Western Newts

Dart Frogs

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Poison Dart Frogs
By Jessica Berti


Poison dart frogs are part of the family Dendrobatidae (8). These colorful dwellers of rainforests and other warm moist climates are called poison dart frogs or poison arrow frogs because South American Indians use the potent toxin secreted from granular skin glands on the back of some frogs to poison darts used for hunting (3, 8, 10). In the mid 1960s, this toxin known as batrachotoxin, was discovered in skin extracts of Columbian frogs in the family Dendrobatidae (5). Of the many poisonous frogs, only those from the genus Phyllobates secrete batrachotoxin. High levels of this toxin are found in the western Columbian Phyllobates terribilis, Phyllobates bicolor, and Phyllobates aurotaenia while trace amounts are found in the Central American Phyllobates vittatus and Phyllobates lugubris (4). The Noanama, Choco and Cuna Indians of Columbia use batrachotoxins from the three frogs with the most toxins to poison the tips of darts used for hunting (7).

Picture of Phyllobates terribilis (6)

Batrachotoxin is a steroidal alkaloid found in the granular skin glands of frogs in the genus Phyllobates (1). It protects the frogs by producing noxious effects in the mouths of predators (3). Although the toxin helps protect them, some snakes and large spiders are predators of adult frogs. Most other animals cannot tolerate the poison secreted from the skin glands. Predators of tadpoles, which are unprotected by toxins, include snakes and dragonfly larvae (8). Batrachotoxin is very potent and selective (1). The skin of Phyllobates terribilis contains about 1mg of batrachotoxin: more than any other species in that genus (5). This toxin is not produced by the frogs themselves, but is instead taken up through diet and secreted through the skin of the frog (6).

Structure of batrachotoxin (9).

John W. Daly, a leading expert in this field, studied captive frogs of the family Dendrobatidae, which did not produce toxins initially. These frogs were fed a diet of arthropods containing alkaloids. The alkaloids were accumulated unchanged in the skin of the experimental frogs (6). Usually captive frogs are not toxic because the alkaloid rich diet those in the rainforest normally eat is not available to the average pet owner (4).

Because of their diets, Phyllobates terribilis and the other species of poisonous frogs are toxic in the wild (5). The toxin found in these frogs, batrachotoxin, is very potent and specific. According to Jiri Patockaa and Ladislav Stredab, at doses less that 0.1 µg, symptoms observed in laboratory animals include convulsions, salivation, muscle contractions, and death. In mice, the subcutaneous LD50 is 0.2 µg/kg; though doses as low as 0.01 and 0.02 µg/kg have proven lethal (9). Myers et al. estimated a lethal injection dose of 2.0 to 7.5 µg for humans. When administered orally, the toxicity is much lower, making safe for Indians to consume animals hunted with the poison darts. The toxins may also be destroyed by cooking (9). Other symptoms of intoxication include muscle and respiratory paralysis, strong muscle contractions, convulsions, salivation, and death (7, 9).

In their book Principles of Toxicology, Thomas Brown and Karen Stine state that batrachotoxin acts by preventing voltage-gated sodium channels from closing in nerves. An action potential is not allowed to developed, and the nerve impulse is not sent on. Sodium channels are also made less selective; therefore, ions other than sodium can enter the cell through the channels (2). This toxin has been used to study the function of sodium channels because it is both potent and specific (5). Interestingly, tetrodotoxin is a noncompetitive inhibitor of this sodium channel activation. It binds to a separate site on the channel and acts as an antagonist (1). John W. Daly states that Phyllobates are not susceptible to the alkaloids because the sodium channels in their nerves and muscles are modified (3).

Poison dart frogs are not the only species that contain batrachotoxins. Some birds found in New Guinea as well as insects there and in South America also have been shown to contain this toxin. Like Phyllobates, the birds are thought to consume the toxin and secrete it through their skin. New research suggests that Choresine beetles are the source of batrachotoxin for the birds while relatives of this arthropod might be the source for frogs (5).

Nature has given these frogs a way to protect themselves. While beautifully colored and seemingly harmless, these toxic creatures secrete a very toxic substance from their skin. When traveling to South and Central America stay clear of these dangerous frogs.

(1) Bartels-Bernal, Eva, Terrone L. Rosenberry, and John W. Daly. “Effect of batrachotoxin on the electroplax of electric eel: Evidence for voltage-dependent interaction with sodium channels.” Proceedings of the National Academy of Science of the United States of America. Vol. 74, No. 3, pp. 951-955, March 1977 <http://www.pnas.org/cgi/reprint/74/3/951>

(2) Brown, Thomas M., and Karen E. Stine. Principles of Toxicology. New York. Lewis Publishers, 1996.

(3) Daly, John W. “Thirty Years of Discovering Arthropod Alkaloids in Amphibian Skin” Journal of Natural Products, 61, 1, 162 - 172, 1998, 10.1021/np970460e. <http://pubs.acs.org/journals/jnprdf/>

(4) Dumbacher John P., Thomas F. Spander, and John W. Daly “From the Cover: Batrachotoxin alkaloids from passerine birds: A second toxic bird genus (Ifrita kowaldi) from New Guinea” Proceedings of the National Academy of Science of the United States of America. Vol. 97: 12970-12975. <http://www.pnas.org/cgi/content/full/97/24/12970>

(5) Dumbacher, John P., Avit Wako, Scott R. Derrickson, Allan Samuelson, Thomas F. Spander, and John W. Daly. “Melyrid beetles (Choresine): A putative source for the batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds.” Proceeding of the National Academy of Sciences of the United States of America 2004 November 9; 101(45): 15857–15860. (2004). 5 December 2005 <http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=15520388#top>

(6) Grundsoe, Thomas and Mona. <http://www.grundzoo.dk/billedsider/froer.htm>

(7) Habermehl, Gerhard G. Venomous Animals and their Toxins. New York: Springer-Verlag, 1981.

(8) National Aquarium in Baltimore. 2005. Blue poison dart frog. 5 December 2005 <http://aqua.org/animals_bluepoisondartfrog.html>

(9) Patockaa, Jiri and Ladislav Stredab. “Brief Review of Natural Non-protein Neurotoxins.” The Applied Science and Analysis Newsletter. (2002). 5 December 2005 <http://www.asanltr.com/newsletter/02-2/articles/Neurotoxins.htm>

(10) Schafer, Rachel. 1999. "Dendrobates auratus" (On-line), Animal Diversity Web. 5 December 2005 <http://animaldiversity.ummz.umich.edu/site/accounts/information/Dendrobates_auratus.html>