Frogs: The Rainbow Connection
by Stanley Rand
On warm spring evenings, swamps, urban ponds, and even jungle movies are full of amphibian voices. Today, as when Mary Hinckley wrote in the Memoirs of the Boston Natural History Society in 1884, "As you approach a locality where (frogs) are in full voice, the air seems to grow gradually dense with this ear-deafening, all-pervading sound."
For some people, frogs provide a delightful natural serenade. For others, they are an intolerable nuisance. In Panama, where I studied frogs and other amphibians for years, a friend stuffed rocks into the rain gutters around his house to stop the incessant singing of the túngara frogs (Physalaemus pustulosus), while I installed a pond outside our windows to attract these vocal creatures.
Frogs live in a wide diversity of places, and have evolved often extraordinary adaptations in appearance and behavior to face the challenges of their surroundings. While about 3,500 species of frogs (I use "frogs" to include toads, as I explain later) exist worldwide, including 25 in the state of Virginia alone, many people have actually seen, or heard, only a few. But their livesand songsdeserve our full attention, as these marvelous creatures leap into an ever-changing world.
Perhaps to fully appreciate frogs, you need to understand why they are singing. Only male frogs sing. They do so to attract mates and warn off rivals. Female frogs use these songs to find males of their own species and select from among them the most attractive. Each species sings its own unique song, and with a little practice anyone can recognize all of the species in ones area.
The male frogs calls, given to attract females, may also attract the attention of predators. The fringe-lipped bat (Trachops cirrosus) in tropical America, for example, specializes in eating frogs. This bat locates its prey by listening for songs and, when it hears one, swoops down and picks the singer out of the water. A male túngara frog is caught between the risk of not attracting a mate if he does not call, and the risk of being caught by this bat if he does. As one line of defense, the frog adjusts his calls so that he takes the greatest risks from predators only when the risks of losing his mate to rivals singing nearby are highest.
Not all frog courtship is by singing. Frogs that live along mountain torrents, where calls can be lost in the sounds of the rushing water, may rhythmically wave their legs or arms to attract attention. Others emit a shrill whistle to be heard through the noiselike a doorman summoning a taxi. Once a pair has come together, the male and female touch and squeeze each other in a mating embrace known as amplexus, synchronizing the laying and fertilizing of eggs.
Competition for mates may involve fights between rivals. An unmated male English toad (Bufo bufo) will try to displace a more fortunate male from the back of the female that he is clasping, but the intruder does not persist indiscriminately. If the warning squawk of the clasping male is high pitchedsignaling that he is a small animalthe intruder perseveres, but if the call is deepindicating a large malethe intruder leaves to search for a smaller rival. Fights over mates usually involve males, but in several species, like Panamas tic frog (Eleutherodacylus diastema), a female may attempt to displace another female during courtship by forcing herself under the male.
Males often defend not only their mates, but also their calling sites, against rivals. A male may use a special call to warn off another male that begins calling too close to him. If the intruder continues, the resident may attack him. Usually a trial of strength quickly determines who wins the calling site, but male gladiator treefrogs (Hyla rosenbergi) have sharp bony spurs on their thumbs with which they can injure a rival, even puncturing his eardrum.
Mating toad (Bufo) males are particularly impetuous, occasionally forming a ball of squirming, croaking males, each attempting to force his way in toward the center. At the center of the toad ball is of course a femalebut often a dead one killed by her overly enthusiastic suitors. Toad males at the height of their breeding ardor seem especially undiscriminating, and may grasp any nearby object of appropriate size: a frog, a dead rat, or the toe of a frog watchers rubber boot.
Almost all frogs call by forcing air from their lungs through their larynx, causing the vocal folds to vibrate. This is much the same mechanism that we use to speak. But instead of exhaling to the outside as we do, a calling frog forces the air into an inflatable vocal sac. After the call is completed, the air is returned to the lungs to be used again. The vocal sac saves energy as well as aircritical to a male frog because calling is the most energetically expensive thing he ever does. Calling is so physically taxing that some male treefrogs abandon calling completely, instead sitting quietly next to a male that is calling, ready to intercept any female that approaches the caller. On any one night, these satellite males are less successful at getting mates than the real callers, but they reduce the energy they expend, and lower their risk of predation, so that they can keep trying on future nights.
