Wisconsin's Tree Frog Species
Two Unique Types Call Our State Home
by Annie Shao, age 16
Shy, swift, and elusive, tree frogs leap from tree to tree with amazing agility, making them difficult for humans to observe. Wisconsin is home to two different species of tree frogs: the eastern gray (hyla versicolor) and the Cope’s gray (hyla chrysoscelis).
The eastern gray and Cope’s gray tree frog are almost identical, with a few subtle differences. The eastern gray is about two and a quarter inches larger than the Cope’s gray and also has bumpier skin. The two species also prefer different habitats; the eastern gray lives in wetter places like marshes, while the Cope’s gray can be found in drier hardwoods near water. Finally, the frogs’ songs differ. The eastern gray is known for its three-second trill, while a faster, more nasal song belongs to the Cope’s gray tree frog.
The life cycle of a tree frog begins when a female lays one to two thousand eggs. She lays her eggs in groups of 10 to 40 on plants, where they hatch in three to seven days. In about two months the tadpoles become mature frogs.
After the frogs spend their summers eating insects and trying not to become food themselves, they search for a safe place to hibernate during the winter. Usually they settle down on the ground in leaves, under logs or loose bark, and in animal burrows. When winter arrives, snow covers them, giving the frogs insulation from wind and the cold air.
Their bodies also prepare for winter. Glycerol, an alcohol in their tissues and fluids, increases. Since glycerol has a lower freezing point than the water in their bodies, it acts as an antifreezing agent for tree frogs. The rest of their bodies will freeze until the spring when they revive again.
Many people wonder how tree frogs are able to stick to smooth surfaces. Some may guess that their feet have suction cups on them or that they can cling to small irregularities on surfaces.
Both of these guesses are incorrect. Tree frogs’ toe pads have flat, hexagonal-shaped cells surrounded by miniscule openings. These openings let each cell move around and stick to the surface at a relatively flat angle. The cells then release a mucus-like liquid that flows between and over the cells, creating surface tension. This surface tension helps the frogs bind strongly to any surface, but not so securely that they are unable to break the bond.
Although we may not always see them, the eastern gray and Cope’s gray tree frogs live alongside us year-round. As eggs hatch into tadpoles and then grow into frogs, humans marvel at their climbing agility. Of course, we also enjoy their chorus in the spring after a long, silent winter.
[Source: Wisconsin Natural Resources]