The weasel family

carnivores of many shapes, sizes, and lifeways

Ermine in summer coat. Photo by Kerry Howard

This taxonomic family (Mustelidae) of carnivores is familiar to Juneau folks in the form of weasels, wolverines, martens, mink, and otters. Mustelids are a species-rich family, with fifty or sixty or seventy species, depending on what source you read. They originated over twenty million years ago from a wolf-like ancestor and have evolved into a variety of ecological niches. They are distributed naturally over all the continents except Australia and Antarctica.

Mustelids range in size from the tiny least weasel, weighing up to about 250 grams (8.8 oz) to the sea otter, weighing up to forty-five kilos (99 lbs) or occasionally more. A long-extinct mustelid is estimated to have achieved the size of a black bear or a jaguar, the biggest mustelid on record. Known as Megalictis, it once roamed the North American plains and probably preyed on many kinds of animals. 

Members of this family generally have rather elongate bodies,relatively short legs, and short muzzles. Most are highly carnivorous, typically pursuing rodents, fish, and other small critters, and most are solitary predators, but some are quite social. Certain members of the family occupy diverse, fairly specialized ecological niches—A few very social, some very arboreal, others fossorial (burrowing), some highly aquatic. It would be tidy and convenient if such specialized habits were associated with branches of the evolutionary tree of mustelid radiation (social species on one branch, fossorial ones on another, etc.). But that’s not generally the case—those habits have often arisen independently on the several known branches.

The long, slender body form of weasels is well-suited for these predators to pursue voles and mice into narrow tunnels and tightspaces. Similarly, that body shape enabled the North American black-footed ferret to chase prairie dogs down into their burrows. But that’s a sad story: thanks to habitat destruction and poisoning programs by uncaring agriculturalists, prairie dog colonies are few and scattered; the ferrets were declared extinct in the wild; captive-raised ferrets have been reintroduced in several places.

American badgers originated early in mustelid evolution, in a lineage distinct from other extant mustelids, and are quintessential burrowers. Much heftier than the weasels and ferrets, they have powerful forelegs with strong claws that enable them to dig deep and extensive burrow systems. Largely solitary, they are major predators of rodents and other small creatures of the prairies, but they too are far less common than they were.

American badger. Photo by Kerry Howard

The European badger is not closely related to the American badger, despite the shared name. A mighty burrower, it is quite social, living in multi-family, complex burrow systems. It’s a predator of worms, insects, hedgehogs and other small vertebrates, but also eats some tubers and roots.

Honey badgers (or ratels) of Africa and Asia evolved early in mustelid evolution and are not closely related to any other mustelids (including other “badgers”). They are typically solitary predators, fiercely chomping up all sorts of prey and raiding bee nests for honey, and they are good burrowers.

Arboreal species tend to be more omnivorous than the others, often eating lots of fruits in addition to small animals. The tayraof Central and South America is probably related to the martens of North America, but it’s somewhat bigger and longer-legged. It can cavort very capably in the treetops, using its big tail for balance. However, the only one I ever saw was on the ground; it burst from the brush beside a trail, giving me (not knowing that such things existed) quite a startling!

River otters. Photo by Kerry Howard

Aquatic living is typical of mink and the distantly related otters. These species eat chiefly fish and invertebrates such as crabs and crayfish, in some cases also eating birds’ eggs and nestlingswhen available. The sea otter is the most aquatic of all, seldom coming ashore. It’s the biggest extant mustelid, with large, webbed hind feet. Its dentition is much modified from the basic carnivore plan—the molars are broad and flattened, made not for cutting but for crushing shelled invertebrates.

Next in size is the giant otter of South America, which is not very closely related to the other otters. It is very social (unlike most but not all other otters), living and hunting in cohesive, extended family groups and defending the family territory with loud choruses. With webbed feet and a muscular tail, it is a strong swimmer. Group members hunt together: fish are the primary prey, but sometimes also taking snakes and small caiman. It is now endangered.

