Wood frogs (Rana sylvatica) can be distinguished from other local frogs by their dark “mask” extending back from each of their eyes. The rest of their body can vary from light tan to reddish brown; in fact, a single individual can vary their coloring seasonally. While other species, like the green frog, are often found in close proximity to water year round, wood frogs are rarely seen in association with water other than during the spring breeding season when they migrate to and congregate in vernal pools en masse. For the rest of the year, wood frogs can be found in upland forests. The individual pictured here was found in a dry upland pine-oak forest habitat.  

Because amphibians are cold blooded they cannot internally regulate their body temperature; during the cold winter months most frogs bury themselves in the soil below the frost line to hibernate. Wood frogs, however, are especially adapted for colder climates. In New England, they are typically the last frogs to hibernate in the fall, and are often the first to emerge in the spring (I have seen wood frogs breeding in vernal pools that still have plates of ice fringing the edges). During hibernation, wood frogs typically burrow no deeper than under the leaf litter or just below the surface of the soil, rather than below the frost line as do other species. This surface hibernation is possible because wood frogs can actually tolerate the freezing of their blood and other tissues. Wood frogs accumulate glucose (sugar) in their bloodstream, which acts as a natural antifreeze to protect them through this period of freezing. Laboratory studies have shown that wood frogs can survive the freezing of up to 60-70% of the water in their bodies. Given their unique ability to withstand freezing it is unsurprising that wood frogs are the only frog found north of the Arctic Circle. Despite being essentially frozen solid, wood frogs are 100% functional within hours of thawing out in the spring.