Harbinger of Spring

Oplinger, Carl S., The World and I

It is mid-March, somewhere in the mid-Atlantic states. Little, if any, snow is on the ground. But a person who ventures forth at dusk may hear the sound of distant sleighbells. In fact, it is a chorus of tiny frogs: spring peepers.

Literary scholar and naturalist Joseph Wood Krutch, in a collection of essays titled The Twelve Seasons, expressed in delightful fashion his opinion that a spring peeper chorus, not the arrival of the first robin or appearance of the first crocus, was the truest harbinger of spring.

Male spring peepers, no bigger than the first thumb joint, produce the loud, bell-like calls each of which is a single note ending with an upward slur. In t his manner the males urgently beckon the slightly larger females to the ponds--permanent, or more generally, temporary--that serve as breeding sites. The inch-long males have hopped their way from wooded tracts through varied landscapes, perhaps over a distance of half a mile (about 32,000 inches!) to reach these ponds. Now they wait as females make the same impressive trek.

Sounds of this chorus carry almost half a mile. This fact supports the view of a pioneering herpetologist, G.K. Noble, who believed that amphibians in temporary ponds have louder voices and shriller calls than species (such as bullfrogs) that dwell in permanent ponds or lakes. Mating and larval development must be completed before the ponds disappear with the heat of the summer.

Cross bearers

The spring peeper is one of roughly 600 species of frogs that have been given the family name Hylidae. Tree frogs, whistling frogs, and chorus frogs are common names of other members of this family. All of them have distinctive expanded toe tips with sticky adhesives pads. Cartilage between the terminal two bones of each toe facilitates swiveling motion sideways and backward.

The peeper can generally be identified by a black, X-shaped mark on its back. This feature inspired its Latin species name, crucifer. The genus name was Hyla until a few years ago, when taxonomists changed it to Pseudacris.

The oblique dorsal cross is most readily visible on those peepers whose bodies are tan, olive, or gray and less visible on individuals whose body color is darker brown. Irregular, black or brown bars and blotches mark the limbs, especially the hind limbs. Among the northern subspecies, P. crucifer crucifer, the belly is an unmarked white to pale yellow, while individuals of the southern subspecies, P. crucifer bartramiana, have a spotted belly.

The family Hylidae belongs to the ancient clan Amphibia. This class of vertebrates was the first group to solve the challenges of life on land. Fossil evidence indicates that amphibians emerged in the late Devonian period, about 350 million years ago. Today, the three main groups of amphibians--salamanders, frogs, and toads--are found worldwide but, as is true of other poikilotherms (animals with variable body temperature), the majority dwell in tropical and temperate regions.

The range of the northern spring peeper extends from the Canadian Maritime Provinces to Manitoba, south through the northern and mid-western regions of the United States to the Gulf Coast and most of the southern United States. The southern subspecies is found in southeastern Georgia and adjacent northern Florida.

Male peepers call from secluded sites, positioning themselves among grass tufts or woody shrubs just above the surface of temporary or permanent ponds, water-filled ditches, or shallow pools. In the southern regions, choruses begin at dusk from November through March. But for residents of the mid-Atlantic states and northward, the chorus that signals spring begins in mid-March and continues until early June. Early in the season, the chorus seems to coincide with warm evening rains. Later, desperate males sing day and night in hopes of mating with late-arriving females.

Take time to listen to the chorus. The careful listener may detect notes of different pitch, with measured rests between the notes of individual callers. When a male peeper calls, the single median vocal sac distends like a bubble, to about one and a half times the size of the head.

A perfect union

Among species that reproduce sexually, the urge to mate is the strongest drive of all. To ensure successful mating, male peepers team up to form duos and trios. The females head toward such groups in preference to solo singers. After approaching a group, the female selects the most persistent caller, seemingly deducing that he is in the best physical condition.

