Blanchard's Cricket Frogs (Acris crepitans blanchardi) in the central portion of their range show minimal capacities for freezing tolerance and survive overwinter by using terrestrial hibernacula where they avoid freezing. However, frogs may exhibit greater freeze-tolerance capacity at high latitude range limits, where winter climate is more severe. We studied freezing tolerance, glucose mobilization during freezing, and hibernacula microclimates of cricket frogs in southeastern South Dakota, at the northwestern limit of their range. Cricket frogs from South Dakota generally survived freezing exposure at −1.5 to −2.5°C for 6-h periods (80% survival), but uniformly died when exposed to these same temperatures for 24-h freezing bouts. Hepatic glucose levels and phosphorylase a activities increased significantly during freezing, but hepatic glucose levels during freezing remained low, only reaching levels approximating those prior to freezing in freeze-tolerant species. Moreover, muscle glucose and hepatic glycogen levels did not vary with freezing, suggesting little mobilization of glucose from hepatic glycogen stores during freezing, contrasting with patterns in freeze-tolerant frogs. Temperatures in soil cracks and burrows potentially used for hibernacula were variable, with some sites remaining above the freezing point of the body fluids throughout the winter, some sites dropping below the freezing point for only short periods, and some sites dropping below the freezing point for extended periods. These data suggest that cricket frogs in South Dakota, as in other portions of their range, survive overwinter by locating hibernacula that prevent freezing, although their toleration of short freezing bouts may expand the range of suitable hibernacula. These data also suggest that overwinter mortality may be high at the northern range boundary and might limit cricket frogs from expanding their range northward.
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Vol. 2010 • No. 2