Crested rat slobbers tree poison on its fur, dares predators to bite it

By Ed Yong | August 2, 2011 7:00 pm

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The African crested rat is a thief, but its loot only becomes obvious if you take a bite out of it. Doing so would give you a mouthful of ouabain, a poison so strong that it can kill an elephant. The rat doesn’t make the poison itself. Instead, it pilfers it from the local Acokanthera schimperi tree. It gnaws on the roots and bark, chews them up, and slavers a coarse toxic gel onto the special hairs on its flanks. Local people use the same poison to coat their arrowheads. The rat uses it as a chemical shield.

The crested rat is found throughout eastern Africa. It is normally sluggish but when threatened, it puts on a vivid display. It pulls its head back, turns sideways onto its attacker and parts the grey fur on its flanks using special muscles. These actions draw attention to a leaf-shaped crest of brown hairs on its side, which are encircled by a “target” of black and white. It’s almost as if the rat is daring a predator to bite it.

Any animal that takes up the invitation is in for trouble. Domestic dogs do so from time to time, and they end up stumbling about and frothing at the mouth. They often die of rapid heart failure. In two cases where the animals survived, they took weeks to recover. For these reasons, people have long thought that the rat is poisonous; now Jonathan Kingdon from the University of Oxford has proved them right.

By chemically analysing the hairs, Kingdon found that they were loaded with ouabain, a poison found in the tree bark. The name comes from the Somali for “arrow poison” and local tribesmen use ouabain to coat their hunting arrows. The chemical blocks a protein that pumps sodium ions out of muscle cells. In doing so it causes the muscles to contract with unusual force. A large enough dose triggers a fatal heart attack that can bring down even large animals like hippos and elephants.

The rat’s flank hairs are beautifully adapted to store and deliver the poison. Under the microscope, each shaft is perforated with tiny holes, like the nozzle of a machine gun, and is filled with several thin fibres. The shafts behave like wicks, rapidly drawing up any liquid that comes into contact with them. Kingdon tested this with red ink, but it works equally well with a gel of ouabain and saliva. By slobbering on its sides, the rat saturates its fuzz with a killer defence.

The long, grey fur that normally sits over the poisonous hairs, and probably help to screen them from rain or bright light. When the rat it attacked, it parts these covering hairs to draw attention to its defensive ones. The hairs don’t deal any injury themselves. Even though the crested rat looks like a porcupine, its hairs aren’t that sharp and they never actually pierce the mouths of its attackers. But their open structure ensures that light contact is enough to send poison into a predator’s mouth.

This is a costly gambit. The rat could suffer a nasty bite in the process of teaching its attacker a lesson. But its body is well adapted to withstand such assaults. The bones of its skull have extended around its eyes and brain to create a thick helmet. It’s so heavily reinforced that some zoologists have compared it to the head of a turtle.

Its skin is also tough, dense and “resistant to all but the sharpest of teeth, claws and beaks”. The skin of one dead specimen looked like it had been savaged by dogs, but it didn’t have a single puncture wound. With adaptations like these, the crested rat can easily afford to be bitten once. The second bite is very unlikely.

Other adaptations probably help the rat to deal with the poisonous payload that it so often chews up. Its stomach is exceptionally large, similar in size to animals that have to break down tough, fibrous diets. But the rat only eats leaves and fruits that are easier to digest. Instead, Kingdon thinks that its massive stomach might help it to detoxify some of the poisons that it inevitably swallows. The rat also has unusually large salivary glands – perhaps its saliva also helps to process ouabain.

The natural world is full of animals that steal poison from the things they eat. The tiger keelback snake absorbs the defences of its toad prey, the sea slug Glaucus repurposes the stinging cells of its jellyfish prey, the southern vole steals poison from fungi, and poison arrow frogs pilfer the poisons of ants, beetles and mites. Among mammals, the only other example of poison-theft is the hedgehog, which will chew the poison glands of toads and slobber the mixture onto its spines. But this is a pale shadow of the crested rat’s defence. The hedgehog anoints its spines to make them more painful and irritating, but there’s no evidence that they can actually kill.

Reference: Kingdon, Agwanda, Kinnaird, O’Brien, Holland, Gheysens, Boulet-Audet & Vollrath. 2011. A poisonous surprise under the coat of the African crested rat. Proc Roy Soc B

Images from Jonathan Kingdon and Kevin Deacon

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