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Hilton Jr., B. 1993. Amphibian acrobats. South Carolina Wildlife 40(2):6-10.

(Note: The draft below was submitted to South Carolina Wildlife magazine; the article that actually appeared in print may have been edited.)

The Place: A low-lying field on the outskirts of Rock Hill (smack in the middle of South Carolina's Piedmont)

The Time: December 21, 1967 (shortest day of year and the beginning of winter)

The Sound: A shrill, repetitive, ascending "peeyp . . . peeyp . . . peeyp"

The Source: Some organism in a shallow, temporary puddle that formed along a field edge after several days of rain

My Response: Complete bewilderment

In that winter of 1967 I was 21, a Pittsburgher recently transplanted to upstate South Carolina so my dad could take a new job in Rock Hill. As a child I had roamed the urban woods of western Pennsylvania, chasing butterflies or climbing sugar maple trees when I wasn't out imitating Pittsburgh Pirates baseball players. I was especially enthralled with turtles and snakes, but--much to my mother's chagrin--my ability at catching critters far exceeded my talent at building escape-proof cages.

One of my big regrets as a budding naturalist in Pennsylvania was that I could only pursue my beloved reptiles from late spring through early fall. These so-called "cold-blooded" creatures couldn't stay active during deep freezes that often accompanied northern winters, so after Halloween had come and gone it was useless to go looking for garter snakes or box turtles that were already hibernating.

With all this in mind, I was hardly prepared for things I witnessed my first year in the South--overwintering warblers, no predictable snows in late October, and soil that stayed unfrozen from November through early April. These were remarkable situations for a former Yankee used to six months of frigid weather, but none of them prepared me for discovering the source of the "peeyp . . . peeyp . . . peeyp" I heard that first day of winter in 1967.

As I approached the puddle that was the source of the shrill calls, the sounds suddenly stopped. I stopped, too, waited for the calls to resume, and then moved forward slowly, a step at a time, careful to avoid dead leaves and twigs whose snap would betray my presence.

It took nearly 15 minutes to move 25 yards, and just as I could finally see the puddle margin through the broomsedge, the sounds stopped again. Having come this far, I decided not to leave until I identified their source, so I sat down, propped my elbows on my knees, and squinted through my binoculars at the shallow water at the puddle's edge. In the meantime, I wondered whether these calls could be from a new songbird, some large insect, or a southern mammal with which I was unfamiliar.

Within five minutes the calls began again, first one, then another, and then two more, and I was startled by one of the more memorable sights of my life in the woods--a quartet of small, quarter-sized frogs swelling up their vocal sacs and calling out "peeyp . . . peeyp . . . peeyp."

"Spring Peeper" was the name that ran immediately came to mind, a species I had read about but never encountered in the field in urban Pennsylvania. But because it was late December, the very name of this diminutive creature made me wonder how it could be vocalizing on a day when I thought it should be hibernating. What I've learned since then is that spring peepers, like many reptiles and amphibians in South Carolina, never enter true long-term hibernation; winters in the Palmetto State are often so mild that peepers--driven by a powerful inborn urge--can ignore the calendar and take every opportunity to mate and spread their genes to future generations.

Spring Peepers are one of eight small frogs in South Carolina classified as "true treefrogs" in family Hylidae. Hylids comprise a loosely-related group of frogs whose ancestors became adapted for tree-climbing by virtue of each toe having an extra length of cartilage and an enlarged adhesive pad at its tip. Over evolutionary time, some members of the family--including Carolinas species such as cricket frogs (genus Acris) and chorus frogs (Pseudacris)--lost their toe pads, but these structures are retained by true treefrogs (Hyla). Thus, retention of the toe pad, along with an extendable, rounded vocal sac at the base of the throat, separate true treefrogs from their relatives.

