Lifeform of the week: Resourceful African crested rat picks its poison

Outside of our own species, it is rare for animals even to use tools, but animals fashioning weapons is almost unheard of. A striking exception to this – the African crested rat – was just presented in the August 2, 2011 edition of the Proceedings of the Royal Society B: Biological Sciences. The small, slow and seemingly defenseless rodent has a trick up its sleeves – it’s highly toxic to predators. So much so that just taking a bite at the animal can result in severe illness and death.

You might argue that poison as defense is not uncommon in nature, and you’d be right. Various plants and small animals manage to avoid getting eaten by being inedible and by advertising this unpalatability with bold coloration. What makes the crested rat special is that it isn’t inherently toxic. Instead, it renders itself poisonous by coating its fur with extracts from chewed-up tree bark.

Locals in the eastern portions of Africa where the crested rat is found have long observed the rodent’s interaction with domestic dogs. Dogs who try to attack the rats emerge with a battery of miserable symptoms that can sometimes last for weeks. Those that survive the ordeal are not inclined to give crested rats a second try.

There was no doubt that the rats were poisonous to their attackers. But until now, nobody really knew how this poison was being delivered. To shed light on the question, researchers studied the behavior of crested rats in the wild as well as analyzing their fur and other anatomical details back in the lab.

Jonathan Kingdon, one of the authors of the study, described how they witnessed the animals anointing themselves with the protective poison:

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.

The animal’s poison of choice – ouabaïne – is a doozy. Its source is the same tree (Acokanthera schimperi) that indigenous elephant hunters use to craft their poison arrows. The chemical has a powerful effect on the heart. In minute doses, it can actually stimulate a weak heart. But larger doses create overly forceful heartbeats and can even cause heart failure.

With the aid of microscopy, the scientists found that hairs on the crested rat’s flank – where it delivers the mixture of saliva and deadly bark – are ideally textured to sponge up as much toxin as possible. To give potential predators fair warning, the rats bare their illness-inducing flanks when threatened. Muscles in the skin part the fur, making the dangerous part more conspicuous and highlighting its high contrast black and white coloring.

Further adaptations such as a reinforced skull and a thick skin allow the rats to take a good thrashing in a dog’s mouth without sustaining life-threatening injuries. They come away battered but otherwise intact, while the unfortunate dogs retreat to learn a painful lesson about which foods should be avoided.

While one mystery has been solved, scientists are still befuddled over how on earth this tiny mammal can regularly chew a plant lethal enough to take down an elephant.

Opossums influence evolution of snake venom

August 5, 2011

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Alex Reshanov

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