TRACKING A WEASEL - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)


Scientific discoveries, like most surprises, come by luck, and luck comes by keeping moving and having a keen nose to detect anomalies. One may keep moving, to look for a crossbill’s nest, but, idiosyncratically, may find a raven’s winter food cache and much more instead. And that is also how weasels find and capture chipmunks in the winter woods, which I will get to shortly.

Here in New England two species of weasels molt from brown in summer into a new white coat in winter in response to day-length changes. They retain, however, the black tip on the end of their tails. (In other warmer parts of their range—weasels are found as far south as Florida—both species remain brown year-round.) Three other northern weasels are also native to New England: the mink, the pine marten, and the fisher. None of the three changes its pelage color. All are members of the Mustelidae, the remarkable family that includes wolverines, otters, skunks, ferrets, and badgers (including the memorably named African honey ratel). Of those occurring locally, all remain active during winter, except the striped skunk, which becomes semidormant and lives off its fat.

Of the two species of weasel that turn white in the winter, the long-tailed weasel (Mustela frenata) does not extend far into Canada, whereas the ermine (Mustela erminea, also known as the stoat, in England) has a more northerly and circumpolar distribution. In the field it is practically impossible to distinguish these two. Males of both species weigh about twice as much as females, and M. frenata are about twice as large as the ermine. Undoubtedly at least some of the “ermine” that used to decorate the coats of European aristocracy was in fact long-tailed weasel. Both came from the trappers in North America.

Weasels, at first glance, seem to be designed wrongly. They are beautifully camouflaged in white, yet that conspicuous black tip of a tail seems an odd, inexplicable anomaly—until experiments showed that hawks easily captured fake weasels that had no black-tipped tail. When the hawks were baited with fakes that had black-tipped tails, however, the birds grew confused, either momentarily hesitating or attacking the tails as though they were the head-end. Other small animals also use such deception-evolved tails. Many lizards, for example, have colorful, conspicuous tails that divert or distract predators. The tail is easily detachable, and starts writhing and flailing after being detached, to divert the predator even more from the rest of the animal that slinks away. Lycaenid butterflies also have similarly distracting and detachable “tails” on their wings that fake out a predator in much the same way a good basketball passer fools his opponent on the court. The butterflies’ tails imply to a predator that its prey is about to head in one direction, when it then turns and escapes in the opposite.

Weasels are consummate mouse predators, but they are not restricted to a diet of mice. A 1999 study of least weasels (Mustela nivalis), which are native to North America north of New England, and to Alaska, Europe, and Siberia) shows that small rodents (mostly voles) constitute 41 percent of their diet in summer when they also eat birds, eggs, and insects. In winter they subsist primarily on small rodents.

The flexibility of these predators is noteworthy. Weasels readily climb trees; the pine marten in particular is known for its tree-hunting of squirrels. So skilled are weasels that in Wytham Woods, near Oxford, England, the population of tree-hole-nesting great tits was reduced by 50 percent before predator-proof nest boxes were constructed. I’ve been told by Maine woodsmen that in the winter, weasels will even catch snowshoe hares, a feat I find difficult to imagine as the largest weasel that I have weighed is a Mustela frenata male of 283 grams. An adult snowshoe hare weighs five times as much. However, weasels are not intimidated by a size disadvantage. My neighbor near my Maine cabin told me of seeing a white weasel in November in close pursuit of a hare.

The mustelids’ ability to kill large prey may involve more than brawn, as evidenced by the finesse fishers display while preying on porcupines, which no dog can subdue or eat. They display a curiosity and willingness to acknowledge the new. I suspect they are fairly intelligent. The long skulls of all mustelids, from weasels to otters, indicate a remarkably large brain volume for such a small animal. According to at least one anecdote that I trust, weasels can count (or at least have a sophisticated concept of quantity) up to six.

For about a year my father had a pet weasel that, when still young, used to ride around in his coat pocket. The weasel came from Bulgaria. Papa had been hiking in the country one spring when he heard a rustle in the dry fallen leaves. He thought he had encountered a brown snake. Instead, it turned out to be a mother weasel followed closely by a train of her seven young, one right behind the other. He picked up all of the cute weaselets and put them into a cloth bag, tied it shut at the top, and put the bag into his knapsack. I presume he’d been trapping small mammals and catching fleas; the cloth bag was standard equipment for retrieving the fleas that jumped off the animals, which he sold to the Rothschilds in London for their famous flea collection.

Papa continued his hike and eventually sat down and opened the knapsack to get his lunch. The mother weasel followed him in pursuit of her stolen offspring. She entered the open knapsack. Papa opened the cloth bag. The mother weasel went into the bag, pulled out one of her young, and ran off with it in her mouth. Even more curious now, he waited. The weasel mother returned, entered the knapsack again, and rescued her baby number two. And so it went, until baby number six. But for number seven she never came back, and it became my father’s favorite pet. For a long time after it grew up, it amused and amazed houseguests with the speed with which it could catch a dozen mice simultaneously released into the bedroom. That weasel got even those mice that climbed the curtains. Ultimately the weasel was killed in an accident, the victim of the very features that give these mustelids their edge in hunting success with mice. The weasel, being small and eager to explore hidden spaces, had crawled under a blanket on the bed, and someone had inadvertently crushed it.

Unlike my father, I prefer not to keep weasels free in the house, but opportunities to see them in the woods are rare, and usually fleeting. I recorded a few encounters, including the following. The woods had just been blanketed by a fresh snow on top of a recent crust, and the boughs of the balsam firs were bent low. The oaks and maple twigs formed horizontal lattices on which thick, white pillows of snow had accumulated in hours of windless silence. On this Christmas Day 1995, the forest floor was still clean of tracks and the immaculate snow surface sparkled with pinpoints of sunlight glinting off mirrors made of millions of individual snow crystals.

