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Frogs
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The typical frog has long hind legs, a large head, a short body, and no tail; an amphibian, it may live entirely in water or may spend much of its life on land. Frogs are found on every continent except Antarctica. They are thought to have evolved from a tailed, four-limbed, amphibious ancestor; their means of locomotion has changed since the time of that ancestor as a result of the loss of the tail, reduction in the number of trunk vertebrae, and lengthening of the hind legs and feet. Their feeding system has also altered, either because the use of the tongue was virtually abandoned or, conversely, because its prey-catching use was stressed. Frogs have adapted to a great variety of ecological situations.
CLASSIFICATION
The classification of frogs at the family level is currently debated. It is usually determined by characteristics of skeletal and muscular structures in larvae and adults, features of the frogs' life histories, and certain genetic and biochemical components. One current theory states that the Leiopelmatidae of New Zealand and the Pacific Northwest are the most primitive living frogs. The Discoglossidae of Europe, Asia Minor, China, northwestern Africa, and the Philippines are also considered primitive.
The family Pipidae of South America and Africa south of the Sahara is a highly specialized group; the African clawed toad, Xenopus laevis, is a member of this family. The southern Mexican Rhinophrynidae, with its single species, is related to pipids.
The Pelobatidae, spadefoot toads of Europe, North America, and Southeast Asia, and the Pelodytidae, with only two species, are a distinct group. The family RanidaeÑthe true frogs, such as the leopard frog and bullfrogÑis large and widely distributed in North and South America, Europe, Asia, and northern Australia. Ranid relatives include the following: the Hyperoliidae of Africa, Madagascar, and the Seychelles; the Mantellidae, also of Madagascar; the Hemisotidae and Arthroleptilidae of Africa; and the Rhacophoridae of Africa and Southeast Asia.
The Bufonidae, or true toads, inhabit all landmasses except Australia, New Zealand, New Guinea, and Greenland. Several Central and South American groups are closely related to bufonids, as are the Sooglossidae, two genera on the Seychelles Islands and Madagascar. The Heleophrynidae of South America and the Myobatrachidae of Australia and New Guinea have affinities with the Leptodactylidae of the New World. Also related are the Hylidae, widespread only in the Americas but with one genus, Hyla, occurring nearly worldwide; the arrow poison frogs, Dendrobatidae, and the green-boned frogs, Centrolenidae, of Central and South America; the Pseudidae of South America; and the Rhinodermatidae of Chile. The Microhylidae of the Americas, Southeast Asia, and Africa constitute a separate group.
HABITAT
Frogs live in diverse habitats. Pipid frogs and many other types spend their entire lives in ponds and streams in the tropics. Other species require only the moisture found on leaves or under rocks or logs to survive. Desert-dwelling spadefoot toads live buried in sand much of the year and emerge to breed only when it rains. They lay their eggs in puddles, and tadpole metamorphosis must be completed before the puddle dries up. Frogs are widely distributed from lowland tropics to high mountains and high latitudes and from very wet to seasonally dry habitats. Most frogs practice external fertilization in waterÑthe female lays eggs and the male sheds sperm over them. After hatching, the tadpole lives in water, at least until it metamorphoses into adulthood. Only a few species lack the tadpole stage.
STRUCTURE AND FUNCTION
Frogs may be easily identified and classified into species by various external characteristics. These include the proportions of the head, body, and limbs; color patterns; placement of the tympanum, or eardrum; toe webbing and shape; and such ornamentation as cranial crests, the so-called claws, and skin structures. Features unique to frogs include their skeletal structure, their hearing systems (used to perceive the species-specific mating calls), tongue feeding, and modifications of the larynx and vocal sacs that serve to produce sound. Unlike adults, frog tadpoles have tails, respire through internal gills, have cartilaginous skeletons, lack teeth but have horny mouthparts, are limbless until late in development, are usually herbivorous, and have long, coiled intestines.
Frogs, like other amphibians, are cold-blooded (poikilothermic). Scientists formerly believed that these animals had to avoid excessive heat or cold to survive. Recent studies, however, have found that some frogs, like the wood frog (Rana sylvatica) and the spring peeper (Hyla crucifer) have a natural ability to survive cold temperatures for weeks with as much as 65 percent of their total body water as ice. When ice forms on the frog's skin, it triggers enzymes to convert glycogen to glucose in the liver. The glucose travels in the blood to the major organs where it protects them from freezing while ice fills up the frog's body cavities.
