Amphibians and Humans
Amphibians and humans
Amphibians have figured in the lives of humans since antiquity. Frogs and salamanders are richly represented in mythology, culture, art, and literature, and even today they are seen as attractive characters in commercial advertising and as whimsical stars on television. In contrast to these roles, which in some cases are rather superficial, amphibians are of real importance as food and the source of compounds of medicinal value. They are key organisms in research and teaching and for purposes of natural control of insects. Despite the importance of amphibians, humans' actions have negative effects on them in numerous direct and indirect ways—through introductions of exotic species, in the loss or alteration of habitats, and even by overcollecting. Several species are believed to have become extinct within the past two decades, probably owing to human activities.
Mythology and culture
Frogs and salamanders have appeared in the legends and folklore of many cultures throughout history. Certain beliefs, such as the connections of amphibians to water, rainfall, earth, and the underground, recur in diverse cultures, but other beliefs are more localized. Such linkages often are depicted in indigenous art. For example, the Zuñis of New Mexico even today decorate their water-holding pots with frog tadpoles. In ancient Egypt frogs were associated with water and mud because of their sudden reappearance and reproductive activity following the annual flooding of the Nile River Valley. Thus, frogs came to symbolize birth and resurrection. Among the hieroglyphics found on the walls of the Egyptian funerary temple of Hatshepsut (queen of Egypt during the fifteenth century b.c.) are images of the god of creation, Khnum, and his wife, the frog-headed Heqet, forming children on a potter's wheel. Indeed, several Egyptian gods were depicted with the heads of frogs.
Half a world away, in the Mayan culture of the Yucatán Peninsula of Central America, frogs and toads were believed to announce the rains with their choruses. Today's Maya still perform rain dances, rituals that are thought to be of great antiquity. At one point in the ceremony, four boys are tied to the altar and mimic the calls of two different species (Bufo marinus and Rhinophrynus dorsalis). The Maya also associated frogs with agriculture. The Madrid Codex, a fifteenth-century Mayan almanac painted on plaster-coated bark paper, shows frogs making furrows with sticks and sowing seeds. One frog, which the Maya called the uo, was thought to come from the sky with green corn grains in its intestines. The uo was probably Rhinophrynus, which breeds only during heavy rains. The name uo is onomatopoetic—the name represents the sound of the frog's call. Uo is also the name of the Mayan month of greatest rainfall.
In other cultures amphibians were believed to have mystical powers, and shamans used their images in various rituals. Several ancient cultures in Egypt, Greece, Turkey, and Italy had images of frogs as amulets for good luck or to ward off evil; this is true even in present-day Myanmar (Burma). The Itelmens, aboriginal people of the Kamchatka Peninsula of eastern Siberia, considered hynobiid salamanders (Salamandrella keyserlingii) to be spies sent by Gaech, lord of the underground, to find and capture them for their master. Another Siberian nation, the Selkups, thought that frogs protected them from evil spirits, and frog images thus were used widely by shamans in ceremonies. In medieval Europe, much interest was attached to the toad because of its poison glands, and extracts from the glands were employed in witchcraft. It was believed that the toad could withstand its own poison by carrying around an antidote in the form of a stone located in its head. Shamans used so-called toad stones—in practice, any stone the size and shape of a toad—to neutralize poisons from snakebites or bee stings.
Another common belief in European culture, since at least the time of Pliny the Elder in the first century a.d., is the myth of the invulnerability of salamanders to fire. The common name of the European Salamandra salamandra—the fire salamander—is directly traceable to this legend. Images of salamanders emerging from fire have led to an otherwise inexplicable association among the common dictionary definitions of the term salamander. Among these definitions are "a mythical being thought to live in fire," "a portable stove or burner," and "the mass of iron that accumulates at the bottom of a blast furnace." When asbestos, an incombustible mineral, was discovered, it was believed to be the hair of the salamander and sometimes was referred to as "salamander's wool." The basis for the association between salamanders and fire is thought to be that salamanders seemed to emerge from the flames when the logs in which they were hiding were
thrown onto a fire. Recent observations of California newts (Taricha torosa), which cover their bodies with slime secreted by their glands and then walk unaffected through the flame fronts of brush fires, demonstrate that these amphibians have a greater tolerance for fire than was previously understood.
