Mar/Apr 2009For Wildlife Watchers: Barking Tree Frog by Rob Simbeck, photography by Phillip Jones
We think of wildlife watching as a visual activity, but it is often just as much about listening. As I work, I'm always attuned to the fussing and calling outside, ready to interrupt what I'm doing if I hear some avian drama begin to unfold. The legendary American ornithologist Roger Tory Peterson once won a friendly birding competition from a sickbed while his companions hiked the nearby woods. His finely tuned ears heard more than they saw.
Birds aren't the only multimedia extravaganza out there. Crickets, grasshoppers, cicadas and smaller insects all add their own music. And then there are the frogs. Every spring, as the rains massage new life into the dormant earth, the frogs take up their choruses. Bullfrogs, peepers, green frogs and tree frogs, like strings and reeds in an add-water orchestra, fill the air with sounds as utilitarian as they are magical.
We hear the males, calling in voices ancient and insistent, working to attract females who will join them in rendezvous choreographed by nature to create the next generation. Each species has its pitch and timbre, and each has its story.
The largest of the Southeast's native tree frogs is a plump, two-and-a-half-inch hunk o' love called the barking tree frog, named for an earnest call that can sound a little like a hound in the distance. Launched from a perch on a high branch, it may be - its purpose is not known for certain - an announcement call, the equivalent of, "Ladies, the other gentlemen and I will be congregating at the pond. We would be delighted with your company."
The mechanics involve air passing over the larynx and through the distended throat, which acts as a resonating chamber, and then out of the mouth, a soundhole like that of a saxophone. The males finally descend and move to the pool of water where, floating, they take up another call, sometimes rendered as "toonk," prior to mating.
In South Carolina, the process begins in April, at about the time the overnight low holds at 60 degrees, and can continue in rainy weather into the summer. The male clasps the female from behind as she deposits up to 2,000 eggs in shallow water, and he fertilizes them as they emerge. Gestation takes about a week, and the tadpoles, which feed on algae and pond detritus, grow for 6 to 10 weeks to a length of two inches or so. Metamorphosis sees the tadpole swap its tail for legs and gills for lungs, shorten its intestines as it moves from a diet of plants to insects and worms, and even shift the position of its eyes.
The adult has a short head and snout with a conspicuous tympanic membrane, or eardrum. The throat, belly and inside of the hind legs are golden in color, and the male has a green or yellow throat.
Barking tree frog - Hyla gratiosa
Description: 2½ inches long, plump. Green to brown with blotches; variable.
Range and Habitat: Coastal plain and Gulf states. In or near trees close to ponds, streams and other permanent waters.
Reproduction: April to August. Female lays up to 2,000 eggs. Gestation 1 week; tadpole stage 6 to 10 weeks.
Viewing Tips: Spring nights, particularly during or after rain. Your ears will guide you.
The adult's bumpy, textured skin, with dark blotches on the back and yellowish stripes along the sides, is a marvel in several ways.
"The skin color is very changeable," says Steve Bennett, herpetologist with the S.C. Department of Natural Resources. "Like many other amphibians, these frogs can move melanin around. If they're out in the light, they tend to become lighter. In a darker setting, they will be darker. They may be responding to temperature and other environmental factors and possibly to behavioral cues."
Those changes make possible both camouflage and, since darker colors absorb more light and heat than lighter ones, temperature control.
The skin is also highly permeable, allowing it to absorb water and oxygen, as well as chemicals, something that helps explain why, worldwide, we are seeing precipitous declines in the numbers of some amphibians.
"Amphibians are first responders," says Bennett. "They soak up and absorb everything. Frogs are sensitive to waterborne toxins, particularly when they're tadpoles, and a lot of pathogens - viruses and the like - seem to be wreaking more of a havoc on larval frogs. Among the worst are herbicides and pesticides, substances that break down into estrogen-mimicking compounds. Amphibians get hammered by them."
The skin is also noteworthy in that it has toxin-producing glands that make these frogs less palatable to most potential predators, although snakes still find them attractive prey. Tadpoles are very vulnerable to being preyed on by fish if they happen to share habitat with them.
On land, they spend their time in the treetops, eating insects and resting during the day in the crotches of limbs. In dry weather, they may hide under leaf litter, and in winter, they will hide beneath loose bark or under litter in tree cavities or stumpholes. These are, by the way, remarkably cold-tolerant animals - a relative, Hyla versicolor, can actually withstand the formation of internal ice crystals.
Barking tree frogs are fairly widespread along the coastal plain and into portions of the Piedmont, particularly along the Savannah River, inhabiting isolated wetlands, particularly pine woods and pine savannahs. As with most other amphibians, they are affected by loss of habitat as well as problems caused by environmental change and pollution, including acid rain, and there are fewer places now to enjoy their symphony. It is still worth seeking out those places where it is possible to become immersed in their calls, surrounded by one of nature's aural spectacles, relishing the experience of watching with our ears.