The African crested rat, a rodent from East Africa, applies a toxin from tree bark to make itself poisonous, reports a new study published in Proceedings of The Royal Society B..

The discovery is the first known instance of a mammal acquiring its poison from a plant, according to the Wildlife Conservation Society, whose scientists were involved in the research. All other known poisonous mammals — including the duck-billed platypus, shrews and the solenodon — produce their toxins themselves.

“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. Tim O’Brien, Senior Scientist of the Wildlife Conservation Society and a co-author on the study, in a statement. “The animal and its acquired toxicity are unique among placental mammals.”



The African crested rat is the only mammal known to acquire its poison from a plant. The rodent masticates the poisonous bark of the Acokanthera tree and applies the slaver to its flank hairs, which absorb the poison like candle wicks. Photo credit: Susan Rouse.

The African crested rat gets its toxin — called ouabain — from the Acokanthera tree, a tree used by East African hunters to poison the tips of their arrows.

Scientists have long suspected that the species is poisonous due to its behavior, which includes “exposing a black-and-white coloration on its flanks when threatened by predators” and “reports of dogs becoming ill or dying after encounters with rats,” according to WCS. But the study marks the first time scientists have been able to prove the African crested rat is toxic and that it derives its toxin in a novel manner for a mammal.

WCS explains how the scientists made their discovery:

The researchers confirmed the hypothesis by presenting a wild-caught rat with branches and roots of the Acokanthera tree. The rodent proceeded to gnaw and masticate the bark (avoiding the leaves and fruit) and apply the “slaver” on its flanks. Further, the research team employed electron microscopes to examine the unique structure of the flank hairs. In doing so, they found that the perforated cylindrical structure of the hairs facilitates the rapid absorption of the poisonous saliva. Interestingly, ouabain has also been used by doctors for centuries as a clinical treatment against congestive heart failure.

The rodent has characteristics that increase its odds of surviving attack, including “a thick reinforced skull, thick vertebrae, and unusually tough skin.”

The researchers say they still don’t know how the rodent uses the toxin without being adversely affected by it.

Jonathan Kingdon, et al. A poisonous surprise under the coat of the African Crested Rat. Proceedings of the Royal Society B. Available online Aug 2, 2011.

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