A Wintery Walk

a wonderful weasel encounter

Days are rapidly getting shorter, and the peanut butter junkies at my feeders are voracious. The familiar Oregon juncos that thronged the feeders all summer are now scarce. But I’ve begun to see slate-colored juncos, here from the Interior for the winter. Closely related to the Oregon types (both are usually classed as subspecies of dark-eyed junco), but the new arrivals took at least a week to figure out how to exploit the peanut butter feeder. Maybe they watched the chickadees and got the idea.

On a lovely, sunny but cool and windy, day in mid-October, a little group of friends strolled up the road at Eaglecrest to go off onto the upper ski loop. Darting in and out of the angular rocks that line the road toward the Black Bear lift was a small white critter that disappeared almost as soon as it showed its head. So it took us a little time to ascertain who it was: of course, a short-tailed weasel or ermine (called a stoat in Europe). It then gave us many chances to see it as it explored both sides of the road, around the turn and past the Kimball memorial bench, popping up its head every so often to look around (and perhaps to check on us). The weasel was presumably hunting for something edible, such as a vole or shrew, but we saw no evidence of success.

Photo by David Bergeson

The white winter fur was certainly conspicuous against a background of gray rocks and brown fern fronds. When there’s snow on the ground, of course it’s a different matter—the white fur is great camouflage then, only the black tail tip and a beady black eye marking the beast on a white background. “Our’ weasel apparently had turned its coat from summer brown to winter white well before snow would cover the ground (although a little dusting fell a few days later). Molt is said to be initiated by changes in photoperiod (day length), and modified by temperature, but southern ermine don’t change to white at all. In northern populations, the physiology of molting to winter color is not closely timed to reliable seasonal snow cover; in fact, on snow-free Haida Gwai’i, ermine still acquire a white winter coat. This leaves open a question about why molt is not better synchronized with background color everywhere.

Weasels are built long and slender, which enables them to slip into narrow tunnels, even into vole hideouts in pursuit of prey. However, that elongated body has a lot of surface area (where heat is lost) relative to body volume (where heat is generated), so weasels have a high metabolic rate that generates heat– but necessitates lots of food. An active hunting style presumably provides more encounters with prey (than a sit-and-wait style, for instance), but has its own energetic costs. They need to eat several times a day (taking in about thirty percent of their body weight!) and have a well-insulated nest (often stolen from a victim) in which to rest between hunts. If the usual prey of small rodents is scarce, weasels may hunt hares, squirrels, birds, and even eat worms and bugs and carrion if necessary. They cache dead prey for future meals.

Mating season is in spring, but fertilized eggs are not implanted in a female’s uterus until nine or ten months later. Then the embryos develop into babies in about a month, the newborns staying in the nest for a couple of months or so. Litter size is variable, usually four to eight kits, but well-fed females can produce much larger litters (up to eighteen kits!). Juvenile females become sexually mature while still in the natal nest and may (if a mature male was nearby) already carry fertilized eggs when they disperse to establish their own home ranges. Males can’t inseminate their female litter-mates because they don’t mature until about a year old.

Here are a few more interesting tidbits about weasels: They have excellent color vision, unlike most mammals. They climb well, with reversible ankle joints (like squirrels) so they can descend a tree head-first. If threatened by a superior predator, such as a cat, they may pretend to be dead; if not eaten, they quickly revive and run away.

At this time in October, there was a thin sheet of ice on my home pond in the morning and the ponds at Eaglecrest were ice-covered. Nevertheless, we saw caddisfly larvae in their cases, hanging out on the bottom of a few ponds or moving extremely slowly. They probably spend the winter as larvae, feeding on detritus when possible, but otherwise quiescent; pupation and metamorphosis into flying adults would occur the following year.

A few blueberries clung on their bushes, and I was informed that they were exceptionally tasty. The bright green fronds of deer fern and fern-leaf goldthread stood out on a background of brown, dead and dying vegetation. On the surface of some old skunk cabbage leaves, tiny pools of water had coalesced and frozen solid, forming jewel-like, nearly spherical beads that gleamed in the sunlight.

We ate our lunches in warm sunshine, all spread out in the lee of a grassy bank. The first wintery walk of the year turned out to be a good one.


weasels and their kin

One of the treats of a snowy winter is wandering around looking for animal tracks. When I counted up the species for which we’ve found tracks, I saw that one taxonomic family was disproportionately represented—the Mustelidae. Five species of mustelids are likely to leave tracks in snow in our region: ermine, mink, marten, river otter, and wolverine. I’ll first present some basics about mustelids in general, and then some specifics about each of these five species.

