Extended stretches of subfreezing temperatures can be challenging for animals who live in temperate climate zones. For some animals avoiding this problem is simple - migrate to a warmer location (birds), burrow beneath the freeze line (some snakes, mole salamanders), take up residence in a deep body of water that won't freeze completely (fish, turtles, some salamanders) or hibernate in a secluded den like a rock cavern or hollow log (bear, small mammals, terrestrial arthopods). What about animals in which none of the above choices are possible? Are they cruelly left to fend for themselves in the icy depths of winter with nowhere to go? Well, yes.
Certain species of frogs and caterpillars have a unique way of dealing with winter's frigid conditions - they simply freeze, or so it may seem. Gray treefrogs, wood frogs and chorus frogs have highly elevated levels of glucose in their bloodstreams that acts as a cryoprotectant in subfreezing temperatures. These frogs will take up residence under loose pieces of bark, rocks and leaf litter where they remain the entire winter - sometimes frozen solid. Higher concentrations of blood glucose enables them to survive by limiting ice formation in their tissues, as long as the duration and severity of cold conditions are not too extensive. Amazingly these organisms are able to respire anaerobically in a frozen state allowing them to make limited amounts of energy in the absence of bodily function. To see a picture of a frozen wood frog and gray treefrog click here.
Cryoprotectants not only allow these frogs to survive winter conditions, but also give them an advantage when it comes to breeding strategy. It's not surprising that the first frogs to breed in late winter and early spring are those with highest amounts of cryoprotectant in their bodies - wood frogs followed by spring peepers followed by the rest of the chorus frogs. Depending on the weather, wood frogs will breed in vernal pools with ice still on their edge at a time where few predators are prowling about. By being able to tolerate colder temperatures, wood frogs and chorus frogs have the advantage of arriving at vernal pools in large numbers without having to worry about mass predation from snakes and other warm-weather pool inhabitants.
*Side Note: Though we just passed the winter solstice and are in the midst of a multi-week cold snap, amphibian breeding season is not far away. I've seen wood frogs in pools as early as February 22nd in one of the warm winters of the early 1990's.