WOOD FROG Rana sylvatica
The wood frog is widespread throughout northern North America, and is the only amphibian that lives north of the Arctic Circle. This abundant frog is the most widely distributed amphibian in Alaska, ranging from the mainland of Southeast Alaska north to the Brooks Range, and is the sole amphibian found north of Prince William Sound. They inhabit a variety of habitats including mixed forests, open meadows, muskeg, tundra, and even landscaped spaces in urban and suburban areas. Wood frogs are highly terrestrial, and are only found in water during breeding and early development. Although adults can be as long as 3 inches (7.6 cm), they are frequently smaller. This smooth skinned frog may be brown, tan, grey, or green above, with a uniformly cream colored underside. Distinguishing characteristics generally include a prominent dark eye mask and a contrasting light colored lip line running from the snout tip to the rear edge of the mask. Their toes are incompletely webbed.
The wood frog is capable of surviving the frigid Arctic winter because it is one of the most freeze tolerant species on Earth; it has the amazing ability to freeze solid and thaw out as temperatures warm in the spring. Wood frogs hibernate in shallow bowl-shaped depressions under a layer of dead vegetation (duff), with snow cover providing additional insulation. At the onset of freezing temperatures, wood frogs begin pumping much of the water out their cells and organs and into extracellular spaces and body cavities. At the same time, they pump large amounts of glucose (a sugar created in the liver) into their cells. The syrupy glucose solution inside the cells serves as a cryoprotectant (antifreeze), protecting the cells themselves from freezing and from desiccation. Within a few hours, ice fills the abdominal cavity and encases all internal organs. Flat ice crystals form between layers of skin and muscle, and the eyes turn white because the lens and fluids freeze. Nearly 70% of the frog's total body water is converted to ice. The blood freezes, the heart stops beating, all breathing and muscle movements cease, and the wood frog remains in a virtual state of suspended animation until it thaws.
In early spring (April and May in Alaska), as ice begins to melt along the shores of ponds or lakes, wood frogs thaw and re-animate. Within hours, male wood frogs make their way to shallow breeding ponds where they begin attracting females with loud, staccato calls that sound like duck quacking. Wood frogs will breed virtually anywhere that has standing water for at least part of the summer, including ponds, bogs, marshes, temporary pools, tire tracks, or roadside ditches. Calling and breeding are explosive and last only a few weeks. The globular masses of hundreds or
thousands of eggs are fertilized externally, and are usually found attached to vegetation just below the surface. After fertilization, adults abandon the eggs and disperse over land, where they feed on insects, worms, and other invertebrates and, in turn, are preyed upon by birds and larger animals. Development from egg to tadpole to frog occurs very rapidly to ensure complete metamorphosis before the water body either dries out or freezes over. Although development and growth rate depend on water temperature and food availability, eggs generally hatch in about a week, and tadpoles metamorphose into little froglets in about eight weeks. In Alaska, look for masses of nickel-sized froglets emerging from their watery nurseries late July and August. Recent studies of wood frogs in National Wildlife Refuges in Alaska have found some of the highest rates of physical abnormalities (missing, shrunken, or misshaped limbs, or abnormal eyes) documented in the published literature. The cause for the high prevalence of abnormalities is unknown, but hypotheses include chemical contaminants, parasites, ultraviolet radiation, predators, extreme temperatures during development, or a combination of these factors.
(Alaska Dept. of Fish & Game; Kevin Broderson, revised by Dave Tessler 2008)
(Wood Frog Photo by Barbara Logan; Wood Frog Tadpole Photo by )