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A species of rat has evolved an ingenious method to foil any predators that try to eat it, scientists report.

They found that the African crested rat chews the roots and bark of a highly toxic tree, and then smears the lethal mixture on its specially adapted fur.

Any animals that attack receive a mouthful of potentially deadly poison.

It is the first time that this behaviour has been reported in a mammal, the researchers write in the .

Jonathan Kingdon, lead author of the paper, from the University of Oxford, said: "The need to deter predators has led to one of the most extraordinary defences known in the animal kingdom."

But the team, from the UK, Kenya and New York, are still puzzled by how the rodent is able to survive a dose of the toxin, which comes from the poison arrow tree (Acokanthera schimperi).

Traditionally, this poison has been used by hunters to kill elephants.

Deadly secret

The African crested rat (Lophiomys imhausi) is found in the north east of continent, and has long been thought to be poisonous: there have been several reports of domestic dogs that have dropped dead after trying to bite one.

But until now, nobody realised that it employed the help of a plant to make itself deadly.

When undeterred by predators, the rodent looks like a long-haired, grey rat.

However, when it is under attack from animals such as jackals, wild cats and leopards, it does not run off as it is extremely slow. Instead, it freezes, and then exposes a bold, black-and-white-striped tract of hairs that run down its flank - and it is this fur that is slathered in poison.

Professor Kingdon said: "The poison is an organic poison. We all have minute quantities of it in our bodies, and it controls the strength of the heart beat.

"But if you have too much, your heart beats so hard that you have a heart attack.

"It is the source of the deadly arrow poison that is used to kill elephants."

Toxic tactics

Closer analysis revealed that these hairs on the rat's flank have an unusual structure.

Professor Kingdon said: "It is of a structure that is completely unprecedented in any other animal. There is not another animal that has hairs shaped like the hairs on that tract."

Microscopy showed that the hairs on its back draw up the poison much like the wick of a candle, ensuring that each hair remains saturated with as large a dose of poison as possible.

The rest of the rat's hairs have a more typical mammalian structure. The researchers noted that it does not apply the poison to these - only to its crest.

The team thinks that the elaborate display is enticing predators to bite the rat's most poison-laden spot.

"As the predator rushes in and tries to make its first bite, the rat ensures the flank is the first thing the predator encounters: it advertises this visually with its colouring," explained Professor Kingdon.

The tactic seems successful: if the predator does not die from their toxic encounter, it is unlikely to ever want to take a bite of the rat again.

The only animal, said Professor Kingdon, that uses a similar trick is the hedgehog. The prickly beast will sometimes kill a toad and bite into its glands, then smear this toxic mixture on its spikes.

However, the poison seems to boost any discomfort its spikes might cause, rather than have a lethal effect on its predator.

The researchers now want to find out more about this unusual evolutionary relationship between predator, plant and prey.

They are also keen to discover how and why this rat is able to withstand this poison when so many other animals cannot.

The team said this could have interesting medical implications.

Isolating the part of the poison that helps the heart beat and identifying the physiological components of the rat that prevents it from dying could help to lead to novel treatments for heart problems.