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New Research Shows that Toxic Birds and Poison-dart
SAN FRANCISCO (October 12, 2004) - In the mid 1960's, scientists identified a potent neurotoxin in the skin of a Colombian poison-dart frog. This toxin, called a batrachotoxin, was so powerful that Native Americans used it to coat the tips of their blow darts. Since the initial discovery, researchers have identified several species of frogs in the genus Phyllobates that contain batrachotoxins, but no one has been able to determine where the poison comes from. Poison-dart frogs that are raised in captivity do not contain detectable amounts of batrachotoxins, so scientists have assumed that they acquire their toxins from a dietary source in Colombia. However, restrictions on field work in Colombia have prevented scientists from tracking down the batrachotoxin source. The search seemed to have reached a dead end until Dr. Jack Dumbacher, Assistant Curator and Chair of the Ornithology and Mammalogy Department at the California Academy of Sciences, discovered batrachotoxins in another animal - a poisonous songbird from New Guinea.
In 1992, after hearing from local people in New Guinea about a bird called a Pitohui that caused burning or numbing sensations if it was eaten, Dumbacher tested the skin and feathers of the species and detected the presence of neurotoxins. This finding presented the first known example of chemical defense among birds. The results became even more intriguing when Dumbacher and his colleagues learned that the toxin in question had only been found in nature once before - in Colombia's poison-dart frogs. Over the next several years, Dumbacher found varying levels of batrachotoxins in five species of birds from the genus Pitohui, as well as in one species from the genus Ifrita. His data showed that the concentration of toxins in a bird's skin and feathers varied not only by species, but also by geographic location, suggesting that the birds - like the poison-dart frogs - were acquiring batrachotoxins from an environmental source. Since then, Dumbacher has been working with colleagues from the Smithsonian Conservation Research Center and the National Institutes of Health to determine which plant or insect may be responsible for producing the poison.
Their mission has been mammoth by any measure - New Guinea is home to over 700,000 known species of insects and about 15,000 plant species. Dumbacher attempted to probe the most promising suspects first by conducting stomach analyses on poisonous Pitohuis and Ifrita birds and then testing any identified species for toxins. He also talked to local villagers and asked them to point out any plants or insects they knew to cause burning or numbing sensations. He found the proverbial needle in the hay stack when villagers from Herowana pointed out a beetle from the genus Choresine that they called a nanisani. According to these local naturalists, the name "nanisani" refers specifically to the unusual numbing and tingling sensations to the lips and face that are caused by contact with one of these beetles. They also use "nanisani" as a name for the poisonous Ifrita bird. Dumbacher sent some of these beetles out for analysis, and test results confirmed that they contained batrachotoxins, making them a possible direct source of toxin for New Guniea's poisonous birds.
Dumbacher and his colleagues have yet to collect extensive stomach analysis data that would directly link the beetles to the birds, but in the tests they have run so far, they have already found a Choresine beetle in the stomach of a Pitohui. They have also found a number of Choresine-sized insects in both Pitohui and Ifrita specimens, suggesting that both types of birds likely acquire their poison from the beetles. The alternative hypothesis that both the birds and the beetles are acquiring their poison from a third source, such as a plant, is unlikely, since the Ifrita seems to be exclusively insectivorous. Choresine beetles belong to the Melyridae family - a family that is also found in the rain forests of Colombia. Thus, relatives of the "nanisani" beetles are a likely source of the batrachotoxins found in Colombia's poison-dart frogs. A paper describing these results will be published in the Proceedings of the National Academy of Sciences.
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