Mustelids are a widespread family, occurring on every continent except Australia and Antarctica. There are over fifty-five species, ranging in size from the diminutive least weasel, weighing as little as one or two ounces, to the sea otter, reported to reach over a hundred pounds. They tend to have relatively long, thin body shapes, although some, such as badgers, are stockier; legs are generally short. The claws do not retract (unlike most cats), but again there is an exception: the claws of the fisher are partially retractable. Males are generally larger than females of the same species; their home ranges are larger and tend to overlap those of several females.

They are typically carnivores, preying on a variety of small or middle-sized animals, and sometimes scavengers, although some, such as marten, also eat fruit (and serve as seed dispersers). Much of their ecology is related to availability of food: population abundance, litter size and survival, frequency of reproduction, rate of maturation of juveniles, adult survival (starvation is reported to be a common cause of death in wild populations).

Along with males of many other placental mammals, male mustelids possess a baculum or penis bone. The size of this bone in different species has been suggested to relate to the length of the copulatory act: long bacula are correlated with long copulations. Extended copulations are not generally possible when penile erections depend entirely on hydraulics, i.e. blood pressure. This begs the question of when and where long copulations are adaptive or, conversely, when and where short ones are adaptive. As you might imagine, the subject has attracted some discussion but with no definitive answer.

Most mustelids also share a reproductive habit that (to humans) seems odd: After copulation, the fertilized egg divides just a few times and then rests; it does not implant immediately in the uterine wall, so no placenta is formed and the embryo does not develop further for some time. The delay of implantation lasts several months, during which the few-celled embryo just floats around in the uterus. Eventually, however, it does implant, a placenta develops, and active gestation (just a few weeks long) begins. Thus, the time of mating and the time of active pregnancy are well separated, and birthing is therefore postponed to a season well after the mating season. Delayed implantation is typical of numerous other mammals, including bears and seals.

The adaptive value of a seasonal separation of mating and birthing is often discussed. Most explanations address the importance of rearing young at times of year when food and other conditions are optimal.

This leaves unanswered the reason(s) why mating occurs so long before the season for rearing offspring, and I have not discovered good explanations. In some cases, other aspects of the life history may have created limitations on the convenience of getting male and female together; for instance, bears hibernate for the winter and sexual encounters shortly before birthing are not likely, summertime is surely more convenient; or males take advantage of freedom from child care to go roaming and foraging while females tend the young. I’d like to find a serious analysis of the conditions that favor the seasonal separation of mating and birthing.

Here are a few interesting factoids about our resident mustelids:

River otter—They are very aquatic, eating mostly fish, other aquatic animals in open water or tide pools, and sometimes capturing floating birds from underwater. They can dive to about twenty meters, staying under up to four minutes or so. Being heavy-bodied, they must tread water or scull with the tail to stay afloat. Otters are reported to forage cooperatively in some locations (e.g., Prince William Sound). Otters often travel overland between bodies of water, sometimes sliding over the snow. Their home ranges are said to be smaller on the coast than in the interior, presumably because food sources are more abundant. Otters usually mature at age two years.

Wolverine—They often favor remote areas but use a variety of habitats. In winter, this large mustelid mostly forages by scavenging carcasses left by other predators; its powerful jaws can crack the bones of moose. It is sometimes said that wolverines are too big to survive very long on small prey, too small to kill large game animals regularly, and too slow to chase fast prey. So scavenging becomes a good way to feed. In summer, carrion is less available and wolverines eat more small mammals and birds. They commonly den under deep snow in alpine areas, but commonly travel widely (many miles) to find food. If they get lucky, they will cache surplus food in a handy location. They are slower to mature than our other mustelid residents, usually maturing when three to five years old.

marten. Photo by Matt Knutson

Marten—Denizens of old-growth and mature forests, they are highly arboreal. They commonly feed on small mammals, as well as birds, eggs, and carrion, and are said to need the equivalent of at least three voles per day. However, they can also kill hares and marmots. They mature at age one or two years, depending on food supply. There are two species in Southeast; detailed genetic studies have shown that Kuiu and Admiralty islands are home to a distinct and strictly coastal species (which also occurs on Haida Gwaii and Vancouver).

