ferrets, members of the weasel family, before being released
(credit Shattil and Rozinski/NPL/Minden Pictures)
At birth, the least weasel is as small and light as a paper
clip, and the tiny ribs that press visibly against its silvery pink
skin give it a segmented look, like that of an insect. A newborn
kit is exceptionally underdeveloped, with sealed eyes and ears that
won't open for five or six weeks, an age when puppies and kittens
are ready to be weaned.
A mother weasel, it seems, has no choice but to deliver her
young half-baked. As a member of the mustelid clan a noble
but often misunderstood family of carnivorous mammals that includes
ferrets, badgers, minks and wolverines she holds to a slender,
elongated body plan, the better to pursue prey through tight spaces
that most carnivores can't penetrate. Bulging baby bumps would jeopardize
that sylphish hunting physique.
The solution? Give birth to the equivalent of fetuses and then
finish gestating them externally on mother's milk.
"If you want access to small environments, you can't have a
big belly," said William J. Zielinski, a mustelid researcher with
the United States Forest Service in Arcata, Calif. "You don't see
For Dr. Zielinski and other mustelid-minded scientists, weasels
exemplify evolutionary genius and compromise in equal measure, the
piecing together of exaggerated and often contradictory traits to
yield a lineage of fierce, fleet, quick-witted carnivores that can
compete for food against larger celebrity predators like the big
cats, wolves and bears.
American river otters, also mustelids, at Yellowstone National
Park in Wyoming. (credit Barrett Hedges/NG Creative/Minden
Researchers admit that wild mustelids can be maddening to study.
Most species are secretive loners, shrug off standard radio collars
with ease, and run close to the ground "like small bolts of brown
lightning," as one team noted. Now you see them, no, you didn't.
Nevertheless, through a mix of dogged field and laboratory studies,
scientists have lately made progress in delineating the weasel playbook,
and it's a page turner, or a page burner.
Researchers have been astonished to discover that the average
mustelid is like a fur-covered furnace, its metabolic rate exceeding
not only that of other carnivorous mammals but also that of its
twitchy, ever-gnawing rodent prey.
"If you compare a least weasel to a meadow mouse, they're the
same weight, but the weasel has the higher metabolic rate," said
Roger Powell, an emeritus professor at North Carolina State University
and doyen of weasel studies.
"The weasel heart beats at up to 400 pulses per minute," said
Mark Linnell, a faculty research assistant who studies mustelids
at Oregon State University. "They're geared to run at full speed,
and they're always high-strung."
That keyed-up metabolism is another example of a grand mustelidian
compromise. "If you have a high metabolic rate, you can be more
active and search farther for food in more places and in more diverse
ways," Dr. Powell said. "But you have to catch more food in order
to do that."
Big cats must eat the equivalent of roughly a third of their
weight each week; weasels must eat a third or more of their weight
each day. "They're living life on the edge," Dr. Powell said.
Weasels also have big brains relative to body mass, and they
apply their neuronal bounty to continuously fine-tune their movements
during a hunt, a strategy that allows them to attack prey up to
10 times their size.
The fisher, a particularly fearless weasel in the marten branch,
may be the only North American carnivore to have mastered the art
of dining on adult porcupine a large rodent that, in addition
to being protected by a formidable quill sheath, weighs a good 12
pounds more than the eight-pound fisher.
"It's got to be one of the great predator-prey matchups in history,"
said Roland Kays, a biologist at the North Carolina Museum of Natural
Sciences and North Carolina State. The fisher must encounter the
porcupine on open ground, at which point it can start running circles
around its quarry. The fisher tries to dart in and bite the porcupine's
vulnerable face; the porcupine pivots to turn its shielded back
toward its attacker. Dart and spin, dart and bite.
After several deep wounds to its face, the porcupine grows weak,
loses its footing and match over. The fisher will then flip
the punctured, pincushioned animal onto its back and carefully tear
into a quill-free patch of belly, gaining access to desirable organs
like the small intestine, which is not only rich in protein and
lipids, but also contains the partially digested plant matter that
even carnivores need.
