Illinois Natural History Survey - University of Illinois

 

Gray Tree Frogs

 

Susan Post

 

It’s a perfect April day and I have just gotten out of the van at La Rue/Pine Hills snake road. Tiger and zebra swallowtails are nectaring on the abundant larkspur and all around cricket frogs are clacking. Yet I also hear a trill. Who could be making that noise and where is it coming from? The call resonates.  I’m searching, searching. Finally one of the instructors calls, “Look at the oak. Go over to the knot, where the branches cross." Bingo! A well-camouflaged gray tree frog that for the untrained looks like just another knot on the tree.                             —Illinois Wilds Institute for Nature, Field Notes, April 12, 2006

Illinois has two species of gray tree frogs—Hyla versicolor (common gray tree frog) and Hyla chrysoscelis (Cope’s tree frog)—which can be distinguished only by studying their voices and chromosomes. H. chrysoscelis’s mating call (trill rate) is faster and it is diploid (has 24 chromosomes). H. versicolor is tetraploid (has 48 chromosomes). These frogs are found throughout Illinois and range east of the Great Plains. The two species are found in mixed woods and temporary wetlands where they prefer to remain out of sight in woodland trees, sleeping by day. They may be glimpsed near suburban or rural homes, perched in tree cavities, hollow stumps, or on decks and swimming pools

Hyla versicolor in Latin means “color-turning.” Common names include dusky tree toad, chameleon tree frog, common tree toad, tree toad, and changeable tree toad.  Hyla chyrsoscelis comes from a Latinized word meaning “gold-colored,” and common names include northern tree toad. The common name of tree toad for both species refers to the frog’s skin, which has a rough, granular, dry, warty appearance that people usually associate with toads.

The skin color of both species varies from brown to greenish gray with darker blotches and bright orange on the undersurfaces of the hind legs. They have a white spot just below each eye.  The frogs’ overall coloration disguises it among the lichens and rough bark. Another common name —chameleon tree frog—refers to the frogs’ ability to change colors. These color changes are brought about by the change in shape of the pigment cells. Low temperatures or subdued light will cause the cells to expand and the skin to assume a dark color. High temperatures or bright light has the opposite effect.

Large, rounded, adhesive toe disks enable these frogs to climb and also distinguish this species as a tree frog. These toe pads allow the frog to cling to rough and smooth surfaces, even glass!

The mating season is usually from April to August. The males will seek out shallow ponds or creeks and begin to sing. Their calls are short, resonant trills that are lower pitched and of shorter duration than those of American toads. These calls have been described as “the charm of contentment; in fact, it is much like the purring of a cat only louder. At a distance, it sounds something like the bleating of a lamb.”  They give their calls from perches, which are thought to enhance the males’ chances of attracting females. Horizontal branches with few leaves arched over a pond are strategic perches. Females attracted to the males’ calls will walk, not hop, toward them. A female will choose a male by touching him with her nose or even leaping upon him.

While the courtship and breeding season may last for several weeks, a female will mate only once. Females lay up to 2,000 eggs attached, singly or in small groups, to vegetation at the surface or beneath the water. The eggs hatch after two to five days. The tadpoles are an olive-green color with a bright, orange-red tail. The tadpole stage lasts from one to two months. When gray tree frog tadpoles emerge from the water as froglets, they are about 1.5 inches long and a bright shade of green, which helps camouflage them on their journey from pond, through grass, to a tree. The frogs are sexually mature after two years and will be 2.0 to 2.5 inches in length.

Gray tree frogs usually hibernate on the ground beneath fallen leaves, rocks, or in underground crevices. They have the ability to withstand subfreezing temperatures—to as low as -20o F—by manufacturing glycerol in their blood.

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