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Critter Chemistry

October 31, 2000

Birds and Frogs: An Alkaloid Connection

Studies Find Bees Are Potential Chemical, Biological Agent Detectors

Maureen Rouhi

Unusual toxic alkaloids belonging to the family of batrachotoxins--previously known only in poison-dart frogs and birds of the genus Pitohui--have been found in a second bird species of a different genus. The finding suggests that chemical defenses among birds might be more widespread than previously thought and raises questions about the origin and ecological roles of the batrachotoxins.

The discovery of the alkaloids in Ifrita kowaldi birds was reported by John P. Dumbacher, a researcher at the National Zoological Park, Washington, D.C., and chemists John W. Daly and Thomas F. Spande of the National Institutes of Health (Proc. Natl. Acad. Sci., Early Edition, Oct. 17).

Dumbacher's involvement in the research dates back to 1989, when he was a graduate student studying the courtship and mating behavior of a species of birds in New Guinea. He sometimes found other kinds of birds trapped in his nets. Those of the genus Pitohui were nasty.

The first time Dumbacher took a Pitohui out of a net, the bird scratched his fingers. "My fingers started to burn," he recalls. "And if you put your finger in your mouth after handling a Pitohui, your mouth begins to tingle. It's a lot like tasting hot chili peppers or touching a 9-volt battery." He says the natives of New Guinea knew the birds were poisonous, calling them trash birds because they are useless as food.

The New Guinea songbirds Pitohui dichrous (left) and Ifrita kowaldi contain batrachotoxin alkaloids such as homobatrachotoxin, previously found only in poison-dart frogs. [Photos by John Dumbacher]

Dumbacher tried to get chemists to find out what in these birds caused such reactions. Only Daly and his colleagues at the NIH Laboratory of Bioorganic Chemistry agreed to help. "As soon as I saw the mass spectra," Daly tells C&EN, "I recognized a compound that I had worked on 20 years before, from a frog."

What Daly saw was the signature of batrachotoxin alkaloids. These unusual alkaloids are potent neurotoxins, previously found only in poison-dart frogs. Their discovery in Pitohui birds was reported in Science in 1992 (258, 799).

"People were stunned by that first report," says Paul J. Weldon, a research zoologist at the Conservation & Research Center of the Smithsonian Institution, in Front Royal, Va. "No one thought that birds had anything like alkaloids on them. It floored the heck out of me."

The unusual alkaloids are found mostly in the feathers, the first line of defense for birds, Weldon notes. Many predators, he observes, pluck the feathers before feeding on a bird carcass.

Chemical defense among vertebrates other than poison-dart frogs is little studied. Many anecdotal reports of toxic or unpalatable birds can be found in the literature, but no toxin had been isolated and identified other than the bactrachotoxins in Pitohui, and now I. kowaldi, birds. That the same type of toxins has turned up in two diverse organisms--birds and frogs--is "quite interesting," Daly says, but the bigger mystery is their source.

Work in Daly's lab suggests that the bactrachotoxins in poison-dart frogs are dietary in origin. But the exact source still has not been established, Daly says. It is likely that Pitohui and Ifrita birds get their alkaloids from dietary sources, too. But, he notes, "we have been looking now for the past eight years at what these birds eat. We have not discovered the alkaloids in any of the food samples."

"It's beginning to look like birds are more interesting chemically than anyone had thought," Weldon says. Researchers have assumed that the only compounds birds had on them were lipids, he explains, because work on avian natural products had been mainly on the oils birds secrete to waterproof their feathers. "I don't think anyone seriously thought about taking polar solvents and applying them to bird feathers."

Toxic birds harboring unusual natural products may be just the tip of the iceberg, Weldon says. Reptiles and mammals, too, could turn up interesting compounds. "People just have not taken the time to look at vertebrates."

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