|A gray tree frog. (Photo by Mary Holland)
Melodic, birdlike trills lasting up to several seconds emanate from the woodlands this time of year. They are produced not by birds, but by gray tree frogs (Hyla versicolor), resident amphibians often heard but seldom seen. Singing begins while the males are still in the forests, near where they spend the winter hibernating under leaf litter and rocks (their body produces glycerol, a form of antifreeze which allows 40 to 80 percent of their body to freeze with the frog still surviving). Within a week or two, movement to the perimeter of breeding ponds begins, where their singing becomes very concentrated and extremely loud, especially at night. The females’ preference for males with the most prolonged and frequent calls certainly contributes to the noise level. (To hear and see a gray tree frog singing, go to www.youtube.com/watch?v=2k5CTLNw04w
The reason this very vocal frog isn’t seen more often is a combination of the type of habitat it seeks out while vocalizing and its camouflaged appearance. Unlike other frogs, the males tend to stay in and on shrubs and trees close to water while they court potential mates, not out in the open water or on the banks of the pond. In addition to being hidden in the shrubbery, gray tree frogs are extremely well camouflaged due to their mottled skin, so much so that they can be approached quite closely and still blend into the background so well they can’t be detected. As their species name implies, gray tree frogs have the ability to change the color of their skin, from grey/brown/black to green, depending upon environmental factors such as humidity, temperature, and color of their surroundings.
We have only two species of true tree frogs in eastern Massachusetts—the gray tree frog and the much smaller spring peeper. These two frogs share several universal traits of tree frogs: well-developed discs at the tips of their toes, extra cartilage between the last two bones of each toe, allowing them to swivel their toes and keep them flat against all surfaces, relatively long limbs, and horizontal pupils, to name a few. The gray tree frog is larger than the spring peeper and has a little white spot under each eye, as well as yellow-orange on the under side of its hind legs. Males have darker throats than females.
It is the ability of all tree frogs—and gray tree frogs are no exception—to adhere to smooth, vertical surfaces that sets them apart from other frogs. In fact, they can stick to surfaces even when the surface is tilted way beyond the vertical, to the point where the frogs are upside down. How is this possible? Each toe pad is coated with a thin layer of mucus, which adheres to surfaces much like wet tissue paper sticks to glass. However, when they are walking or jumping, the frogs can detach their toe pads easily by peeling themselves off the surface. Researchers at the University of Glasgow wondered how these two apparently diverse skills were possible, and discovered that the change from adhesion to peeling is a gradual process, with adhesive forces weakening at angles above 90 degrees. Thus, gray tree frogs maintain a grip by keeping the angle of their toes with respect to the surface at an angle of 90 degrees or less. Clever creatures that they are, tree frogs that wish to cling to the substrate actually spread their legs out sideways to minimize the angle between their feet and the surface, thereby increasing the probability of adhering to the surface.
Each mating pair of gray tree frogs produces about 2,000 eggs, which hatch in less than a week. In a month or two, the green tadpoles (with orange-red tails) will metamorphose into four-legged frogs, and leave the water. At this stage of their life, young gray tree frogs are usually a fairly brilliant shade of green, but this color lasts only a day or two before they turn gray, the color they will remain for the rest of their life, and for which they are named.
Readers can find a daily nature note and photograph at Mary Holland’s blog, http://naturallycuriouswithmaryholland.wordpress.com.