Even though the Spring Peeper is most often encountered on the ground, in puddles, or near pond margins, its toe pads make it an efficient climber; in fact, "efficient" may be an understatement when it comes to describing the arboreal abilities of true treefrogs. Anyone who has seen a treefrog poise on a low-hanging branch, bob its head a few times to gauge distance, and then cut loose with a trigger-quick, spindly-legged leap into space has probably been reminded of the trapeze artist at the circus. High above the crowd in the circus tent, when the "daring young man" leaves his swing and arches upward, there's always that twinge of uncertainty over whether he'll successfully execute his spin and reach the another trapeze. Somehow, though, treefrogs and circus performers almost always complete their leaps--a feat certainly made easier for the amphibian acrobat by its unique toe structure. The "intercalary" cartilage of the treefrog provides an extra flexible joint and allows each toe to swivel for exact placement, thus optimizing the toe pad's gripping surface and keeping the little amphibian out of the jaws of some lunker bass waiting patiently in the pond below.

South Carolina's eight true treefrogs have a delightful complement of scientific names that, if you know a little Latin, are often descriptive and enlightening. The Spring Peeper, for example, is called Hyla crucifer for the dark cross-like marking on its back. This treefrog is found in every county of the state and is active year-round whenever temperatures approach the 60's. Members of this species vary greatly in appearance, but they are always some shade of brown or gray, never green. The belly is unmarked--except in the ventrally-spotted H. c. bartramiana subspecies with limited distribution in Georgia and perhaps the Savannah River basin. In northern states, Spring Peepers hibernate under logs and bark, but they breed from October to March throughout most of South Carolina (February to May in the highest elevations). Each female can lay up to 900 tiny jelly-covered eggs that she attaches one at a time to submerged rocks or vegetation--not bad production for a vertebrate with a body only an inch long!

The Bird-voiced Treefrog (aptly named Hyla avivoca for a loud, rapidly- repeated "whit-whit-whit" vocalization that resembles a bird call) occurs in heavily-wooded swamps in the southern half of South Carolina's Savannah River bottomland. Amphibians and Reptiles of the Carolinas and Virginia (Martoff et al., UNC Press)--arguably the best book of its kind ever published--describes this species' vocalization as "reminiscent of an osprey's call, or of whistling for a dog." The Bird-voiced Treefrog has a grayish or greenish back covered with darker irregular markings and a light-colored spot below the eye. Individuals may grow to two inches in length. It, like many treefrogs, has pigment along the concealed surfaces of the long, slender hind legs; in this case it is a pale green or yellowish swath of color. The female lays up to 650 or so eggs in packets of six to 15; these hatch in only a day and a half and metamorphose into tiny froglets within a month. Some observers report that skin secretions from this species cause watery eyes and noses in humans who handle it.

Gray Treefrogs resemble the Bird-voiced Treefrog but usually have more granular skin; markings on the inner thighs are bright orange and there is a white spot below the eye. Interestingly, there are two Gray Treefrogs (Hyla versicolor and H. chrysoscelis) that look exactly alike to human observers, but because they do not interbreed where their ranges overlap they are separate species. Despite their identical appearances, the two species are very different at the genetic level. According to Martof, "H. versicolor has 48 chromosomes, twice as many as H. chrysoscelis," a difference "readily revealed by microscopic examination of cells of the inner eyelid." The two species can also be identified by their calls, which are some variation of a "short, vibrant, flute-like trill." The call of H. chrysoscelis is "shorter, harsher, and more forceful" containing "an average of 45 trills per second, whereas that of H. versicolor has only 25." In other words, if you've got a microscope or a sound-analyzing computer, you, too, can correctly identify the species of Gray Treefrog in your area! (Geographically speaking, H. chrysoscelis apparently occurs throughout the state, while H. versicolor may be absent from the mountain regions.) During the summer breeding season (May through August) each female gray treefrog lays clumps of five to 50 eggs on the surface of shallow standing water.

