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Poison Dart Frogs
Poison Dart Frogs
"Are those frogs real? I thought they were plastic!" A comment most common when people see these frogs. Poison Dart Frogs, poison-arrow frogs, poison frogs and dendrobatid frogs are all names for small, brightly-colored frogs from Central and South America. These are members of the Dendrobatidae family, which secrete poisons from glands in their skin. They are also known as "poison dart frogs" because the secretions of some species of the genus Phylobates (some of which are so poisonous they are dangerous to handle) are used by the Choco in Colombia as a coating for blow darts used to tranquilize potential prey. The secretions from all dendrobatid frogs are poisonous, but only a few are toxic enough to kill a person. However, once they are fed a diet of fruit flies, they soon loose their skin toxins.
The family Dendrobatidae has eight genera with 179 species of poison dart frog known already. Obviously the most visible adaptation of the poison dart frog is its distinctive coloration, which is believed to warn off predators that see colors. The boldly contrasting patterns are thought by some to be aposematic, that is, being conspicuous and serving to warn even those animals that cannot see color.
These frogs have another defensive mechanism, alkaloid toxins that they secrete through canals in their skin. If touched or bitten, they expel the toxic substance. Typically, the toxins have a particular odor and a bitter, peppery taste that can induce vomiting, thereby forcing a predator to spit out the frog.
It turns out that the alkaloid poisons in the skins of poison dart frogs -- called batrachotoxins -- are not made by the frogs themselves but by their prey. Some frogs simply store their prey toxins in their skins, but others may metabolize them to produce even more toxic forms. And when they shed their skins (as they do occasionally), they eat the skin recycling the toxin.
Poison dart frogs tend to become specialists in the specific insects -- mostly ants-- that make the toxins they store. These prey animals may in turn have stored toxins from plants that they feed on! The trick that allows these frogs to consume toxic alkaloids, convert them to even more toxic compounds, and then store them without themselves being poisoned has not yet been worked out. What's interesting is that at least three species of the Dendrobates frogs of the New World tropics modify an alkaloid to create one that's about five times as poisonous, according to a team led by John W. Daly of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in Bethesda, Md. The souped-up poison, one of a class called pumiliotoxins, ends up as a protective agent in the frogs' skin. "... It shows how chemistry connects the life of one organism to another, " comments chemical ecologist Jerrold Meinwald of Cornell University.
These frogs are diurnal (which means day active), 20 to 50 mm from snout to tail, and come in a rainbow of "warning" colors. Females are generally bigger (2 to 5 mm) and fatter, and some males in certain species have broader plates on their forelegs. Males also have an enlarged neck sack that is used to croak and pick up chicks.
A male in his own territory will inflate his vocal sac to call a female that is ready to court. The female comes and follows the male to the site of egg laying and the two frogs may dance around one another or even rub noses. Mating procedures vary even within the same species. Sometimes the eggs are fertilized after the female has left. The male then may move the eggs about to make sure they are all fertilized. Clutches of eggs are small -- from 2 - 30 -- and the parents guard the eggs and keep them wet.
A "nurse frog" collects hatching tadpoles on her back a few at a time, where they adhere to a sticky mucus. The tadpoles are delivered to a pool of water in a cup of leaves or in some other protected area. Only a few tadpoles are placed in each location, because crowded tadpoles become cannibals. As the tadpoles become frogs, the gills initially used to breathe are replaced with lungs. Legs are grown and tails are lost in preparation for leaving their watery habitat for that of the rainforest floor.
Although small, poison dart frogs play an important role in their ecosystems. Their diet helps control populations of various rainforest pests. Research indicates that the alkaloids found in poison dart frogs may have medical applications for humans as non-addictive painkillers and cardiac stimulants for heart attack victims. Much of the basic research on tropical poison frogs has been conducted by scientists in the Department of Herpetology at the American Museum of Natural History in collaboration with colleagues at the National Institutes of Health. Their work in jungle camps and modern laboratories has led to the discovery of dozens of new species of frogs and about 300 new alkaloids from frog skin. Many of these toxic alkaloids, including those from the skin of the poison dart frog, have unique effects on nerve and muscle. For that reason, the frog toxins have become important tools in biomedical research aimed at better understanding and treatment of neurological and muscular disorders. Someday, your life may literally be saved by a frog.
Sources— www.amnh.org/exhibitions/expeditions/treasure_fossil/Treasures/Poison_Dart_Frogs/frogs.html?50; www.peabody.yale.edu/explore/disc_frogs.html; http://department.txwes.edu/bio/mclark/Javafiles/jendex3.html; http://animaldiversity.ummz.umich.edu/site/resources/Grzimek_herps/Dendrobatidae/
Dendrobates_auratus.jpg/view.html; www.asanltr.com/ASANews-99/995frogs.htm; www.hilozoo.com/animals; http://library.thinkquest.org/C007974/2_2poi.htm; www.sciencenews.org/articles/20030906/fob5.asp; www.aquariumofpacific.org/ANIMAL_DATABASE/animaldb.asp?id=84