Frog - Uses

Uses

See also: Animal testing on frogs

Frog legs are eaten by humans in many parts of the world. Originally they were supplied from local wild populations but overexploitation led to a diminution in the supply. This resulted in the development of frog farming and a global trade in frogs. The main importing countries are France, Belgium, Luxembourg and the United States while the chief exporting nations are Indonesia and China. The annual global trade in the American bullfrog (Rana catesbeiana), mostly farmed in China, varies between 1.2 and 2.4 thousand tonnes.

Frogs are sometimes used for dissections in high school and university anatomy classes, often first being injected with coloured substances to enhance the contrast between the biological systems. This practice is declining with increasing concerns about animal welfare and "digital frogs" are now available for virtual dissection.

Frogs have served as experimental animals throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system through studying frogs. In 1852, H. F. Stannius used a frog's heart in a procedure called a Stannius ligature to demonstrate that the ventricle and atria beat independently of each other and at different rates. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy assays in the first half of the 20th century. A sample of urine from a pregnant woman injected into a female frog induces it to lay eggs, a discovery made by the English zoologist, Lancelot Hogben. This is due to the fact that a hormone, human chorionic gonadotropin, is present in substantial quantities in the urine of women during pregnancy. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer. This was the same technique later used to create Dolly the sheep and their experiment was the first time that successful nuclear transplantation had been accomplished in higher animals.

Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy assays have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable and can easily be kept in the laboratory. Xenopus laevis is increasingly being displaced by its smaller relative, Xenopus tropicalis, which reaches its reproductive age in five months rather than the one to two years taken by X. laevis, thus facilitating faster studies across generations. The genome of X. tropicalis is being sequenced.

Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than morphine, is found in some species of poison dart frogs. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection. Dart poisons are under active investigation for their potential as therapeutic drugs.

It has long been suspected that pre-Columbian Mesoamericans used a toxic secretion produced by the cane toad (Bufo marinus) as a hallucinogen, but it is more likely that they used substances secreted by the Colorado River toad (Bufo alvarius). These contain bufotenin (5-MeO-DMT), a psychoactive compound that has been used in modern times as a recreational drug. Typically, the skin secretions are dried and then smoked. Illicit drug use by licking the skin of a toad has been reported in the media but this may be an urban myth.

Exudations from the skin of the golden poison frog (Phyllobates terribilis) are traditionally used by native Colombians to poison the darts they use for hunting. The tip of the projectile is rubbed over the back of the frog and the dart is launched from a blowgun. The combination of the two alkaloid toxins batrachotoxin and homobatrachotoxin is so powerful that it has been estimated that one frog contains enough poison to kill about 22,000 mice. Two other species, the Kokoe poison dart frog (Phyllobates aurotaenia) and the black-legged dart frog (Phyllobates bicolor) are also used for this purpose. These are less toxic and less abundant than the golden poison frog. They are impaled on pointed sticks and may be heated over a fire in order to maximise the quantity of poison that can be transferred to the dart.

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