The Squirrel Treefrog (Hyla squirella) lives in the southeastern half of the state, from the coast to the fall line and a bit further inland. Only an inch-and-a-half long, it is seldom seen in the natural setting because of its ability to quickly change its color and markings. Sometimes the back is pale brown with dark round spots, but in a few minutes it can be light to bright green and spotless. There is often a more-or-less complete dark bar between the eyes and a poorly-defined yellowish or whitish stripe along the edge of the upper jaw. The underside is white, with some yellow toward the flanks. This species is more "urban" than many treefrogs and often dozens of them can be found huddled together under roof overhangs or in backyard shrubs. Sometimes called the "rain frog," the Squirrel Treefrog gives its mating call before and after summer storms. Martof reports the call as a "flat, nasal, ducklike "waaaak," 0.25 second long and repeated every 0.5 second." Females lay up to 1,000 eggs on pond bottoms.

Our largest treefrog is the Barking Treefrog (Hyla gratiosa), a chunky amphibian almost three inches long. Its green back color ranges from pale to dark, with spots of black or gray; the underside is creamy white or light yellow. This species occurs in the southeastern half of the state from the fall line to the coast, where it lives in wet bottomlands or swamps and around ponds in savannah areas. According to Martof, "breeding occurs in shallow ponds after heavy rains. As the males move into the ponds from bushes or trees, they utter series of loud, distinctive, doglike barks, but when the water is entered, the call changes to a hollow-sounding "doonk" repeated at short intervals." A large female can produce 2,000 eggs in a season, each deposited separately on the pond bottom. Barking Treefrogs are sometimes sold as a terrarium pets and will take food from the hand, but declining populations dictate that humans leave this species--and our other wild treefrogs--in peace.

The Green Treefrog (Hyla cinerea) may be the most commonly-observed treefrog found in South Carolina, primarily because it is attracted to insects that gather at porch lights. I remember watching these brilliant green frogs on my first trip to Myrtle Beach, where several of them clung to the the patio doors of my motel room while consuming the local population of night-flying insects. Green Treefrogs have smooth skin, white bellies, and a whitish or yellowish stripe down each side of the body. They occur in the southeastern half of the state from the fall line to the coast, and breed among cattail stands in ponds, drainage ditches, and marshland. The two-inch-long female lays about 350-450 eggs between clumps of floating vegetation. Martof describes the male's call as a "bell-like, nasal 'queenk' repeated once a second."

The other two South Carolina treefrogs have the word "pine" in their names, but they are very different creatures. One, the Pine Woods Treefrog (Hyla femoralis), occurs in the southeastern half of the state from the fall line to coast. It lives near water throughout the pine woods and savannahs of the Low Country, and less commonly in hardwood bottomlands. With its mottled markings on a pale gray to dark brown back, the inch-long Pine Woods Treefrog superficially resembles the Gray Treefrog but lacks the white spot below the eye. When the hind leg is extended, there is a noticeable row of yellow or orange spots contrasted against a dark brown background. The mating call, according to Martof, "emitted by males on trees in the water, is a low-pitched, guttural trill, which sounds somewhat like 'getta, getta.' It is often heard in pine savannahs [in late spring and summer] on dark overcast days." Another observer claimed that a chorus of Pine Woods Treefrogs sounds like an "office full of industrious typists." The female deposits about 100 eggs in a thin layer, usually on the surface of the water but sometimes on nearly-emergent vegetation.

The last of our treefrogs is of particular significance because it is an endangered species. The Pine Barrens Treefrog (Hyla andersoni) occurs in small colonies within the northern Sandhills of South Carolina and the adjoining coastal plain of North Carolina, primarily in swamps containing hardwoods, white cedar, or pine. (There are also isolated populations in the New Jersey pine barrens for which this frog is named.) Most human observers consider this two-inch amphibian to be the most attractive of our treefrogs, with its bright green back contrasted with a purple stripe than runs through the eye to the base of the hind legs. The belly is white, and the inner thighs are a brilliant orange highlighted by yellow spots. Its call is a nasal "quonk" repeated many times at irregular intervals. Females lay about 500 eggs. Breeding from April through July, Pine Barren Treefrogs seem to be surviving where there is appropriate habitat, but constant pressure to drain inland bays and associated wetlands keep this rare species on the edge of extinction.