Weasel in its winter coat.

A movement caught my eye as I scanned the scene. It was a weasel. Against this glistening background, the white weasel, with its black-tipped tail, looked almost lemon-yellow, especially toward its hindquarters. I froze as the animal came closer, dragging something. Within ten or fifteen feet of me, the weasel dropped its prey, raised itself tall on hind legs, and eyeballed me for a few seconds. Apparently satisfied that I wouldn’t interfere with its operations, it then dropped down, grabbed the still-limp chipmunk in its mouth, and continued on its way over a rise among the trees.

The local chipmunks had been denned for two months already. Like other ground squirrels, they would be immobile and in torpor, at least some of the time. In such a state they would be unable to escape any predator that could reach them.

The eastern chipmunk builds a twelve-foot burrow system that includes a nest chamber some three feet underground, food-storage chambers, escape tunnels, and one main work hole. The weasel, however, has a long skinny body and very short legs that allow it, like a dachshund (a German name that translated means badger-hound) bred to capture badgers (dachs) in their dens, to enter rodent tunnel systems, such as those of chipmunks.

Chipmunks lay up winter food stores, allowing them to avoid or reduce the amount of time they spend in torpor, which is when they are most vulnerable to predators. Every fall the chipmunks near our house spend days on end running to the bird feeder, filling their cheek pouches to bulging capacity with sunflower seeds, running into their burrow and unloading, and returning for more. In years of plenty of oak, beechnut, and sugar maple mast, the chipmunks also haul in the seeds of those trees. The more food stored, the more time the chipmunk will remain warm-bodied, awake, and vigilant in winter. Indeed, chipmunks are not nearly as prone to spend the entire winter in torpor as other ground squirrels, which don’t lay up a larder. In 2000, a heavy mast year in northern Vermont, I saw the local chipmunks frequently emerge above the snow, making many trips to the bird feeder throughout the winter. The next year, when there was almost no beech or oak mast in northern Vermont (but a large crop in southern Vermont), they were already absent above ground in late September. It seems that like many a millionaire, a chipmunk in winter facing unlimited amounts of food can never quite get enough, yet when they have little they know how to get by.

I waited about ten minutes for the weasel to get out of view and settled with its prey before I started following its tracks. Why had it bothered to drag its heavy load? Why didn’t it feed on the chipmunk right where it caught it, presumably inside its snug warm den?

Weasels live throughout the Northern Hemisphere and even up into the Arctic. They are active all winter. They must contend with intense cold, yet they are small, skinny, and poorly insulated, all of which facilitate rapid rates of heat loss. To compensate, their resting metabolism is twice that of other animals their size. Yet they have small stomachs and unlike their cousins, the striped skunk, they put on little body fat. As a result, they have to eat more food per day than any other winter-adapted animals.

Yet for all their seeming design flaws for retaining heat, they are in fact superbly designed rodent predators. Weasels need to be small and skinny to enter the chipmunk’s tunnel, and balance their energy budget by behavior. Radiotracking studies show that most of their time in a typical twenty-four hours in winter is spent eating and resting. Weasels need no permanent den, nor do they need a large stomach, because after reaching the rodent’s nest they use their victim’s nest for their own and curl up into a ball to conserve energy supplies while feeding about five to ten times per day. Finally after finishing their meal and again in need of energy supplies, they sally forth on their next hunt.

After dragging its prey about thirty yards in a fairly straight line, the weasel I was watching had climbed with its freshly killed chipmunk up onto a small knoll. There the tracks suddenly circled and zigzagged back and forth in a small clearing. Clearly, the predator had been searching for something in that several-square-yard area; the tracks still showed the drag marks of the dead chipmunk. As I said, the crust was thick, and the fresh snow on top was soft, maybe an inch or two deep. I suspected that the weasel was searching for a very specific hole, possibly the entrance to a den now covered with crust.

I noticed the tracks leading off from the knoll to the base of a nearby small hop hornbeam tree. Along the trunk of that tree, where the ice crust was thinner, the weasel had finally descended into the snow. I next waited for about fifteen minutes, hoping for it to come back out. First I sat quietly; then I squeaked like a mouse in distress. Still no weasel. I then dug down through the nearly two feet of snow to the unfrozen leaf-covered ground by the hornbeam tree, continuing to dig and following a tunnel in the snow. The snow tunnel (much like tunnels I later learned were routinely made by chipmunks) led to the trampled area where the weasel had left its many circling tracks on top of the crust, and that was where I found the entrance hole of a tunnel leading into the ground itself.

I put some loose snow over this entrance and left. When I returned later in the day the hole was indeed opened and the fresh weasel tracks, without accompanying drag marks, led away into the woods. The well-fed carnivore had left.

I checked once again the next afternoon following another morning snowfall when the old tracks and the holes were obliterated. There were still no new tracks. Nor did new weasel tracks appear later.

This was apparently not the weasel’s den. It was probably the usurped den of a previous chipmunk victim that this weasel remembered. The most recent victim was likely caught outside its den; why else would the weasel have dragged it so far through the snow? That is, this chipmunk had probably not been in torpor and it got caught anyhow, because if it had been torpid it would have been inside its snug nest, which the weasel would have used while consuming its meal. Thus, staying warm to remain alert does not necessarily guarantee survival for the individual, at least not for this chipmunk. An individual’s odds are determined by its own specific circumstances, and small idiosyncrasies in the life circumstances of different species almost guarantee different strategies as well. Each animal’s existence is balanced on its own often conflicting mix of contingencies.