Mammals that pack a toxic punch

How animals use venoms and repellents as a survival strategy,deterring predators with chemical weapons

The African crested rat,or Lophiomys imhausi,is a creature so large,flamboyantly furred and thickly helmeted that it hardly seems a member of the international rat consortium. Yet it is indeed a rat,a deadly dirty rat,its superspecialised pelt permeated with potent toxins harvested from trees. As a recent report in the journal Proceedings of the Royal Society B makes clear,the crested rat offers one of the most extreme cases of a survival strategy rare among mammals: deterring predators with chemical weapons.

Venoms and repellents are hardly rare in nature: Many insects,frogs,snakes,jellyfish and other phyletic characters use them with abandon. Skunks and zorilles mimic the sulphurous,anoxic stink of a swamp. The male duck-billed platypus infuses its heel spurs with a cobralike poison. The hedgehog declares: Dont quite get the point of my spines? Allow me to sharpen their sting with a daub of venom I just chewed off the back of a Bufo toad.

Other mammals chemically gird themselves against smaller foes: Capuchin monkeys ward off mosquitoes and ticks with extracts gathered from millipedes and ants,while black-tailed deer rub themselves liberally with potent antimicrobial secretions produced by glands in their hooves.

According to William Wood,a chemistry professor at Humboldt State University in California,these secretions have been shown to be effective against a broad array of micro-organisms,including acne bacteria and athletes-foot fungus,which could explain why teenage deer are especially diligent with the hoof-rubbing routine right before the annual deer prom.

For each newly identified instance of a chemical fix,researchers seek to identify its benefits,drawbacks and evolutionary back story,and to compare it with other known cases of chemical arms. Distinctive themes have emerged.

For example,whereas poisonous insects tend to advertise their unpalatability in bright colours like red,orange and yellowthe better to warn off their major predators,the diurnal,keen-eyed birdsmost mammals and their mammalian predators are nocturnal or crepuscular,dawn and duskular. Colour is wasted on them,but strong contrast between dark and light is not. This is why skunks,zorilles also known as polecats and the African crested rat have independently converged on a similar pelage theme of black against white. The pattern is unmistakable in very low light,and its message is too: Youve seen me. Im noxious. Now buzz off.

In their fetchingly titled paper,A Poisonous Surprise Under the Coat of the African Crested Rat, Jonathan Kingdon and Fritz Vollrath of Oxford University and their colleagues described the complex of traits that give rise to the rodents rottenness. The researchers determined that the rat spends many hours gnawing on the bark and roots of the Acokanthera tree,from which it extracts the same curare-type heart toxin that African hunters have traditionally used to kill elephants. The rat then slavers the toxic masticant onto tracts of specialised hairs running along its flank.

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Those hairs,when observed under a scanning electron microscope,look very different from ordinary fur,Vollrath said. Each outer shaft is stiff and full of holeslike a dead cactus,he saidand inside are a series of long,fluffy microfibres. The researchers showed that the applied toxin seeps through the outer holes of the hairs and is wicked up and stored by the fibers,lending the rat twinned flank strips of doom.

One little nip is all it would take to sicken or even kill a predator,and the crested rat is well equipped to endure exploratory bites,Vollrath said: Its hide is unusually thick,and its head is helmeted like a turtles. Whether through trial and error or by following an enlightened elders example,Africas many carnivores give the rat a wide berth.

The researchers dont yet know why the rat is itself immune to the toxin,or how its fate came to be bound up with the Acokanthera tree. Vollrath looks to basic rat nature for ideas. The rat eats a lot of things that other animals wont, he said. If it eats something disgusting,it tries to spit it out,clean it off,using its skin as a napkin.

In contrast to the crested rat,skunks synthesise their toxins from scratch,yet they,too,have taken chemical defense to a highly derived,almost mannered extreme. Skunks stand alone in mammaldom,and though they once were considered a kind of weasel,the worlds ten or so species have recently been assigned a family plaque of their own,the Mephitidae,from the Latin for bad odour. Through anal scent glands just inside the rectum at the base of the tail,skunks generate an extreme version of the familiar spray with which carnivores mark their territory,wildly accentuating the chemical components that we and most other mammals judge to be very bad news. At the heart of skunk spray is a thiol,the signature of nasty environments high in lethal hydrogen sulphide and low in oxygenplaces like mines,swamps,and oil and gas wells. Our nose is able to detect thiols at extremely low levels,parts per billion, Wood said. We needed to stay away from areas with low oxygen,where we could die. Skunks,he added,have come along and capitalised on this.NATALIE ANGIER