Introduction
Wood Frogs are famous for their ability to survive freezing during winter hibernation. They are no ordinary animals — whereas most ecotherms (cold-blooded animals) would die if subjected to below freezing temperatures, Wood Frogs can survive temperatures as low as -6C! How do they pull off this trick? Read on to find out…
Photo of Wood Frog (Rana sylvatica) egg masses courtesy John White – Virginia Herpetological Society.
Featured image atop page is by Lindsey Swierk from the 2017 SAVE THE FROGS! Photo Contest.
Habitat
Wood frogs are amphibians; they spend part of their lives as tadpoles and part out of water as adults. This species has a distinctive dark “mask” extending back from the eye, meanwhile dorsal coloration varies from nearly pink to shades of brown to nearly black. It is the only North American frog found north of the Arctic Circle. They occur from the southern Appalachian Mountains of Georgia, north into Canada above the Arctic Circle, and west to Alaska. They are found in a variety of habitats including tundra, subalpine woodlands, willow thickets, wet meadows, bogs, and temperate forests (both coniferous and deciduous) of various canopy species associations.
Photo of Wood Frog (Rana sylvatica) in shallow water courtesy John White – Virginia Herpetological Society
Reproduction
Wood frogs lay their eggs in early spring in the water, normally in ponds that do not support large predators such as fishes. The females will lay their eggs in clutches of between 1,000 and 3,000 eggs, which is why you may find large numbers of tadpoles in close proximity. The eggs hatch about one month later if the water is cool, but in warm water, hatching can occur as soon as nine days. The tadpoles develop over the next two months. The tadpoles are brown or green in color, depending on their environment; some species have black spots along their backs for camouflage purposes. The tadpoles eat algae and other small creatures that live in or near the water. They are omnivores, so they can also eat insects. The adult wood frogs are carnivores and eat insects, but they may also consume detritus, eggs of other amphibians, and plant matter.
Photo of Wood Frogs (Rana sylvatica) in amplexus mating embracing courtesy John White – Virginia Herpetological Society
Dealing With Freezing Temperatures
Wood frogs (Rana sylvatica) have a special way of dealing with freezing temperatures. This frog has developed a strategy to fight against inclement weather. Specifically it can produce antifreeze that protects it from temperatures as low as -6ºC. They accumulate glucose as a cryoprotectant that is derived primarily from massive hepatic glycogen stores built up before hibernation. Glycogenolysis is triggered immediately when ice begins to form on the skin of the frog and glucose is distributed to all organs by the action of the cardiovascular system before heartbeat and blood flow finally stop. Glucose is produced in the liver and it begins to synthesize it at the onset of freezing. Glucose is concentrated inside cells and acts as an antifreeze, preventing cell fluids from freezing.
On the other hand they have a large amount of nucleoproteins (nucleotides + proteins) in their bloodstream. These compounds enhance the formation of ice and prevent it from being organized in the form of large crystals that would damage cells.
Photo of Wood Frogs (Rana sylvatica) egg masses underwater courtesy Kristiina Hurme
Conclusion
Wood Frogs (Rana sylvatica) are one of the most fascinating amphibians on Earth. Although they are listed as Least Concern, there are some threats that affect their survival, such as the destruction of their habitat (especially the felling of wood for commercial purposes) and draining of their wetlands for agricultural purposes.
Additional Resources
Virginia Herpetological Society Wood Frog Page
Wild Adirondacks Wood Frog Page
