After more than a decade of searching, researchers have found a promising explanation for the defensive chemicals that have been identified in poisonous birds of New Guinea. When the birds eat certain tiny beetles, they may be stocking up on the toxic substance, the scientists suggest.
Birds in the genera Pitohui and Ifrita carry batrachotoxins, the same compounds found in some of the poison frogs of the Americas. Neither the birds nor the frogs are likely to make the toxins themselves, says John P. Dumbacher of the California Academy of Sciences in San Francisco.
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He and his colleagues say that after chasing several false leads for sources of toxins in the birds’ diets, they have a good candidate: beetles in the little-studied genus Choresine. These beetles, each only about 7 millimeters long, live in the same regions as the birds do and carry batrachotoxins, Dumbacher and his colleagues report in the Nov. 9 Proceedings of the National Academy of Sciences.
They haven’t yet traced the poison from beetle to bird, but they note that the birds eat Choresine-size insects. The scientists have also identified a Choresine beetle—though not one of the species tested for toxins so far—in a bird’s stomach.
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“That’s very good circumstantial evidence,” comments chemical-ecology pioneer Jerrold Meinwald of the Cornell Institute for Research in Chemical Ecology in Ithaca, N.Y. The batrachotoxins are “very rare compounds, and they occur spottily in nature,” he says.
In 1989, Dumbacher discovered the first instance of toxins in bird feathers when he and his colleagues were catching and releasing forest birds in fine nets. Pitohuis, birds the size of jays, scratched and bit the researchers’ hands. The cuts stung, and “the first thing you do is put the cut to your mouth,” Dumbacher recalls. When he did this, his mouth tingled and began to go numb.
Chemists Thomas Spande and John Daly of the National Institutes of Health in Bethesda, Md., and their colleagues figured out in 1992 that the birds carry steroids called batrachotoxins, which were first identified in the 1960s as poisons in some frog skins.
Some other frog poisons have been traced to a dietary source, but batrachotoxins have remained a mystery in both birds and frogs.
Dumbacher asked New Guinea villagers to suggest insects or plants that cause tingling or numbing. Avit Wako, a resident of Herowana, not only called the researchers attention to the Choresine beetles but also organized a collecting program. The local beetle’s name, nanisani, refers to the unpleasant effects. The researchers subsequently isolated batrachotoxins from the villagers’ collection.
It’s still possible, although unlikely, that the birds and the beetles independently get batrachotoxins from some third source, says Dumbacher. He also points out that beetles are not known to synthesize steroids, so the Choresines may be borrowing toxins from plants or live-in bacteria.