Sunday, 12 December 2010

Poison dart frog

Most species of poison dart frogs are small, sometimes less than 1.5 centimetres (0.59 in) in adult length, although a few are up to 6 centimetres (2.4 in) in length. They weigh about 2 grams, depending on the size of the frog. Most poison dart frogs are brightly colored, displaying aposematic patterns to warn potential predators. Their bright coloration is associated with their toxicity and levels of alkaloids. Frogs like the ones of Dendrobates species have high levels of alkaloids, whereas the Colostethus species are cryptically colored and are non-toxic.[5] Unlike most other frogs, they are diurnal, rather than being primarily nocturnal or crepuscular.[6] When born and raised in captivity, poison frogs do not produce the skin toxins which they retain in their native habitat.[7]

They lay their eggs in moist places, including on leaves, in plants, among exposed roots, and elsewhere, and allow the tadpoles to wriggle onto their backs shortly after they hatch. They then carry the piggy-backed tadpoles to water, where the larva remain until metamorphosis. The water is typically a pool, but some species use the water gathered in bromeliads or other plants; and some species provide food, supplying the tadpoles with unfertilized eggs to eat.[6]

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Many poison dart frogs secrete lipophilic alkaloid toxins through their skin. Alkaloids in the skin glands of poison frogs serve as a chemical defense against predation, and they are therefore able to be active alongside potential predators during the day. About 28 structural classes of alkaloids are known in poison frogs.[3][16] The most toxic of poison-dart frog species is Phyllobates terribilis. It is argued that dart frogs do not synthesize their poisons, but sequester the chemicals from arthropod prey items, such as ants, centipedes and mites. This is known as the dietary hypothesis.[17] Because of this, captive-bred animals do not contain significant levels of toxins. Despite the toxins used by some poison dart frogs, there are some predators that have developed the ability to withstand them, including the Amazon ground snake (Liophis epinephelus).[18]

Chemicals extracted from the skin of Epipedobates tricolor may be shown to have medicinal value.[19] One such chemical is a painkiller 200 times as potent as morphine, called epibatidine, that has unfortunately demonstrated unacceptable gastrointestinal side effects in humans.[20] Secretions from dendrobatids are also showing promise as muscle relaxants, heart stimulants and appetite suppressants.[21] The most poisonous of these frogs, the Golden Poison Frog (Phyllobates terribilis), has enough toxin on average to kill ten to twenty men or about ten thousand mice.[22] Most other dendrobatids, while colorful and toxic enough to discourage predation, pose far less risk to humans or other large animals.

Like many frog families, dendrobatids have also been affected by the worldwide decline in amphibian populations. Habitat loss (due to logging and farming) and predation by introduced species are among the more common causes, but the cutaneous chytridiomycosis has struck dart frogs the hardest in the past 25 years.[25] Zoos have tried to counteract this disease by treating captive frogs with an antifungal agent that is used to kill athlete's foot in humans.[26]

taken from wikipedia

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