Fall 2002

Gray Treefrog

Master of Camouflage

On summer evenings at my home, we like to watch the occasional gray treefrog clinging to the window under the porch light, snapping up insects that fly too near. Treefrogs use their round padded toes to climb and actually adhere to the glass.

	Photo by Rob Curtis, The Early Birder

From this vantage I can see their characteristic traits. They range from one to two inches in length and have long, slender legs with yellow-orange markings on the inner thigh that are only visible when the legs are extended. The belly is a dirty white color, and males have a black throat. The body color can change from dark gray to pale green, but they all have a silvery-white or light green patch of skin under each eye, and an irregular dark pattern on the back that is outlined in black.

If they did not climb on the window, I probably wouldn't have noticed the treefrogs at all. I have seen a couple resting on and under my lichen-covered cedar roof shingles, but they are not easy to spot. These masters of camouflage are able to change color to match their background — gray when on tree bark, green when perched among lichens. The yellow patch across the thighs is thought to confuse predators because it appears and then disappears as the gray treefrog jumps and lands. Juveniles are bright green and usually lack the dark pattern on the back. This allows them to blend in while among shrubs or herbs.

There are actually two species of gray treefrog, however they can only be distinguished by their calls or  number of chromosomes. The eastern gray treefrog or Hyla versicolor (Hyla means "belonging to the forest," and versicolor means "to change color") has a soft, musical trill. This is one of my favorite frog calls because it is a pleasant yet unworldly sound. In fact, some sci-fi movies use the eastern gray treefrog call as background noise on alien planets or as the voice of creatures.

The other species is the Cope's gray treefrog, or Hyla chrysoscelis (chrysoscelis means "golden leg"), which has a faster and more shrill call. My students at Saint Joseph's College think that it sounds like a chimpanzee screaming. The eastern gray treefrog also has twice as many chromosomes as the Cope's gray treefrog. While easterns are typically more common in Chicago Wilderness, both species are considered common in Illinois and Indiana. Though few local studies have been done comparing the ecology and distribution of these species, a recent study by herpetologist Mike Redmer and the Illinois Natural History Survey is finding that the eastern treefrog is common throughout the Kankakee and Des Plaines River watersheds, while the Cope's treefrog is prevalent in the Fox River drainage.

Like most amphibians in Chicago Wilderness, gray treefrogs prefer breeding ponds that do not have fish, including small isolated and temporary wetlands. My research in northwest Indiana indicates that gray treefrogs most often breed in wetlands that are semipermanent (meaning they hold water throughout wet years but dry up during dry summers), have low acidity, are unpolluted from road salt and fertilizers, and are in or near woodlands.

	Photo by Mike Redmer .

The adults spend most of their time in trees, often hiding under pieces of peeled bark. Being freeze-tolerant, they even hibernate in the trees. The breeding season is from late April through late July, and the males will often sing from trees near ponds at heights of up to 20 to 30 feet. It is fun to try to locate them with a flashlight during an evening chorus. Their eggs hatch in a few days, and the tadpoles can be easily identified by the bold red-orange and black tail.

Area amphibian and reptile biologists have identified the eastern gray treefrog as one of six species that indicate high-quality savanna habitats in our region. (The others are the green frog, spring peeper, northern leopard frog, eastern tiger salamander, and Blandings turtle.) Volunteers will monitor Chicago Wilderness savannas annually for these indicator species in an effort to improve habitat for amphibians and reptiles.

— Bob Brodman

To get involved with amphibian and reptile monitoring, call (847) 965-1150.