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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

A mucous secretion in their toe pads allows treefrogs to cling to smooth surfaces. © RJ and Linda Miller
A mucous secretion in their toe pads allows treefrogs to cling to smooth surfaces.

© RJ and Linda Miller

December 2006

Window stickers

Treefrogs adapted to scurry up bark and fend off the big chill.

Anita Carpenter

Gray treefrogs spend their time in the timber. So as November deepens into December and winter approaches, where do they go? Do they remain in the leafless trees? Seems unlikely. Do they return to their breeding ponds like the leopard frogs? Do they seek shelter in the ground? If so, where?

Treefrogs are so at home in their treetop canopy that they are devilishly difficult to observe. These shy amphibians are active in the darkness of night and hide out, resting, during the day. As protection from predators, they are true experts at camouflage and readily change color to blend in with their surroundings. A gray treefrog may look gray to brown when resting on an oak limb or green when sitting on a maple leaf.

Two look-alike species call Wisconsin home: the eastern gray treefrog (Hyla versicolor) and Cope's gray treefrog (Hyla chrysoscelis). Distinguishing the two is difficult and definitive identifications are left for the gurus in a genetics lab. The eastern treefrog is slightly larger (2 ¼ inches) with slightly warty skin and seems to prefer wetter habitats like swamps. The Cope's has smoother skin and outside of the breeding season has a solid lime green back. It prefers drier hardwoods and brush, but still near water. It is somewhat easier to distinguish in spring when the eastern's love song sounds like a loud musical trill lasting up to three seconds. The Cope's song is faster and harsher with a nasal quality. Whichever species is in the mood, a chorus of singing treefrogs can be deafening on a warm May night.

Males claim territories on the breeding ponds when the water warms up to 50 degrees. The most desirable perches hang horizontally over the water where their calls can be heard far away. They trill incessantly to impress and lure in a female. After amplexus, she will lay 1,000-2,000 eggs in small 10- to 40-egg clusters attached to pond vegetation. Then she leaves the pond and the frisky male advertises for another mate. The breeding season can last from May through mid-July. Thereafter, the males return to life in the trees.

Tadpoles hatch in three to seven days and by six to eight weeks, they've changed into bright green treefrogs that leave the water to begin a life on land. By early August, I often find young treefrogs hugging milkweed stems or resting on their boat-shaped leaves.

Gray treefrogs are expert tree climbers and confident jumpers. Large rounded toe pads enable them to move effortlessly through the trees. Frequently, you may find treefrogs clinging to cabin windows. How do they hold onto such a smooth surface? Do they have little suction cups on their feet or can they grip irregularities on the glass? Neither hypothesis is correct. The adhesive force is surface tension between the two flat surfaces. An electron microscope image would reveal that the toe pad is composed of many flat-topped hexagonal-shaped cells surrounded by minute openings. These openings allow each cell to move independently and adhere to the window with the flattest possible angle. The cells secrete a mucous-like fluid that flows between and over the flat-topped cells creating surface tension. However, the adhesive force is not so strong that it can't be broken by muscular action as the frog walks across or leaps from the window.

After a summer of feeding on insects and avoiding becoming a meal for hungry birds, snakes and small mammals, the frogs look for a safe spot to overwinter. As the days shorten and temperatures drop, gray treefrogs seek shelter on the ground in leaf litter, under logs, loose bark or in an animal burrow. Insulating snow cover provides a protective layer during the prolonged cold.

Treefrogs also undergo a biochemical change, another adaptation to increase their chances of cold-weather survival. They increase the glycerol levels in their tissues and fluids which acts as a sort of antifreeze or cryoprotectant (cryo meaning "cold"). The remaining free water in their bodies can freeze. These remarkable animals can freeze solid as their hearts stop pumping and still revive in the spring.

As you hike or cross-country ski through the woods this winter, think of gray treefrogs that may be buried under the snow, oblivious to the goings-on overhead. In a few months with the arrival of spring's life-reviving warmth, they will reappear to trill again as active participants in the springtime chorus.

Anita Carpenter is in no hurry to warm up her pipes for the spring sing near her home in Oshkosh. Winter is her favorite season.