Frogs of different species lay their eggs in a diversity of sites: at the bottom of a pool or stream, attached to aquatic vegetation, floating in a foam nest, attached to a leaf overhanging the water, or away from the water deep in a burrow or high in a tree hole. For many frogs, this is the extent of parental care. Once the eggs have been laid and fertilized, they are left on their own to develop. But some frogs exhibit more parental care than this, and do so in a variety of ways. In the completely aquatic Surinam toads (Pipa pipa), eggs are embedded in individual cavities in their mothers back where they develop and hatch as froglets. The mating pair must execute an intricate and precise maneuver to get the eggs laid, fertilized, and positioned correctly, because they do so without using their limbs.
Instead of carrying her eggs on her back, the gastric-brooding frog female (Rheobatrachus silus) swallows them. The eggs develop in her stomach until they are vomited up as froglets. In Darwins frog (Rhinoderma darwinii) in Chile, it is the male that takes the freshly laid eggs into his mouth, but instead of swallowing them, he maneuvers them into his vocal sac where they develop until they are ready to hop out. In the midwife toad (Alytes obstetricans), the male winds the string of eggs around his hind legs as they are laid and fertilized. When the eggs are ready to hatch, he hops awkwardly to the water where his offspring can emerge and swim away as tadpoles.
Female strawberry dart-poison frogs (Dendrobates pumilio) lay eggs in a curled leaf on the forest floor in Central America. The father tends the eggs, but when they hatch the mother carries the tadpoles on her back, one at a time, to tiny pools of water in the axils of large-leafed plants such as bromeliads. In these pools the tadpoles are safe from predators, but have little to eat. To feed them, the mother returns periodically to lay unfertilized eggs in each pool, providing the food the tadpoles need to grow and transform into frogs. African bullfrog (Pyxicephalus adspersus) fathers watch over their tadpoles as they develop, and if the pool in which his tadpoles are living should dry up, he digs an escape channel to a deeper and more permanent refuge.
It is because of their double life that frogs are called "amphibians." Typically they first live in the water as vegetarian tadpoles, and then move onto land as carnivores. (But not all tadpoles grow up in the water; some develop and transform into frogs before they hatch from the egg.) Tadpoles specialize in eating and growing. They feed mostly on plant material, scraping or biting bits off water plants, suctioning up detritus, or filtering microorganisms out of the water. Many take advantage of nutrient-rich temporary ponds that are relatively free of aquatic predators, particularly fish. In a few species, tadpoles are cannibals, specializing in eating other tadpoles, even individuals of their own species. Yet they choose tadpoles to which they are not closely related, in preference to eating their own siblings.
While tadpoles are largely herbivorous, the adult frogs they become generally make their living eating small animals: mostly insects, sowbugs, and worms. A few frogs are big enough to eat other frogs, as well as small lizards, mice, and occasionally a fish or a small bird. Usually, a frog catches its prey by flipping out its long sticky tongue and snatching it. Since frogs have very small teeth (if they have any at all), they don't chew their food, but swallow it wholestill alive and often still moving. I have seen a large grasshopper kicking in protest as it went down the gullet of a ravenous frog.
Almost everywhere that there is food there are frogs. Frogs live throughout the tropic and temperate zonesanywhere from near-waterless deserts, to mountain torrents, to rainforest canopiesand their populations even straddle the Arctic Circle in Scandinavia and coastal Alaska. Because of their permeable skin, frogs are at risk from water loss. To avoid drying out, many frogs live in wet places. Most species are active at night when the air is more humid, and spend the day in damp hiding places. Those living in deserts dig down to moist soil and stay there until it rainssometimes for months. When it does rain, the water-holding frog in Australia (Cyclorana platycephala) absorbs enough waterup to half of its weightto last through months, even years, of drought while sheltered in a cocoon-like chamber dug deep in the earth.
Burying yourself in the earth raises the problem of knowing when to emerge. Dwelling deep underground in the deserts of the southwestern U.S., spadefoot toads (Scaphiopus sp.) recognize that the rains have come by the vibrations of the drops hitting the surface above them. It has been reported that when off-road vehicles run over places where the toads are buried, the toads, tricked by the vibrations, dig up to the surface expecting to find water.
