 3.3-fold higher in liver than corresponding levels in laboratory frozen wood frogs. Under natural conditions, wood frogs undergo multiple freezing-thaw cycles before remaining frozen for the winter. We examined if ecologically relevant repeated freeze-thaw events cause the higher glucose concentrations found in naturally frozen wood frogs. We found that over successive freezing events, glucose concentrations increased stepwise within all measured tissues. Short periods of thawing after freezing did not result in significant decline of tissue glucose concentrations. Wood frogs that experienced three freeze-thaw events had glucose concentrations that approached values of wood frogs frozen in natural conditions. Unlike laboratory wood frogs that only survive frozen for up to 2 months, wood frogs frozen under natural conditions survive frozen for up to 7 months with temperature minima below -18°C. We hypothesize that repeated freeze-thaw cycles allows for greater survival in Alaskan wood frogs through enhanced cryoprotectant production.