Despite variations in appearance, habitat, vocalizations, and behavior, South Carolina's eight species of treefrogs still have many characteristics in common. Most, for example, have "flash colors" on their hind legs. As herpetologist Hobart Smith states in his excellent field guide (Amphibians of North America, Golden Press), these colors, "which are thought to catch the eye of a predator when the frog leaps, disappear from view when the frog lands and folds its legs, thus causing the predator to lose sight of its prey." Incidentally, although true treefrogs can jump and land quite well, they spend most of their time walking and climbing and clinging to vegetation.

All treefrogs reproduce sexually and go through amphibian metamorphosis. Typically, a successful male spends considerable time perfecting his vocalizations to attract a female. In turn, the female exercises choice and seeks an attractive male who mounts her, digs his thumbs into her "armpits," and holds on for up to 2-3 days. By riding the female (a posture called "amplexus"), the male is in the proper position to externally fertilize the eggs as they are released. Amplexus also prevents any other male from mounting the female and fathering her offspring.

Hatch time for treefrog eggs--which are covered by jelly that prevents desiccation, deters predators, and minimizes attacks by bacteria and fungi--varies with the species. In any case, however, a successful egg becomes a tiny tadpole with external gills, no limbs, and a long, laterally-flattened tail. The tadpole feeds primarily on vegetation, especially algae, and has a proportionally long intestinal tract in which microbe colonies help digest vegetable matter. Over time, the tadpole increases its body size, sprouts hindlimbs, and finally develops forelegs. At a critical moment, the respiratory system shifts from gill to lung and the newly-metamorphosed organism becomes an air-breathing treefroglet capable of climbing, jumping, and grabbing with its toe pads. The froglet rapidly resorbs its now-useless tail and changes diet from plant to animal matter, spending the rest of its life trying to catch and gobble protein-rich insects, spiders, and other small invertebrates.

At the global level, tree frogs are often thought of as rain forest animals, but approximately 600 species of hylids occur on all temperate and tropical continents except Australia. Treefrog taxonomy is the subject of intensive study among herpetologists, and it's likely that genetic mapping and other new techniques will result in better understanding of the evolutionary relationships between various kinds of treefrogs.

Treefrogs are superbly adapted for survival as partly aquatic/partly terrestrial/partly arboreal organisms. There are only 14 species of true treefrogs in North America, and South Carolina is home to eight of these. Conditions in the Palmetto State are ideal for treefrogs--mild winters, high humidity, significant annual rainfall that creates standing water, substantial vegetation, and abundant invertebrates for food. Although treefrogs can survive as adults in places where there is sufficient vegetation to provide hiding places, moisture, and insects, they--like many other amphibians--are seriously threatened by the loss of natural wetlands that are essential to their reproductive cycles.

Spring Peepers such as those I heard and saw almost 25 years ago in a Rock Hill puddle seem to be doing well, but other species are less opportunistic and require specialized habitats that are often threatened by development. There is no doubt that many of our treefrogs will survive only if we humans care enough to preserve their wetlands and the natural balance that they bring. Otherwise, those once-mysterious "peeyp . . . peeyp . . . peeyp . . . " calls could cease forever as Spring Peepers and their tiny, toe-padded, true treefrog relatives join the ranks of the Earth's extinct species.


Today's the twelfth-month's twenty-first,
According to Gregorian,
But Winter is one thing it isn't.
Peepers chirp in the backpond while
Terminal buds scratch their metamorphic metaphoric heads
And wonder if they've mistallied sunrises.
Between them, though,
These harbingers of Spring had
Better give some second thoughts as to

Bill Hilton Jr.

Bill Hilton Jr. is a science education consultant, writer, naturalist, and Macintosh computer enthusiast who lives in York, South Carolina.

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