Ermine—They eat almost anything that moves and need to eat almost continually; they are good swimmers and climbers. They make cozy nests, often usurping the nest of a prey mammal (after eating it), even lining the nest with fur of the prey. Well-insulated resting places are necessary for this small, slender predator that needs this help to keep warm in winter. Sometimes they cache their prey near the nest. Juvenile females can become sexually mature while still in the natal nest, at an age of only one or two months. So, when their mother mates after giving birth (which is the custom with these animals), sometimes the same male will fertilize her daughters as well! They have a short life span in the wild, often living less than two years. Ermine are represented by three distinct genetic lineages in our area, and one of them, with a very limited distribution, is considered to be of conservation concern.

Mink—Comfortable on land and in water, they eat fish, crayfish, various other small aquatic critters, and birds—they are said to be especially fond of bird eggs. They make short dives but usually forage in the shallow water or on land. Mink (and wolverines, ermine, and marten) are adept at climbing. They, like squirrels, are able to descend from a tree rapidly and skillfully, because they can rotate their hind ankles so the claws engage with tree bark. They breed as yearlings, and seldom live longer than three years in the wild.

The populations and historical geographic ranges of marten, wolverine, and river otters in North America have been seriously restricted by human activity: habitat loss including deforestation, over-trapping, pollution (especially otters), reduction of their prey populations. In some cases, reintroductions have restored local populations.

Footnote: (There are three other mustelids in Alaska but we don’t generally see evidence of them here. Least weasels live up north and do not occur here. Sea otters live in the sea, yes, and seldom come ashore. Fishers have only rarely been recorded in Southeast and, in any case, are very elusive.)

Snowy tracks

stories written on the winter landscape

Snowshoes crunched over deep snow. The sky was cerulean blue and the sun gradually crept around the mountain peaks. These were fine days to be out, seeing what we could see. We were especially interested in the tracks left by the wild critters as they went about their daily lives.

–A shrew left a long line of tiny marks by the side of a beaver pond. Short digressions led to tufts of grass or a buried stick, where spiders and bugs, slowed by the cold temperatures, might be found. Shrews only weigh a few grams and have a very high metabolic rate, so they have to eat almost continually. We often see their trails running over the snow and plunging into miniscule holes that lead under the snow blanket where prey might be found.

–Mouse trails are much less common. But one day we found a line of hopping prints that went out of the forest and across the upper intertidal zone to the most recent wrack line. The piles of tumbled rockweed might harbor small crustaceans, wayward seeds, or lost insects—all suitable for a snack. Another line of tracks went straight back into the shelter of the forest.

–Snowshoe hares had been busy in some areas. They too were looking for food, maybe willow or blueberry buds. But occasionally there were heavily trampled spots, very localized, as if there had been a dance or other social encounter. Popular routes became hare highways, packed flat along a small ridge or between two dense spruce stands.

–An otter had cruised for hundreds of yards along a frozen slough, making side excursions to visit (briefly) several beaver lodges. The deep trough left by its passage seldom came out in the open but usually stayed under the fringing conifers. Reaching the shore of a well-frozen lake, the otter abruptly turned around and went back the way it had come. The only open water on its route was a very small runnel below a beaver dam—a place not likely to hold good otter food.

–Across some thin pond ice, a great blue heron had gingerly minced its way from one patch of open water, at the inlet, to another, at the outlet. Taking very short strides on its long thin toes, it seemed to have been treading carefully. Little sticklebacks and juvenile coho, beware.

–In several places, we spotted narrow grooves on the snow surface, where a slim body had propelled itself on small feet. These wandering trails led to grassy tussocks, dove under logs, circled a pile of branches, disappeared under the snow and came out again. A mighty hunter was at work: a short-tailed weasel or ermine, whose coat turns white in winter, except for the tip of the tail. The short-legged, long body of a weasel is well-adapted for diving down vole tunnels and other tight places. However, that body form means that a weasel can’t afford to put on heavy layers of fat; the belly would drag when the weasel tried to run—not good for a hunter that has to keep moving for much of the day in search of prey. In addition to their small size, the body shape of weasels gives them a lot of surface area (where heat is lost) compared to the body volume (muscles and organs that generate heat), so they have a high metabolic rate to keep themselves warm. And that means they have to eat a lot. They eat mice and voles and small birds, and carrion when it’s available.

