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UPCOMING OPPORTUNITY:
Join us for another
Winter Hummingbird Expedition to Costa Rica
in February 2006

GREEN TREEFROG:
WAY OUT-OF-RANGE?

We bounded from bed in the pre-dawn darkness on 30 September, aware that a cold front had passed through overnight and eager to get our mist nets deployed to capture migrant songbirds at Hilton Pond Center. Without lighting the lamp we reached into a closet that houses our extensive T-shirt collection and grabbed one, pulling it on after donning shorts and flip flops--our normal haute couture banding attire from early spring through late fall. The odds are pretty high that any randomly selected T-shirt will be decorated with hummingbirds--we have quite an assortment of these, including a half dozen of our original and highly coveted Operation RubyThroat T-shirts--but the one we came up with was adorned with a nice rendering of Northern Leopard Frog metamorphosis (above right). We'd only worn this shirt a couple of times--it was a gift from our "Hummingbird Mornings" presentations last August at Black Hill Nature Center in Maryland--and we like it a lot. Little did we know selecting this particular amphibian shirt would be an omen of things to come that day.

All text & photos © Hilton Pond Center

In morning twilight, we unfurled our nets and went to get the daily papers, which we perused while eating breakfast and waiting for the sun to come up. When it finally rose--which seemed to take longer than usual due to overcast skies--we looked out the window of the old farmhouse at Hilton Pond Center and noticed something odd about the Pickerelweed patch growing in a small water garden (above). At first glance it appeared there was a small Green Treefrog clinging to one of the Pickerelweed leaves--which just couldn't be the case because field guides show this Lowcountry amphibian isn't found in South Carolina's Piedmont. We momentarily concluded it must be some pale leaf that had fallen from an overhanging tree and landed on the Pickerelweed foliage. Just to make sure, however, we trained our binoculars on the spot in question and, quite amazingly, the immobile form of a Green Treefrog sprang into focus--even though this little amphibian was nearly a hundred miles out of its "normal" range.

All text & photos © Hilton Pond Center

We quickly assembled our digital camera, lenses, and tripod and went outside to snap off a few shots, lest the newfound frog hop away. We needn't have hurried, for the treefrog stayed put on the Pickerelweed leaf as we crept ever closer with camera in hand. The frog never even flinched when a treehopper fell onto the leaf and crawled over its back. Overnight temperatures had been in the mid-60s--a bit chillier than recent readings--and the frog almost seemed to be hugging itself as if to conserve whatever heat its "cold-blooded" little body might contain. The frog's forelimbs were folded under its throat region (above), and the hind feet were tucked into its "armpits," giving the impression the frog was merely a lump of green stuff on the Pickerelweed. Its eyes were half closed, so we assume the treefrog was sleeping most of the time we were taking photos--even when we crawled under its perch and snapped a through-the-leaf silhouette while lying on our back (above right). Not until we actually touched the frog did it show any signs of life by beginning to move on the Pickerelweed.

We're familiar with Green Treefrogs, Hyla cinerea, from visits to South Carolina coastal sites, where this species commonly occurs on vegetation around almost any freshwater puddle, marsh, stream, or roadside ditch; it also sometimes hangs out in brackish water locales. Well-camouflaged by its green skin, this arboreal amphibian becomes glaringly obvious when climbing sliding glass doors of condos and beach houses at night--all the better to get various insects attracted to porch lights. Although wetland and semi-aquatic habitats exist in the Piedmont and are often occupied by Gray Treefrogs, Hyla chrysoscelis or H. versicolor, to find a Green Treefrog as far inland in South Carolina as western York County may be unprecedented. Whit Gibbons of the University of Georgia tells us Green Treefrogs are extending their range further northward in his state's Piedmont Region; we'd appreciate feedback from readers about any sightings of this species in the Piedmont of either of the Carolinas.

