African Crested Rat

Originally published in The Manitoban August 17, 2011

I hereby nominate the African crested rat, Lophiomys imhausi, for entry into the Baddest-Ass Mammals In the World Hall of Fame. 

Here's why: when these reckless rodents are faced with a ravenous predator ready to rip them to shreds for breakfast they don't even care. They just stand there daring anyone to mess with them. And for good reason—you totally might die if you try eating a crested rat. They are really poisonous!

Crested rats actively seek out Acokanthera schimperi trees, the so-called poison arrow trees traditionally used by certain African tribes for lacing spears and arrows with deadly, elephant-killing poison. The rats chew the bark of the poison arrow tree and lather the poison-rich spit over a specific region of their body. This organic poison contains a compound called ouabain, which inhibits the sodium/potassium pumps embedded in the cell's plasma membrane. 

The sodium pump maintains the electric resting potential of a cell and is essential to normal physiological function. Ouabain is a cardiac glycoside, increasing the force of heart contractions and cardiac output. Left unchecked, ouabain causes death by heart attack.

It is not yet known how the African crested rat is itself able to resist the normally deadly effects of the poison it has usurped for its own defensive purposes. The rats have enlarged salivary glands and stomachs, which may help to process, dilute and detoxify the poison. 

In the medical world, ouabain has been used in small doses to stimulate weak hearts to contract and beat more strongly. Ongoing research exploring the resistance of the rats to A. schimperi's toxic effects may someday be used to treat humans suffering from heart-related illnesses and to better understand the effects and potential uses of other cardiac glycosides.

The African crested rat, native to northeastern Africa, is the only known mammal on earth which actively seeks out and acquires toxins from a plant for use in its own physical defense. Further, the crested rat advertises its borrowed toxicity by performing a specialized display behaviour when confronted by potential predators.

When exposed to a threat, the rat will hold its ground and expose specialized hairs along its flanks by parting the grey fur that normally covers all of its body, revealing a patch of hairs surrounded by strikingly black and white striped fur. These specialized hairs are highly modified and unique to the African crested rat. 

When viewed under a microscope, it can be seen that the hair shafts contain numerous perforations throughout their length, creating a wicking effect which draws the toxin into and along the full length of the hairs. This allows for maximum retention of the deadly poison-laden spit the crested rats coat these hairs with after chewing the poison arrow tree bark. No other animal is believed to possess hairs of this type.

Other taxa, both vertebrate and invertebrate, have shown comparable poison-stealing behaviours. Some sea slugs steal the poisonous stinging cells of cnidarians, which I wrote about in an article on nudibranchs back in August 2009, for example, but this is the first documented instance of a toxic poison theft in a placental (eutherian) mammal. The only other mammal known to perform a similar behaviour is the hedgehog, which applies toxins from the poison glands of certain toads to its quills; however, this merely causes pain and irritation for potential predators, a long way short of the heart-stopping death which can ensue from messing with the African crested rat!