66.4 Monday, Jan. 6 08:45 Wood frog adaptations to overwintering in Alaska: New limits to freezing tolerance. LARSON, DJ*; MIDDLE, L; BARNES, BM; Univ of Alaska Fairbanks email@example.com
We investigated the ecological physiology and behavior of wood frogs (Lithobates [Rana] sylvaticus) overwintering in Interior Alaska by tracking animals into natural hibernacula, recording microclimate, and determining survival in spring. We measured cryoprotectant (glucose) concentrations in tissues from subsamples of naturally freezing frogs. We also recorded behavior of wood frogs preparing to freeze in artificial hibernacula, and tissue glucose concentrations in captive wood frogs frozen in the laboratory to -2.5°C. Wood frogs in natural hibernacula remained below freezing for 193±11 consecutive days and experienced average (Oct-May) temperatures of -6.3°C and minimum temperatures ranging from -8.9 to -18.1 °C with 100% survival (n=18). In early October, wood frogs underwent 10-15 freezing then thawing events before wood frogs froze for the remainder of winter. Tissue mean glucose concentrations were 13-fold higher in muscle, 10-fold higher in heart, and 3.3-fold in liver in naturally freezing compared to laboratory wood frogs frozen linearly. We mimicked the freeze-thaw cycles wood frogs experience in natural hibernacula in the laboratory and observed a net accumulation of glucose with each re-initiation of freezing. Tissue mean glucose concentrations in freeze-thaw wood frogs were 5.7-fold higher in muscle, 5.4-fold higher in heart, and 2.7-fold higher in liver than glucose concentrations from tissues in linearly frozen wood frogs. Wood frogs in interior Alaska survive freezing to extreme temperatures and durations compared to limits reported in animals collected in southern Canada or the US Midwest. We hypothesize that it is exposure to successive freeze-thaw cycles and the associated elevated cryoprotectant levels that produce the enhanced cold tolerance in wood frogs in Alaska.