By Natalie Angier, The New York Times, June 13, 2016
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 fat weasels.”
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.
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.”
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 improve insulation.
“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.