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| 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,
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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.
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