When the female, swimming in shallow water, approaches her favored caller, the male leaps from his perch and swings onto her, placing his forelimb under hers. Enlarged, swollen thumbs, which are characteristics of the forelimbs of male amphibians, enhance secure coupling.

Egg laying lasts for hours. The cloacal vents of male and female tip toward each other, so that each egg delivered singly from the female's vent meets sperm from the male's cloaca. This process continues until the entire complement of 250-1,000 egg's has been fertilized. The fertilized eggs, resembling black and white poppy seeds, drift away and adhere to underwater debris, grass stalks, or the base of woody shrubs partially submerged at the water's edge.

Depending on the surrounding water temperature, 6 to 12 days pass as the embryo, enclosed in two thin jelly coats, grows into a tadpole. The coats provide partial protection from predators as diverse as leeches and newts. The tadpole, or "pollywog," then wiggles free of its jelly casing and uses its strong tail to swim among the underwater plants and debris. With its several rows of sharp teeth, the tiny creature scrapes off and feeds on diatoms, desmids, and other algae.

The pollywogs are enticing morsels for a wide range of predators. Water beetles, diving spiders, dragonfly larvae, salamanders, other frog species, snakes, crows, and herons are among the many consumers that snap up peeper tadpoles like moviegoers wolfing down popcorn.

Ultimately, the tadpole that has survived the hazards of the wild undergoes dramatic transformation: The external gills are lost, the tail is absorbed, limbs emerge, and changes occur in eye position and jaw shape. All in all, it becomes an animal ready to start life on land. Less obvious are numerous internal changes, including the development of lungs for gas exchange in the air and major digestive tract alterations that permit a shift from aquatic plant-eater to terrestrial flesh-eater.

Sadly, if the pond or pool evaporates under the sun's heat, only blackened stains of untransformed tadpoles are left in the mud, as evidence of the "song and dance" of spring peepers but a few months earlier.

A new adventure

If all has gone well, young froglets, some with vestiges of the tail, start the land phase of their existence. Now a half inch long, the young peepers are most active when the grass is wet with dew or after a rainy spell. But more hazards lie on the road to adulthood. A variety of snakes, frogs, many birds and mammals, and other creatures consume peepers of all ages.

While attempting to avoid their predators, peepers themselves are out and about searching for food, mainly at ground level. Motionless animals remain unrecognized as food. But when the peeper detects motion, its viscid tongue darts out to stick onto and retrieve a prey.

Most prey are slow-moving, tiny crawlers, including minute insects, spiders, mites, ticks, snails, and worms that occupy the leaf litter and shrub bases where peepers do most of their hunting. But numerous studies suggest that peepers and other amphibians are opportunistic feeders that consume whatever small animals are seasonally abundant. With luck, adult size is attained in two year's.

Surviving the winter is a major dilemma for all "cold-blooded" animals--those at the mercy of the external temperature. During the summer and autumn, the peepers have moved away from the ponds and toward the wooded regions from where the preceding generation of adults had started their springtime trek. An overly amorous male may call even as the last leaves fall from the surrounding trees in November. Here among the leaf litter and upper inches of soil, each peeper selects a site in which to burrow, in the hope that the site will not experience severe freezing that could terminate the animal's life.

Recent discoveries have revealed that tree frogs can survive the freezing of as much as 65 percent of the water content of their body during hibernation. The peepers' tissues and fluids accumulate glycerol and glucose, which function as "natural antifreeze" to cope with the cold. In this respect, the frogs resemble Antarctic fishes and numerous other poikilotherms.

Warming temperatures and the onset of rain signal an awakening of peepers from winter's dormancy. It is then another challenging trek to the ponds, where the sounds of a fresh welcoming chorus will provide the promise of another glorious spring.

Carl S. Oplinger is professor of biology at Muhlenberg College in Allentown, Pennsylvania, where he teaches ecology, zoology, and marine biology, Spring peepers were the focus of his doctoral study at Cornell University.

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