As a member of the True Treefrog Family (Hylidae), the Green Treefrog is well equipped to climb emergent vegetation from sedges to cattails to trees, and even vertical walls of glass, wood, or brick that make up beach dwellers' houses. (The species has even been reported from inside the trumpets of Pitcher Plants.) A treefrog's ascent is facilitated by four toes per fore foot (above left) and hind foot (below right)--each tipped by a rounded, almost globular toepad. Folks once thought toepads were little suction cups that gripped surfaces like a rubber dart, but electron microscope analysis shows the toepad tips are covered with cells very different from those on the rest of its epidermis. Each toepad cell is a long, thin column whose outer end is separated slightly from its neighbors; this allows the tips to wedge independently into even the tiniest of cracks on a vertical surface, thus providing a secure grip. In addition, toepads bear mucous glands that produce a substance just adhesive enough to help a frog cling--but not so sticky it can't pull the toepad away as it moves along a substrate. (If you've ever held a treefrog, you probably noticed the toepads leave behind no glue-like residue.)

Green Treefrogs are accomplished hoppers--their long hind legs are testimony to their leaping ability--but they tend to walk from spot to spot in pursuit of their insect prey. (Green Treefrogs seem to have a preference for large, succulent flies.) When they do jump, they sometimes land by grasping a Cattail blade or grass stem with just one or two toes, first sticking to the vegetation with toepads and then wrapping the toes around it. Such acrobatics are facilitated by a piece of intercalary cartilage that lengthens the toe and makes it very flexible; that's why treefrog toes sometimes look "double-jointed." Treefrogs can also swim, although they have almost no webbing between the front toes and a reduced amount on their back feet (see photos above).

All text & photos © Hilton Pond Center

The Green Treefrog is the state amphibian of Louisiana and reportedly the species after which Kermit the Frog was modeled (you can see the uncanny resemblance, below left). Unlike Kermit, Green Treefrogs are primarily nocturnal. Adults are about 1.5-2.5" long from snout to rump, with males slightly smaller. The sexes look alike externally, and both males and females exhibit a great deal of variation in color; even within individuals, color varies with external temperature and during times of stress and sexual arousal. When dormant, Green Treefrogs often turn gray or almost black. The dorsum is usually a solid color with a few golden tubercles ringed in brown or black (above), and usually--but not always--a cream-colored lateral stripe extends from beneath the eye for some distance along the flank; there's a similar line along the posterior edge of each leg. The Green Treefrog's underside is also cream-colored, while the inner surfaces of the legs are pinkish. The external tympanum (eardrum) is relatively small.

Along the South Carolina coast the male Green Treefrog starts calling as early as April, using an extendable, rounded vocal sac at the base of his throat to produce a nasal queenk at one-second intervals; the call has been likened to a distant cowbell. During copulation, the male rides a female and fertilizes eggs externally as she lays them between clumps of floating vegetation. A mature female can yield 400-600 eggs in batches of 40-100; tadpoles hatch in a few days and take about two months to develop.

All text & photos © Hilton Pond Center

We couldn't conclusively determine the sex of the Green Treefrog we found this week at Hilton Pond Center, nor do we expect to see any of its tadpoles swimming in the water garden--no matter how many times we wear our new frog metamorphosis T-shirt. It's amazing we have even ONE of these amphibians in the first place--to have a pair would be almost unthinkable--but how this particular treefrog ever got to York SC is a mystery that may never be solved. The water garden is well-established with no new plants added in more than ten years, and it's hard to imagine the treefrog could have hitched a ride from the coast on one of those small birds that bathe in the garden's shallow water--but there's always the possibility the species is simply underreported away from its "normal" haunts. In any case, as global warming continues and the sea level rises, it's only a matter of time until the Carolina coast is a lot further inland. Maybe then our Green Treefrog won't be so far out-of range in the Piedmont after all.

All text & photos © Hilton Pond Center
Frog metamorphosis T-shirt © Diana Dee Tyler & Liberty Graphics
Kermit the Frog image © Sesame Workshop

ADDENDUM: Two other green-colored treefrogs--the Squirrel Treefrog, Hyla squirella, and Barking Treefrog, H. gratiosa--have almost the same Carolina distribution as the Green Treefrog, H. cinerea, but Squirrel Treefrogs range a bit further into the Piedmont. Squirrel Treefrogs, which can rapidly change color and markings, never have the golden dorsal spots or cream-colored lateral line of the Green Treefrog, while Barking Treefrogs are much stouter than the slender Green Treefrog and have very rough skin. Barking Treefrogs and Green Treefrogs are sometimes confused with each other, not only by humans but by the frogs themselves; in other words, they occasionally interbreed. Again, please report any Carolina Piedmont sightings of Green Treefrogs to INFO.