Researchers from Oxford University and East African Scientists have discovered a species of rats that makes its own poison by gnawing on a poisonous tree and then slathering poisonous spit onto special sponge-like hairs on its flank to defend it.

The Crested Rat is a species of rat that has acquired lethal poison from a plant by chewing the bark of "Poison-arrow trees" called Acokanthera, so called because human hunters extract ouabain from them to coat arrows that can kill an elephant.

"At between 40 and 50 centimeters long, the Crested Rat looks quite harmless as it clambers about in rocky, wooded valleys in Kenya and the Horn of Africa," said Jonathan Kingdon of Oxford University's Department of Zoology.  

"But once disturbed or attacked, the long fur on its flanks parts to expose a vivid black-and-white pattern around a leaf-shaped tract of peculiarly specialized hair, almost as if it is 'daring' a predator to take a bite of these poisoned hairs," he added.

The poisoned sponge-like hairs are very close to the important centers of the rat's head, neck and thorax.  

"We observed the rat gnawing Poison-arrow tree bark directly from the plant, chewing it and then deliberately slathering the resulting mixture onto its specialized flank hairs. These hairs are designed to rapidly absorb the poisonous mixture, acting like a lamp wick," Kingdon said.

"None of us had ever seen complex hair such as these with a latticework for the wall and a bundle of fine fibers for the core," Professor Fritz Vollrath of Oxford University's Department of Zoology and author of the paper, said. "It was surprising how effectively the hair was in 'wicking-up' liquids. ... While the function of a reservoir for the poison was clear, we shied away from testing it by chewing a hair!"

Ouabain is very famous for heart-stopping cardiac glycoside and doctors have used little doses of the substance to stimulate weak hearts.

The Crested Rat has become completely immune to this poison, and scientists hope that finding out more about the Crested Rat's and use of ouabain could lead to new medicines for humans.

When attacked, the rats put on a dramatic, fur-bristling display. Dogs have been reported to die after biting a Crested Rat.

According to the Wildlife Conservation Society, the discovery of the poisonous rat acquiring its poison by gnawing a poisonous plant is the first known instance of a mammal doing so. Other poisonous mammals, which include the duck-billed platypus, shrews and the solenodon, produce poison by themselves.