They secrete batrachotoxin through their feathers, which is one of the world's most deadly poisons.
Cyanide is probably the most famous poison around. Nazi gas chambers, suicide pills, Jonestown Koolaid—they all used cyanide. It takes around 500 mg of the chemical to kill an adult who weighs 70 kilograms. But on the spectrum of toxicity, cyanide is middling at best. It's nothing compared to another compound, known as batrachotoxin.
Secreted from the glands of poison dart frogs in South America, batrachotoxin is fatal at a dosage of just 0.1 milligrams. That's equivalent to around two grains of table salt. After exposure, the toxin jams open the ion channels in its victim's nervous system, forcing muscles to fire continuously. In around 10 minutes, the heart and lungs will seize.
Batrachotoxin just about the most potent toxin on the planet. But killing power aside, the most compelling thing about batrachotoxin is how it reveals large holes in our understanding of evolution.
In 1963, John W Daly was a 30-year-old pharmacist working for the US National Institute of Health (NIH). Like a lot of medical institutions in the 60s, the NIH was interested in South America's potential as a source of new alkaloids. Daly was sent to study tree frogs in West Colombia, which often meant prodding frogs and licking his finger to identify poisonous ones. In this way he isolated and named batrachotoxin, drawing from the Latin word for frog, batrachos.
Something Daly couldn't answer was how the frogs came to be poisonous in the first place. It had been long known that South American frogs were toxic enough for indigenous tribes to smear blow-darts with frog secretions when hunting. But, in captivity, the same frogs produced almost no toxins at all.
Daly had hoped that by isolating the chemical, he would be able to solve the mystery, but he couldn't. It took 30 years for someone to make a breakthrough—a bird specialist in another jungle on the other side of the world.
In 1989, a graduate student from the University of Chicago named Jack Dumbacher was studying birds of paradise in Papua New Guinea. He was trying to catch them in nets but kept getting another bird, called a pitohui, instead. "So I had two or three in a net and was pulling them out, and they scratched my hand," he recalled over the phone. "I licked my cuts and instantly felt my tongue start to tingle and burn. After a moment it went numb and I thought Hey, maybe I shouldn't have done that."
Later Jack mentioned the incident to another researcher who'd had exactly the same experience and they decided to do further testing. As with Daly with his frogs, the method was pretty elementary. "We caught one, plucked a feather and tasted it, and straight away my mouth started to burn," Jack told me. "We knew then the birds were poisonous."
It was odd to find two completely different species from opposite ends of the planet producing the same toxin, but maybe it was possible if they weren't making it.
While poisonous birds might sound like a notable discovery, Dumbacher had trouble arousing any interest. It wasn't until the following year when, back in Chicago, another graduate tasted one of Dumbacher's feathers. Impressed with the sensation he passed it on to his mentor, who just so happened to be Dr John Daly.
Daly immediately recognised the bird poison as the batrachotoxin from his Colombian frogs. While this was bizarre, it also seemed to validate a theory he'd been struggling with for years. It was odd to find two completely different species from opposite ends of the planet producing the same toxin, but maybe it was possible if they weren't making it. Maybe they were each borrowing it from something else.
Here's the evolutionary riddle: For a species to eat something containing batrachotoxin and survive is unlikely, but it's not impossible. But for a species to not just eat it, but appropriate the toxin for their own needs is a giant leap in evolutionary logic.
Animals usually learn not to eat toxins, which is why other species produce them in the first place. So for birds and frogs to keep nibbling on some leaf or mushroom that's killed their parents, for eons and eons until they eventually evolve to use the poison themselves—that seems utterly impossible.
In an effort to get some answers, Dumbacher—now a doctor—launched several expeditions looking for the source of the poison. After one such attempt in 1995, he left a pile of scientific equipment in a remote PNG village which, unbeknown to him, a local named Avit Wako continued using after he'd gone.
"Then, years later, I sent a student intern back to the village and Avit was like, Awesome! Because I've found where the toxin comes from." Avit posted him a sample and sure enough, the mass spectrometer confirmed it. A villager with a year two education had found where the birds obtained their batrachotoxin: beetles.
The beetles are small, from the genus Choresine, and live on other insects. Aside from that and the fact they supply poison to a type of bird, finding the source of batrachotoxin didn't provide any answers. As Dumbacher explained, it actually just raised a bunch of new questions.
One such mystery arose after the team tested all five types of poisonous pitohui birds across the island. "We quickly realised that the others weren't actually pitohuis," he said. "In fact, they'd all been misclassified because they looked the same." But why would PNG have five different varieties of poisonous birds that look identical? Their only theory is that several birds have evolved to eat batrachotoxin beetles, and in the process they've all evolved to look the same. But why? Nobody has a clue.
In the end chasing birds and frogs through jungles didn't bring Dumbacher fame or fortune. But, in a way, his discoveries are quite grand. He's describing a food chain where beetles are sequestering batrachotoxin from something even smaller than themselves, mites for example, and then passing it onto birds and frogs, which use it to defend themselves from threats like humans. And then humans collect it from the frogs to hunt something else. "It's just this incredible story of life," he says. "And it's got everything from toxins to beautiful birds."
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