Many people think of tree frogs as tropical, but we have two nearly identical species of tree frogs in the Chicago Region. Both are commonly known as Gray Treefrogs, but are given different scientific names: Hyla versicolor and Hyla chrysoscelis. Curiously Hyla versicolor has twice as many chromosomes as Hyla chrysoscelis, although how this came to be is not known.
For most purposes, it is sufficient to lump the two and refer to them as "Gray Treefrogs" or the Hyla versicolor complex. Both have the large toe pads which distinguish tree frogs. Both are slightly warty and shiny and are about 1.25 to 2.5 inches long (3.2 to 6.3 cm). They can be bright green, gray or brown and can change colors before your very eyes. Under the hind legs is a flash of bright yellow or orange. They are very well camouflaged for their local environment; shrubs and bushes or trees growing around bodies of water.
Herpetologists can identify the two species by the breeding calls, if they know the temperature of the air and water where the frog was calling. The call of Hyla versicolor has been described as a slow, musical trill or similar to the call of the red-bellied woodpecker. Hyla chrysoscelis makes a faster short, harsh buzzing call. In the Chicago Region, the two species are somewhat geographically separated, with Hyla versicolor occurring mostly south of Chicago and Hyla chrysoscelis in the north and northwest. I used to remember this as the White Sox on the South Side (versicolor because they once were the Black Sox) and the Cubs on the North Side (chrysoscelis "c" for "c").
Gray Treefrogs breed from mid-May to June in our region and can be found in various habitats including savannas, woodlands and floodplain forests. Males tend to be smaller than females and have dark patches under their chins during breeding season. Calling males may be in the shrubs, clinging to the trees or in the water of the breeding pond. Eggs are deposited in packets of from two to three dozen and are attached to plants at the water surface. Each female may lay 2,000 eggs.
Tadpoles hatch in only a few days and transform into frogs from late May to August. Newly metamorphosed frogs tend to be bright green and stay near the pond. In winter, Gray Treefrogs hide in rotting wood, tree crevices or between the bark and the trunk of dead trees. They survive freezing temperatures by turning their body fluids into a natural form of antifreeze (ethylene glycol).
Gray Treefrogs are not common in the Chicago Region, possibly due to the loss of their preferred habitat. When the first settlers arrived in the region, much of the standing timber was immediately logged for construction and fuel. Today, dead trees are cut down and the wood removed; ponds have been drained to control mosquitoes.
Ellin Beltz firstname.lastname@example.org October 26, 2008