Dr. Kays and a former student, Scott LaPoint, have found that
fishers are far more behaviorally flexible than biologists had thought
possible, at least in the Northeast. Hunted and trapped to near
extinction until the 1930s, fishers a misleading name derived
from Dutch colonists' word for polecat, a European weasel
began recovering in their traditional setting of deep forests, where
they could easily avoid humans.
In the last few years, though, the weasels have apparently shaken
off their reserve and begun showing up in suburban and urban areas
a shopping mall in Schenectady, N.Y., a parking lot in downtown
Albany. Two years ago, a sizable male fisher made its way to the
Bronx, startling anybody who saw it slinking along the sidewalk
and raising hope that a solution to the city's rat problem might
have finally arrived.
The fisher, alas, soon disappeared. "I don't know how that one
ended up in the Bronx in the first place," Dr. Powell said, "but
it's no place for a fisher, and I'm sure he wished he'd turned left
when he turned right."
For their part, researchers wish they could overturn the public's
generally poor opinion of weaseldom. To call someone a weasel means
the person is shifty, untrustworthy. Weasel words are those squishy,
defensive qualifiers beloved by, well, journalists.
In a recent "Brewster Rockit: Space Guy" comic strip, a "closet
of nightmares" is opened to reveal, "AAHHH!!! Weasel-juggling clowns!"
Researchers speculate that the negative image may result partly
from the mustelid's serpentine silhouette: In some parts of Central
America, weasels are called "furry snakes." Or maybe it's the distinctive
mustelid musk. Most weasel species communicate with one another
over large home ranges through frequent daubs of a pungent fluid
excreted by their anal glands.
Shihab Shamma, who uses ferrets to study the mammalian auditory
system at the University of Maryland and Descartes University in
Paris, said of the ferrets at his Paris lab, "We give them the names
of smelly French cheeses."
long-tailed weasel in Yellowstone National Park. (cCredit
George Sanker/NPL/Minden Pictures)
But mustelid enthusiasts emphasize the family's beauty and diversity:
some 60 living species across all continents except Antarctica and
Australia, ranging in size from the least weasel, the world's smallest
carnivore (weighing less than half a stick of butter as an adult),
to the mighty wolverine, which can weigh up to 70 pounds.
Many weasels spend time in water, and one species, the sea otter,
is a marine mammal that rarely comes on land. Sea otters are also
among the only nonprimate mammals to use tools, cracking open a
recalcitrant mollusk shell by banging it with a stone. Most of the
time, though, the sea otter's teeth do the job.
"Their teeth are amazing, like no other living carnivore," said
Adam Hartstone-Rose, who studies mammalian bite forces at the University
of South Carolina. "They're big and rounded and with no pointy cusps
that might break off. They look like pillows or gum drops." But
the teeth, with their thick coat of enamel, can easily crush open
a crab, clam or snail.
Most weasels have dentition more typical of carnivores, with
a few sharp, slicing teeth and fewer, smaller molars, which other
animals use to grind plants. As a result of their compact dental
layout, many weasels have foreshortened snouts that make them look
young and cute. They can also act young: Weasels are among the few
animals that play as adults.
If they're well fed, Dr. Powell said, "they'll bounce and ricochet
around, pounce, stalk, wiggle and change shape and just about turn
themselves inside out. They put kittens to shame."
Many weasels live in cold places, and because their long, thin
shape has a high surface area relative to volume, they lose heat
easily. To tackle the cold without relying on fat as an insulator,
many weasels grow luxurious fur coats, some of the densest in nature.
A good head of human hair has about 350 hairs per square inch.
On a mink, the fiber count per square inch is 44,000. Small wonder
that people have historically coveted weasel pelts mink,
sable and ermine, the fur of pomp and royalty taken from the animals
in winter, when their coats turn white.
Weasels also appreciate the value of co-opted fur. In winter,
voles and mice build little dome-shaped nests under the snow. When
a weasel finds one of these nests, it's a genuine jackpot: lunch
and lodging combined. Better still with a few tweaks: After eating
the residents, the weasel lines its new dwelling in rodent fur to
"If you pop open one of these nests in springtime, you discover
a macabre scene," Dr. Zielinski said. "What was once occupied by
a vole is now covered with vole-hair wallpaper."
A rodent's closet of nightmares: no clowns, no juggling, just
one cold and hungry weasel, knocking at the door.