|African Crested Rat|
By Natalie Angier, The New York Times, January 30, 2012
What’s black and white, with a skunkish look to its cover,
And from bark wrests such bite it makes lions fall over?
Meet the African crested rat, or Lophiomys imhausi, a creature so large, flamboyantly furred and thickly helmeted it hardly seems a member of the international rat consortium. Yet it is indeed a rat, a deadly dirty rat, its superspecialized 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. But mammals generally rely, for defense or offense, on teeth, claws, muscles, keen senses or quick wits.
Every so often, however, a mammalian lineage discovers the wonders of chemistry, of nature’s burbling beakers and tubes. And somewhere in the distance a mad cackle sounds.
Skunks and zorilles mimic the sulfurous, anoxic stink of a swamp. The male duck-billed platypus infuses its heel spurs with a cobralike poison. The hedgehog declares: Don’t 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 athlete’s-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 colors like red, orange and yellow — the better to warn off their major predators, the diurnal, keen-eyed birds — most mammals and their mammalian predators are nocturnal or crepuscular, dawn and duskular. Color 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: You’ve seen me. I’m 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 rodent’s 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 specialized hairs running along its flank.
Those hairs, when observed under a scanning electron microscope, look very different from ordinary fur, Dr. Vollrath said. Each outer shaft is stiff and full of holes — like a dead cactus, he said — and inside are a series of long, fluffy microfibers. 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, Dr. Vollrath said: Its hide is unusually thick, and its head is helmeted like a turtle’s. Whether through trial and error or by following an enlightened elder’s example, Africa’s many carnivores give the rat a wide berth.
So, too, do Lophiomys researchers. “Jonathan is a highly enterprising researcher, and he normally eats every animal he studies,” Dr. Vollrath said of his colleague. “But he admitted he would rather not eat this one.”
The researchers don’t yet know why the rat is itself immune to the toxin, or how its fate came to be bound up with the Acokanthera tree. Dr. Vollrath looks to basic rat nature for ideas.
“The rat eats a lot of things that other animals won’t,” he said. “If it eats something disgusting, it tries to spit it out, clean it off, using its skin as a napkin.”
If an early crested rat, while sampling and gagging on a toxic tree, incidentally end up protected against predation, well, evolution has a way of turning a contingency into a necessity. The crested rat is now anatomically and behaviorally dependent on tree toxin for protection, and should Acokanthera go extinct, its little chiseler would soon follow.
In contrast to the crested rat, skunks synthesize 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 world’s 10 or so species have recently been assigned a family plaque of their own, the Mephitidae, from the Latin for “bad odor.”
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 sulfide and low in oxygen — places like mines, swamps, and oil and gas wells. “Our nose is able to detect thiols at extremely low levels, parts per billion,” Dr. Wood said. “We needed to stay away from areas with low oxygen, where we could die.”
Skunks, he added, “have come along and capitalized on this.”
Capitalized and canonized — or maybe cannonized. The skunk’s scent glands have evolved into structures that look like swollen nipples, each able to swivel independently of the other to take perfect aim, and to perfectly calibrated effect (as can be seen in spectacular video on the PBS program “Nature”).
To deter a predator chasing behind at an unknown distance, the skunk goes for the atomized mist effect; if the harasser is within view, the skunk may choose a straight stream to the face.
Skunks are confident in their repellent prowess, but nowadays their swagger can prove fatal. Researchers suggest that one reason skunks constitute a large proportion of roadkill is that they see cars as another predator in need of a lesson: Come ahead, pal, I’ll just stand here and spray.
A good defense means never taking offense. Researchers have been impressed by the ardor with which monkeys in the field prospect for novel forms of insect repellent, and their willingness to withstand extremely irritating chemicals for the sake of rebuffing the bloodsuckers that plague them.
“Capuchin monkeys are notoriously generalist and destructive in their sampling,” said Jessica Lynch Alfaro, the associate director of the Institute for Society and Genetics at the University of California, Los Angeles. “They break everything open, and you have to watch out or they’ll drop branches on your head.”
Every so often, they come upon a product that looks or smells promising, at which point they crack it open and start anointing themselves. They tear up chili peppers to release the capsaicin, rip apart millipedes to procure a few droplets of searing benzoquinones.
If they find a nest of carpenter ants, pay dirt! The monkeys plop down on top and roll every which way, to soak up the ants’ formidable formic acid supply.
Such treatments are clearly painful. “Capuchin monkeys get very agitated when they’re anointing themselves,” said Dr. Lynch Alfaro, who with colleagues recently reviewed capuchin anointing behavior for The American Journal of Primatology. “But they’re keeping off parasites, and they seem to have a high threshold for pain.”
Besides, it’s not all pain and suffering. Anointing is a supremely social affair, and one rubbing monkey soon attracts others.
“They get into such a frenzy that the social order breaks down; everyone is anointing with everyone else,” Dr. Lynch Alfaro said. “It’s like a big, wild party.” They may be black and blue, but the magic potion is spread all over.