Art and literature
Traditional art often has incorporated the likeness of frogs. Among the weaving patterns of the Native South Americans of northeastern South America, frogs appear regularly, often in a highly stylized form that resembles a dumbbell. The pre-Columbian Hopewell Native Americans of the Upper Ohio River Valley smoked ceremonial pipes in the shape of frog effigies. The fleur-de-lis, the traditional iris symbol of French kings and of France, originally was depicted as a group of three frogs.
Amphibians also have figured regularly in literature. The Frogs, a Greek satirical play, was first performed in Athens in 405 b.c. In this play Aristophanes used frogs to make fun of humans when the chorus repeatedly sings out to the god Dionysus, the patron of drama, as he crosses the River Styx to enter Hades and bring back the playwright Euripedes. The call, "Brekekekex, co-äx, co-äx," is thought to be the first use of phonetic imitations of animal sounds in literature. Many of Aesop's famous animal fables dealt with frogs, and the traditional fairy tale of the prince turned into a frog by a wicked witch, only to be restored by the kiss of a beautiful princess, is widely known.
Shakespeare regularly used frog and salamander references in his plays. In Richard III, he derisively referred to the king as "that bottled spider, that foul bunch-back'd toad." The three witches in Macbeth chant, "Eye of newt, and toe of frog," as they stir those ingredients into their evil brew. In As You Like It, Shakespeare made yet another of his many toad metaphors: "Sweet are the uses of adversity, / Which, like the toad, ugly and venomous, / Wears yet a precious jewel in his head." The jewel, often thought to signify the toad's beautiful eye, may well refer to a toad stone.
Later literary references to amphibians include those in Mark Twain's first story, "The Celebrated Jumping Frog of Calaveras County," which featured a frog by the name of Dan'l Webster, and Karel Čapek's science fiction thriller War with the Newts. Through the ages amphibians have held a secure place in mythology, art, and literature, as they still do in today's culture. Frogs are used regularly in commercial advertising—whether for beer or shoe polish—and arguably the most famous anuran of them all, Kermit the Frog, the muppet star of Sesame Street on public television, is loved by children around the world.
Medical and research uses
Amphibians have been employed for medicinal purposes for millennia. The Chinese brown frog (Rana chensinensis) has long been used in traditional medicine in the three northeastern provinces of the country. An oil called "Ha Shi Ma Yu," derived from the dried oviducts, is believed to cure nervous exhaustion. Until the 1970s, as many as 72 million frogs were collected annually for the purpose of obtaining this oil, but the yields have now dropped below five million as the result of habitat loss and overcollecting. Until a century ago frog egg clutches were used as plaster in Russia, frog meat was put on snakebite wounds in western Siberia, and teas made from dried and powdered hynobiid salamanders (Ranodon sibiricus) were used to treat bone fractures and malaria in northwestern China. Amphibians continue to be an important part of traditional medicine in many parts of the world.
More than 200 psychoactive alkaloids have been extracted from the skin of frogs and toads. For these amphibians, they act as natural chemical defenses by affecting the muscles and nerves of would-be predators. Scientists have been able to synthesize many of these alkaloids for research. One of them, batrachotoxin (found only in the skin of the dendrobatid frog Phyllobates), causes ion channels in nerve and muscle cells to fail, resulting in heart failure; when it is labeled radioactively the toxin becomes a very useful tool for medical research. Another alkaloid, epibatidine (from the skin of another dendrobatid, Epipedobates tricolor), is a highly effective painkiller; it is 200 times stronger than morphine, but it is not addictive and has no sedating effects. Epibatidine is produced synthetically and is being tested as a drug for humans. Skin secretions from
the green treefrog (Litoria caerulea), called caeruletide, stimulate activity in the pancreas and intestine, and synthetic versions of it are commercially available for human use for these purposes.