In contrast to the desert frogs, those that live along mountain torrents have too much water to deal with. Some tadpoles in these ecosystems have evolved large suckers on their undersides to climb spray-drenched cliffs and waterfalls, where they can graze on algae without being washed away. One place where you won't find frogs, however, is in the sea. Like slugs, frog skin cant stand salt, although the crab-eating frog (Rana cancrivora) of Southeast Asia can tolerate brackish water in its crab burrow lairs within mangrove swamps. Few native frogs, if any, are found on isolated oceanic islands like Hawaii.
Living in such diverse habitats, frogs have evolved a variety of sizes, shapes, and colors; but all are variations on a single theme. A frog may look like an unusual frog, yet it still looks like a frog. Nothing else looks even remotely froggishnot even frog fish, frog hoppers, or toad bugs.
Every frog has a short body without a neck or tail, and with long hind legs. It is a blueprint designed for jumping, although different kinds of frogs are specialized for different kinds of jumping. Just as you can distinguish professional football players from basketball players by their physique, you can pretty well predict how a frog will jump from its form. A frog's long hind legs provide the propulsion in its jump. Generally, the longer the legs, the longer the jump. If you ever lay a bet on a frog jumping contest, you should pick the frog with the longest legs. However, if you ever wager on a frog marathon, where endurance is at a premium, bet on the short-legged frog.
Among terrestrial vertebrates, frogs and their amphibian cousins are unique in their permeable skin and wide assortment of tightly set glands. These glands keep the skin moist, even slimy, and provide the poisons for defense against both predators and infections. A frog is not completely dependent on its lungs to pick up oxygen and get rid of excess carbon dioxide, as we are. Being permeable, a frogs skin can be used for respiration too. Yet this permeability, while advantageous for respiration, makes frogs particularly sensitive to airborne and waterborne pollution. And pollution is an increasingly serious problem for amphibians around the world, even for those in apparently pristine environments (see following article).
The answer to a question I am often asked, "What is the difference between frogs and toads?" lies in their skins. "Frog" and "toad" are both names that developed in England, where there exist only two sorts of tailless amphibians: slippery, long-legged ones called "frogs," and dry-skinned, short-legged ones called "toads." However, elsewhere in the world are found many other kinds of tailless amphibians that don't look either like English frogs or toads. I label them all "frogs," and call only the dumpy, dry-skinned ones "toads." So the short answer to the question is: "A toad is a dry frog."
Most frogs are camouflaged to avoid capture by predators, or detection by prey. Frogs that sit on leaves in the canopy, for example, are usually green, while those that burrow in the ground are earth-colored, and those that live on the floor of a tropical forest look like dead leaves. Individuals within a species can also vary greatly in pattern and color, making it hard for a predator to learn to recognize its prey. Even a naturalist can be fooled. A primatology student, who spent his days in Panama following white-faced capuchin troops, often brought me frogs for identification. For about a week straight, he presented me Panamanian common toads (Bufo typhonius), one after another, each individual so distinct in color and markings that he mistook them for separate species.
People have been interested in frogs for a long time. During excavations of a pre-Columbian village in the central provinces of Panama, my colleague, Smithsonian Tropical Research Institute archeologist Richard Cooke, found a collection of marine toad bones (Bufo marinus). To find out if the people who made the pits could have been eating the toads and then tossing out the bones, one of our graduate students organized a tasting. He deep-fried both frog legs and toad legs and presented them as canapés to a group of blindfolded Smithsonian secretaries, scientists, and students. After sampling both toads and frogs, we concluded that pre-Colombian gourmands might have eaten toads, but certainly would have preferred frogs.
Even nowadays, people who are not naturalists may encounter frogs in their daily lives. Humans use frogs as food, as pets, as medicine, and as decorations. When you work with frogs, as I do, people give you things in the shape of frogs. Every shelf in my office has frogs on it: a frog phone, a frog stapler, a frog letter opener. A frog mouse pad sits by my computer, and a frog calendar hangs above it.
