Toxic Frogs, Birds May Get Their Poison From Beetles
for National Geographic News
|November 9, 2004|
The Colombian poison-dart frog and six Papua New Guinea birds, mostly jay-sized songbird species commonly known as pitohui, live almost at opposite ends of the Earth. But the animals share one thing in common: They use batrachotoxin, a rare neurotoxin that is 250 times more potent than strychnine.
Researchers believe the creatures use the poison, which laces their skin and/or feathers, as a type of biodefense that protects the animals from predators and parasites.
One enduring mystery, however, has been the source of the poison: Scientists suspect the birds and amphibian can't manufacture batrachotoxin naturally.
Now researchers say they may have discovered how the animals obtain the toxin: They eat beetles riddled with the stuff.
In a study published last week in the Proceedings of the National Academy of Sciences, researchers report that they have found batrachotoxins in a little-known group of beetles from the Choresine genus. The discovery marks the first time the toxin has been found in an insect.
Biologists found the same beetles in the stomachs of the Papua New Guinean birds, which they say suggests that eating Choresine beetles provides the South Pacific animals with the key ingredient for their toxic defense against predators and parasites.
The discovery hinged on the work of local naturalist Avit Wako. Wako, from the village of Herowana in Papua New Guinea's eastern highlands, spent two years collecting biological samples to reveal a possible natural source of the bird-feather toxin.
"The big breakthrough was really his," said Jack Dumbacher, an ornithologist at the California Academy of Sciences in San Francisco and lead author of the study. "It speaks very highly of native naturalists and the value of traditional knowledge," he added.
Batrachotoxin has been known to Western scientists since the mid-1960s, when chemist John Daly and colleagues at the United States National Institutes of Health first identified it in the skin of Colombian poison-dart frogs (Phyllobates spp.). Daly is also a co-author of the new study.
For far longer, however, traditional South American hunters have known about the poison and used it to coat the tips of their lethal blow darts.
Other frog species can produce a range of toxins. But the poison these traditional hunters use to tip their darts is a batrachotoxin, a toxin which affects the neurological system of animals.
Precisely how Colombian poison-dart frogs obtain their namesake toxin has posed a lingering question for scientists, however.
Past studies found that when the amphibians are removed from their native habitat, the toxicity of their skin slowly diminishes. Colombian poison-dart frogs raised in captivity, meanwhile, never acquired the toxin.
This led researchers to theorize that the frogs obtained the toxin from their diet.
During the 1990s Dumbacher, the California Academy of Sciences ornithologist, was a graduate student at the University of Chicago. There he started to study the Pitohui of Papua New Guinea. When eaten, the jay-size bird was known to make people's mouths burn or go numb.
Several years of testing by Dumbacher revealed batrachotoxins in the skin and feathers of five species of Pitohui. In 2000 he found the toxins in another species from the genus Ifrita. The toxin-laced feathers, a suspected defense mechanism, provoke a range of sensations when tasted. "Some people say it's like a hot chili pepper," Dumbacher said. "Others say it's like putting a nine-volt battery on your tongue."
But again researchers faced the conundrum posed by the Colombian poison-dart frog: How do the birds obtain batrachotoxin, if they do not produce it themselves?
Tracking down a possible natural source of the toxin was nothing if not daunting. Papua New Guinea is home to more than 700,000 known insect species and approximately 15,000 plant species.
In 2000 Wako, the local naturalist, started collecting fauna that could be a potential source of the toxin.
By the time Dumbacher returned in 2002, Wako had hundreds of specimens for the ornithologist to look through. Among them, nearly 400 beetles that locals called nanisani, a term that refers to the tingling or numbing sensation in the mouth when the beetles and the birds are tasted.
"If you open your eyes and just talk to local people, they're going to teach you a whole lot," said Bruce Beehler, an ornithologist who led Dumbacher's field research team in Papua New Guinea in the early 1990s.
Testing the Choresine beetles uncovered batrachotoxins. The researchers also found the same beetles in the stomachs of the Pitohui and Ifrita bird species.
The family that the Choresine beetles belong to, Melyridae, is also found in Colombia, where the poison-dart frogs live. The researchers, however, point out that the beetles might not be the ultimate source of the toxin.
"There's a possibility that the birds and the beetles are both getting the toxins from another source," Dumbacher said. For example, birds and beetles could both feast on some as-yet-unidentified plant that provides the starting ingredients for both species' toxic defense.
The ornithologist says it's likely that scientists will discover more bird species that produce toxic feathers and skin.
Dumbacher and research colleagues have been compiling references to toxic birds from around the globe. They include the Australian box poison pigeon and the common starling, which are known to create reactions in humans and other species when eaten.
The Carolina parakeet (Conuropsis carolinensis), which became extinct in the early 1900s, may be a North American example of a toxic bird. During a journey along the Mississippi River in the early 1800s, naturalist John James Audubon met Native Americans who told him that small animals died when they ate the bird.
Scientists theorize that birds could use toxins in their feathers and skin to ward off parasites and insects.
In one study of the Papua New Guinean birds, Dumbacher and his colleagues found that batrachotoxin shortens the life span of bird-infesting lice. They also found that hawks, snakes, and other large predators could be deterred by the toxin. Even native bow hunters on the island leave such birds alone.
The researchers say their next step is to study how the birds respond when they're not feeding on batrachotoxin-rich beetles. The scientists also plan to study how predators fare when hunting the birds.
John P. (Jack) Dumbacher received a grant from the National Geographic Society's Committee for Research and Exploration to study Papua New Guinea's poisonous birds.
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