–Porcupines seem to wander widely, and we’ve found their trails in many places, often still distinguishable under a layer of new snow. One day we found a very fresh trail of footprints and even some quill-drag; we followed it along a little dirt bank until it disappeared over the edge. Looking down, we saw that a small log, sticking out parallel to the bank, had been wiped clean of new snow by the animal’s passage; the trail ended near the end of the log. Of course, we went around to an easier place to climb down the bank and investigated the trail’s end. There we found a deep burrow, with hairs and a few dried-up fecal pellets and a good barn-y smell, that ran into the bank for over two yards: a snug, dry den that had been used repeatedly for some time. Upon close inspection, that little access log had thousands of scratches, evidence of many balancing acts as the porcupine had ventured out and back.

First snows

some tracking discoveries and other observations

One of my favorite activities in winter is to go out looking for animal tracks in the snow. In early-mid November this year, the snow was perfect: not a lot of it, but soft enough to register animal passage and firm enough to hold the tracks’ shapes.

So, off to Eaglecrest I went, with two good friends who like these little explorations too. We found lots to look at. Porcupines had plodded in and out among the trees, in some cases making small highways of repeated use. A few red squirrels had ventured out of their burrows. A weasel had covered a lot of ground, bounding with shorter leaps when it went uphill. It investigated many a fallen log and stump in hopes of nice lunch. Weasels have to eat a lot, just to keep warm and feed their active metabolism.

Voles (or maybe mice – it’s often hard to tell which) had run over the snow from one grass tussock to another or from log to bush and back again. These were the most common tracks, often right out in the open meadows, where they might be easy marks for predators. But we saw no signs of lethal events.

Near the road, we found a spot where an indisputable mouse had hopped across. On either side of its trackway were marks of a tail flick. It couldn’t have been a vole, whose tails are very short, so it had to be a mouse. Why it had flipped its tail from side to side was not clear, however; we speculated that perhaps it was slightly off balance on the coarse cobbles at the edge of the road and used its tail to restore an even keel.

Photo by Katherine Hocker

We found a few lines of tiny tracks that were made by shrews. Emerging from one dime-sized hole, crossing over the snow to an equally minuscule hole, occasionally they tunneled just barely below the snow surface.


Every so often, we looked up instead of down and noted that quite a few trees had long-dead tops. No mystery there, given the howling gales that sometimes whip through this area. But none of the lower, lateral branches had grown upward to replace the missing tops. We’ve all seen conifers whose original ‘leader’ at the top of the tree has been killed but a lateral branch just below it has taken over as leader, creating a kink in the trunk. We puzzled over why this hadn’t happened on the trees in which the entire top was dead.


An answer might lie in the way that hormones control growth. Normally, the leader at the top of a conifer suppresses the growth of lower branches; this is known as apical dominance. But the effects of apical dominance diminish as the distance from the leader increases. So, perhaps, when the entire top of a tree is killed, the distance from the leader was so great that there was no dominance exerted on the remaining branches. Thus, the lower branches had not been suppressed and they did not respond to the loss of the tree top.


A few days later I walked out into the Mendenhall Glacier Recreation Area near Crystal Lake. Tracking was still good and there had been lots of activity. A porcupine had trundled across the ice on the lake, and a weasel (I think) had walked (not bounded) along the footpath. Squirrels and snowshoe hares had crossed the path.


The most interesting marks were made by a bird, whose wingspan exceeded five feet—surely an eagle. Its wing tips brushed the snow in several places around a patch where the snow had been disturbed. Here I could see some heavy-duty bird tracks, confirming the presences of an eagle. All around this area were raven tracks too. But there was no clue about what the eagle was after—unless it might have been a raven (eagles do capture ravens sometimes). It seemed unusual for an eagle to be hunting in a wooded area where the only open ground, where an eagle could spread its wings, was the path itself.


Lots of stories in the snow, so winter was off to a good start for me!