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The Case of the Poisonous Rat

2 August 2011 7:02 pm
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Agatha Christie, meet your tiniest villain yet: the African crested rat (Lophiomys imhausi). Dogs that try to grab a bite of this spiky-haired East African rodent, more closely related to lemmings or voles than true street rats, often wind up violently ill or even dead. Now, scientists have discovered the secret to the crested rat's fatal kiss: A poison once used by African hunters to kill elephants.

When cornered, crested rats don't run or hide like a normal rodent. Instead, they twist to the side and arch their backs, parting their long, gray outer coats, to reveal black-and-white bands that run like racing stripes down their flanks. Like a hornet's yellow-and-black rear or a rattlesnake's rattle, these stripes seem to tell predators one thing: Back off.

The rats' defensive postures are fearsome, but they don't explain the trails of sick dogs left in their wakes. Researchers suspected that the rodents were harboring poison, but they didn't know how.

In the new study, Fritz Vollrath, an evolutionary biologist at the University of Oxford in the United Kingdom, and colleagues have turned Miss Marple and solved the mystery. Crested rats, it turns out, don't make their own poison; they gather it. The team's first clue was observing a captive crested rat diligently gnaw on pieces of bark from the African tree Acokanthera schimperi, also called the arrow poison tree. The animal would then "slather" its short hairs in fibrous spit. That bark carries large amounts of ouabain, a chemical that overstimulates heart muscle, similar to the poison curare, commonly obtained from South American plants. East African hunters once boiled down the bark to coat poisoned arrows for taking down elephants and other big game.

When naïve pet dogs, which don't know to avoid the rodents, bite down, they get a mouthful of those hairs and probably a mouthful of the toxin, which can cause heart failure. Only one other identified animal, the hedgehog, similarly dons itself in poison to fend off predators, Vollrath and his colleagues report online today in the Proceedings of the Royal Society B.

Other mammals similarly "self-anoint" but against much smaller threats, adds Paul Weldon, a zoologist at the Smithsonian Conservation Biology Institute in Front Royal, Virginia. Capuchin monkeys, for instance, rub themselves with millipedes to keep away mosquitoes. But the crested rat can actually kill its large nemeses, which, Vollrath suspects, makes it unique.

The rodents aren't just accidental poisoners, either. In fact, their entire bodies seem built for the craft. The unique hairs on their black-and-white undercoats, for instance, act like "wicks" for the toxin, Vollrath says. These tubes have an outer shell covered in holes like Swiss cheese and an inner core containing small fibers, ideal for soaking up liquids such as ouabain-laden spit.

The crested rat's other body parts, however, seem suited for helping the animals survive attack. In order to deliver their poison and send predators a message, the rodents need to first get bit. To withstand canine jaws, the animals boast abnormally thick skin for a rodent. Vollrath and his colleagues found one crested rat whose hide was covered in bruises, a sign that it had weathered a beating. "These things have been through a bit of action," he says.

"This is a pretty exciting finding," says Jack Dumbacher, an ornithologist at the California Academy of Sciences in San Francisco. Other vertebrates, including the infamous poison dart frog, spirit away toxins from plants and other animals, but the behavior is rare in birds and mammals. That's probably because it's costly: Crested rat ancestors had to evolve not only hefty skin, which isn't easy to lug around, but also an immunity to ouabain. And for Kenya's dog populations, that makes them a unique and unexpected sort of villain.

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