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.
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
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.