A Toxic Warning in Black and White

August 3, 2011

Scientists discover the first mammal, an African rat, to use a poisonous plant to defend itself from predators.

East African hunters who shoot poison-dipped arrows are not the only ones using the Acokanthera tree for its toxin. A small, black-and-white rat also slathers the poison, called ouabain, on its fur to keep predators at bay.

The African crested rat is the first known mammal species to protect itself this way, a fact recently confirmed by a team of scientists that included WCS’s Tim O’Brien. Dogs have been known to fall ill or die after eating the rat but no one knew exactly why. In Kenya, the researchers discovered the answer.

They observed the rat chewing the tree’s bark to concoct a frothy mix of saliva and ouabain, which it then spread along the sides of its body. The two-pound animal’s skull and vertebrae are thick and its skin is unusually tough. But the key survival feature of the rodent, also called the maned rat, is its fur.

The animal’s distinct white-and-black coloration sends a “don’t eat me” signal to predators, and when threatened, the rat typically displays its markings as a warning. And that’s not all. Examining the fur under an electron microscope, the scientists saw that the cylindrical hairs are perforated. This structure helps them rapidly absorb the rodent’s deadly sputum, making for a more potent protection.

 “The African crested rat is a fascinating example of how a species can evolve a unique set of defenses in response to pressure from predators,” said Dr. O’Brien, a WCS senior scientist. “The animal and its acquired toxicity are unique among placental mammals.”

Other poisonous mammals do exist, for example, the solenodon and the egg-laying duck-billed platypus. Those animals, however, produce their toxins themselves with no need for a toxic tree. While the African crested rat avoids nibbling on the Acokanthera tree’s leaves and fruit, just how the rodent keeps from sickening itself remains a mystery.

For more information on this story, see the press release.


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