More practically, frogs can act as biological control agents. They eat lots of insects, including agricultural pests and potential carriers of disease. Indian farmers defend their frogs for this reason. Once, while walking through a flooded rice paddy in central India, I grabbed one of the jumping frogs that dotted the paddy for a closer look. Anxiously, my guide warned me: "Be careful! If the villagers see you hurt a frog, they will stone us to death! They believe that frogs eat the malaria mosquitoes that breed in the flooded fields, and they are very serious about protecting them." I set the creature down gently, thinking it wise not to challenge such frog protectiveness.
Although they have been replaced by kits from the local drugstore, frogs were once widely used in pregnancy tests. In the days when toads (Bufo marinus) were used in Panamas public health clinics, a woman was expected to bring her own toad. I remember seeing small boys at the bus stop outside the Santo Tomas hospital toting cardboard boxes of toads for sale to women who had come without one.
Many frogs have poisonous skin for their own defense, and in the Choco rainforests of Colombia there are a few frog species (Phylobates sp.)bright yellow, and very poisonousthat are actually used by indigenous hunters to treat blowgun darts. Upon finding one such frog in the forest, a hunter pins it to the ground with a stick, and pulls his dart across its back a couple of times. He then has enough venom on his dart to kill a monkey, and probably a man. Scientists on an American Museum of Natural History expedition visiting these Indians in the 1960s reported finding frogs with thin black lines across their backsscars from being used to poison a dart. Medical researchers are studying the toxins produced by some of the "dart-poison" frogs for possible use in the treatment of human disease.
Today, the United States is a major international market for frogs. In addition to frogs imported as food, the U.S. Fish & Wildlife Service import summary for 1998 reported that some 15 million small frogs were imported alive into the U.S. that year, presumably for the pet trade. Over-exploitation of frogs as pets is probably not nearly as threatening to frog populations as habitat destruction is. However, one serious consequence of the frog trade is the risk that imported individuals may bring in diseases that infect wild, native species. Recent studies indeed show that disease is a major factor in the decline of many frog populations.
The current enthusiasm for keeping frogs as pets is relatively new in the U.S., but has existed in Europe for some years. Once, friends took me to visit the apartment of a Parisian frog enthusiast who had covered one wall of his living room with burlap, upon which he hung orchids and bromeliads. A recirculating pump kept water spraying onto the plants, as West Indian treefrogs (Eleutherodactylus johnstonei) climbed and sang amid the verdant flora. When the man asked his neighbors if the frogs bothered them, they told him no, but that they were annoyed by somebody in the building playing loud video games all night. The man chose not to inform them that they had been hearing the electronic-sounding songs of his pet treefrogs.
We frog enthusiasts often have favorite species. Many people find the red-eyed treefrogs (Agalychnis callidryas) so often seen on T-shirts beautiful. I have a personal preference for the less garish frogs with clean lines and bold patterns, like Colostethus talamancae, a small, brown frog with yellow racing stripes. Yet the most dazzling frog spectacle I have ever witnessed was the sight of orange dart-poison frogs (Dendrobates speciosus) hopping over a blanket of bright green moss jeweled with dewy mist, beneath the lush cloud forest canopy of western Panama. Beauty can depend on the beholders circumstances, however. A mudbrown toadlet (Physalaemus sp.) hidden under a dead leaf is truly ravishing when I finally catch him in my headlightafter spending half an hour following his calls in the rainand realize that he is a species new to me, and perhaps new to science.
Frogs fabled tales and appearances as stuffed toys and ceramic ornaments have hopped into the consciousness of cultures around the world. But with their pervasive voices and fascinatingly varied natural histories, frogs themselves well repay study, particularly as their populationsfrom the high Sierras of California to the rainforests of Queenslandare dwindling. I hope that an understanding of the dazzling rainbow of frog species, and of the spectrum of their songs and stories, will help encourage their conservationand help keep them singing.
A. Stanley Rand, Senior Biologist Emeritus at the Smithsonian Tropical Research Institute, studies both amphibians and reptiles. John Nethertons wide array of frog photos are featured in the Voyageur Press book Frogs, on sale in the National Zoo Bookstore.
ZooGoer 29(2) 2000. Copyright 2000 Friends of the National Zoo. All rights reserved.
