From the USGS:
Wood frogs, Rana sylvatica, are the earliest breeding frogs at Patuxent. These frogs are one of the few amphibians that actually produce their own antifreeze within their blood, allowing them to survive cold temperatures while hibernating just under the leaf litter. Like spotted salamanders, these frogs have been a focal species for amphibian monitoring at Patuxent for the past 5 years. Photo by Evan H.C. Grant, USGS. Images Link.