NATURAL HISTORY
The frog's life cycle is complex. Courtship in many species is highly stylized, involving specific calls or prescribed turns and positions before mating occurs. At one point in the ritual, the male clasps the female, and she extrudes her eggs; given this stimulus, he releases his sperm, shedding them over the eggs.
Some females lay several hundred eggs and abandon them. Other frog species lay fewer eggs and protect the developing young in a variety of ways. In the case of the marsupial frog, Gastrotheca, the young develop in a pouch in the skin of the back. Rhinoderma darwini males brood the young in their vocal sacs. Other means of reducing the number of young include direct developmentÑlaying eggs on land, which then develop through metamorphosis without the free-living larval stageÑand maternal retention of developing young. Direct development occurs in the genus Eleutherodactylus, among others. Frequently, only one parent protects the clutch while it develops. A few species retain developing young in their oviducts and subsequently give birth to metamorphosed froglets. The Australian Rheobatrachus silus broods its eggs in its stomach and coughs up metamorphosed froglets at birth.
In aquatic development, the tadpole's mouthparts develop at hatching, gills are covered by the operculum, and the tail structure is finned. Hind limbs often develop quickly. Tadpoles are of different body shapes, depending on their habitat, and their mouthparts also vary. At metamorphosis, forelimbs emerge, the tail begins to resorb, the shape of the head changes, jaws modify, the intestine shortens, and a host of biochemical changes takes place. During metamorphosis the tadpole is vulnerable to predators, and it cannot feed while its body is reorganizing.
Vocalization in frogs occurs as inhaled air is directed into the vocal sacs (pockets in the floor of the mouth) and forced through the larynx at various rates and frequencies. The inner ear of a frog that vocalizes has sensory cells that are specialized to perceive sound at specific frequencies.
The adaptation of long hind limbs has resulted in various locomotor patterns. Temperature, body weight, body proportions, and resistance of the medium all affect the method and rate of locomotion. Frogs can leap long distances but also use their limbs to swim, to burrow, and even to glide from tree to tree. The webbing between the toes makes the feet effective paddles for swimming in the aquatic and semiaquatic species, and makes hands and feet parachutes in some arboreal species. Other tree frogs have modified the tips of the toes as holding devices.
There are two types of feeding: tongueless and tongued. Tongueless aquatic species feed by gulping prey into their mouths; tongued forms extend the tongues to capture prey. The end of the tongue has glands that secrete a viscous material that holds the prey on contact. The tongue and prey are then withdrawn into the mouth. Frogs and toads typically are carnivorous, eating a diversity of terrestrial and aquatic animals, such as insects, other arthropods, and worms. Tadpoles usually eat plant material, scraping algae and grazing on aquatic plants.
IMPORTANCE OF FROGS
Frogs are significant predators in their ecosystems, keeping many kinds of insects under control. They have other uses, such as being subjects for studies in biology, especially of development. As certain frog populations of the United States have been depleted, African clawed frogs have been imported in increasing numbers. This has led to a further problem: when released in nature for breeding purposes, the African clawed frog competes with, and eclipses, the native fauna. This situation is currently under control, but the problem of supply of biological experimental material is not yet solved. Frog legs continue to be eaten by many people, resulting in the overcollecting of frogs in Europe and India.
Presumably because of features of their life cycles (such as having both aquatic and terrestrial stages), many species of frogs are sensitive to environmental perturbations, such as acid rain, increasing ultraviolet irradiation, and changes in land and water through fertilizers and other factors. A consequence of recent human actions is the rapid reduction and even disappearance of frogs in many parts of the world. In the mid-1990s, there was a startling increase in deformed tadpoles and frogs in the United States, and many causes were posited by researchers, including chemical pollution of the water and too much ultraviolet light coming through a thinning ozone layer. A 1999 study found that in some places, parasites were infecting the developing limbs and causing the deformations.
LIFE OF A FROG
Scientists have identified more than 3,700 species, or kinds, of frogs and toads. They belong to a group of animals called amphibians. Like other amphibians, they generally spend part of their life in water and part on land. Most species live in or near ponds, marshes, rain forests, and other wet places.Some toads live in dry places, however, including deserts. Their interesting life cycle involves a drastic change of formÑa process called metamorphosis. The creature that hatches from a frog egg must live in water. It has a finned tail and gills and is known as a tadpole. As the tadpole grows, it gradually metamorphoses, or undergoes changes. The gills become covered by skin. Lungs and legs develop, and the tail disappears. By the time it is ready to leave the water, it has turned into an adult frog, able to breathe air and move about on land. Frogs play important roles in the environment. They have big appetites and help keep insect populations in check. In turn, frogs themselves are an important source of food for snakes and other animals.
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