The large parotid glands of toads of the genus Bufo, located just behind the eyes, produce two substances—bufogenin and bufotoxin—that affect the adrenal and cardiovascular systems in humans. A third parotoid secretion—bufotenin, an alkaloid—is a powerful hallucinogen. The Colorado River toad (Bufo alvarius) possesses the specific enzyme for production of this sub-stance, and the parotoids, which can contain large amounts of the hallucinogen, can produce hallucinations when the skin is dried and smoked. The hallucinogenic properties of toad parotoid glands were well known to the native peoples of Central America, and images of toads with prominent parotoid glands are a common feature on bowls and other objects found at archeological sites.
Frog skin secretions also can have powerful antimicrobial properties. The skin of African clawed frogs (Xenopus laevis) produces peptides called magainins that assist in the natural healing of cuts and bruises. These peptides have potential as a new class of antibiotics. Glues extracted from frog skin can be used to fix crockery, and research suggests that skin secretions may help repair human internal organs.
Among the many medical and research applications of amphibians, frogs and salamanders have been standard laboratory preparations for studies in embryology and physiology. Amphibians are also highly useful model organisms for many field studies of behavior and ecology. Xenopus frogs were the first test organisms to be used for determining pregnancy in humans. Frogs and salamanders are commonly found in biology teaching laboratories throughout the world.
Other human uses of amphibians
Frogs—mainly their large, muscular legs—are eaten regularly by many indigenous peoples, especially in impoverished societies, where they constitute an important source of protein. In the Rift Valley of eastern Africa, clawed frogs (Xenopus) are netted in huge numbers as seasonal supplements to human diets. In affluent countries, frogs' legs are consumed as a delicacy but also as a meat alternative during Lent. The international trade in frogs' legs is enormous and mostly originates in southern Asia and the East Indies. The wild capture of so many frogs, which are insectivores, has resulted in growing populations of mosquitoes and other insects in these countries. Salamanders are eaten rarely, but a major exception is the Chinese giant salamander (Andrias davidianus). This animal, which reaches a length of 5.25 ft (1.6 m) and a weight of 143 lb (65 kg), is raised in farm ponds in China for food. Because of its importance as a food item, an institute devoted to the biology of this species has been established in Hunan Province.
Amphibians are key elements in many ecosystems. They feed primarily on invertebrate prey, especially insects, and thus represent an all-important trophic link between their prey and the larger animals that, in turn, feed on them. Because of their insectivorous nature, frogs and toads, among them, North American bullfrogs (Rana catesbeiana), have been introduced to many parts of the world to control insect populations. A poison frog species, Dendrobates auratus, was introduced to Hawaii from Panama in 1932 to help control mosquitoes. The most famous, and ill-advised, introduction was that of the marine or cane toad (Bufo marinus) to Australia, ostensibly to get rid of a beetle that infested sugarcane. In 1935, 102 toads were released in Queensland. The experiment failed to control the beetle population. Moreover, the toads ate a wide variety of prey, including native frogs; reproduced rapidly in the absence of natural predators; and expanded their range enormously. Today the toads represent a major challenge in themselves and have created a new and destabilizing relationship between amphibians and humans.
Over the years, beginning in Europe in the late eighteenth century, amphibians have become popular as terrarium pets. Many species are regularly kept, including newts, colorful hylids, neotropical poison frogs, Madagascan mantellas and tomato frogs, and aquatic caecilians. Terrarium keepers often have made observations that are of importance to science. Some species have been bred successfully, and an entire industry has developed around amphibians as pets, including public expositions, wholesale and retail dealers in live specimens, veterinarians that specialize in their care, amphibian keepers' magazines, and texts on medicine and husbandry. It seems that we have reached the ultimate in relationships: amphibians as human companions.
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Kraig Adler, PhD