Weasels of the forests

martens and fishers in Southeast Alaska

The weasel family is well-represented in Alaska; of the nine species here, three are associated with water and the others are chiefly terrestrial. Of the terrestrial species, marten and fisher are particularly associated with forested habitats.

Marten range over most of Alaska. There are two species of marten here: the American marten lives in the boreal forests across North America including most of Alaska; the Pacific marten lives on Admiralty and Kuiu islands (and maybe some other islands in Southeast). Marten are highly arboreal, spending lots of time in trees but catching much of their prey on the ground. Voles are a favorite prey in most places, but marten can capture prey the size of a marmot or a grouse. Versatile feeders, they also eat carrion and fruit. They have a slender build and an orange-ish patch of fur on the chest.

Fishers live across North American boreal forests but reach their western limit (for some reason) in the southern Yukon and adjacent mainland Southeast Alaska. This species probably originated in eastern North America and spread westward since the last glaciation (but there is said to be an ice-age fossil in central Alaska…). They occur at low densities here and in the Yukon, although there is some evidence that they may be increasing. Fishers are not only rare here but tend to be elusive as well, so sightings of them in the wild are few—and thus they get much less attention than other species. However, in the limited space of this essay, I choose to write more about fishers.

Fishers tend to be larger and burlier than marten, although small female fishers and large male marten may overlap in size. Male fishers commonly weigh 3.5-5.5 kg (some get much bigger!) and females about 2.0-2.6 kg. The fur is usually dark brown, darker than marten, but it sometimes has pale patches here and there. Like all the terrestrial weasels, fishers can climb well, even coming down a tree head-first, like a squirrel. That’s possible because they (like squirrels) can rotate their feet so the curved claws, when extended, hook into the tree trunk.

Both marten and fishers have short legs, so travelling in soft snow can be difficult. Fishers tend to travel less then, or often use existing trails (their own or those of hares). The foot-loading (i.e., body weight per area of foot) of fishers is greater than that of marten, and male fishers have higher foot-loading than females, so they may have a harder time in soft snow. Fishers, perhaps especially males, often leave body-drag marks in soft snow.

Kits are born in spring, often in a tree cavity. Litter size varies with the food supply: usually two or three kits but sometimes more or none at all. Females mate soon after birthing, although the embryo does not implant in the uterus wall until ten or eleven months later (late in the following winter). Gestation takes five or six weeks and the young kits are nursed by the mother for about ten weeks. All mammalian females spend a lot of energy on lactation; for female fishers, the cost of lactation plus the cost of extra hunting activity to fuel that milk production means that the total cost of reproduction is almost three times the energy needed during non-reproductive times. The kits are competent hunters at the age of four of five months, but most females don’t breed until they are two or three years old.

The home range size of fishers varies enormously, from just a few square kilometers to well over a hundred, apparently depending on prey availability. Males range more widely than females. Hares and small rodents are common prey in most places, but fishers also scavenge carcasses opportunistically and eat fruit and invertebrates at times. They can capture prey as big as a chicken or a porcupine; they subdue porcupines by attacking the vulnerable, quill-less nose until the victim is worn out.

The English common name “fisher” is a misnomer. Fishers may scavenge a few dead fish, but fish are not a common item on the menu. The name might come from a French word for the European polecat.


New Year’s Day 2012

mustelids and lichens in the muskeg lands

Snow was falling, snow on snow, but—unlike the song—this midwinter day was not bleak at all. With two friends, one two-footed and one four-footed, I set out to explore the forest and small muskegs near the Auke Bay school. This was not our original destination, but we got part way out the road, watched a truck slither and spin out over both lanes in the unplowed slush, and decided we’d find a place closer to town. I’d never been in the area behind the school before, so everything was new to me.

No birds seemed to be active there, but we soon found the trail of a short-tailed weasel, also known as ermine, particularly in winter when the fur is white. It had popped out of a hole roughly the size of a fifty-cent piece, looped over the snow for a few feet, and then dived into the snow again. Both of these snow-holes led to open spaces under shrubs bent under the weight of snow, where mice or voles or shrews might provide a snack. The long, narrow bodies of the weasels allow them to follow their prey into small tunnels.

On the surface of the snow, we could easily see their footprints, with the rear feet landing where the front feet had been, as it took off in the next leap. Each leap covered about a foot of distance. They have such short legs that the fastest way to get around is bending the long, sinuous body to extend the stride.

Short-tailed weasels are ferocious predators, dining on mice and other small mammals by preference, but sometimes eating birds, insects, worms, and even young snowshoe hares. Males weigh up to about seven ounces, but females are considerably smaller. They have high metabolic rates and have to eat a lot every day; females with litters may kill four mice a day.

A bit farther on, we found the trail of a bigger relative of the weasel. This path led hither and yon through shrub thickets, briefly into a tiny rivulet, along a log, under some low-hanging hemlock branches, and into still more thickets. Although we occasionally lost the trail for a little way, we eventually followed it for several hundred yards. We decided the trail-maker was probably a pine marten, partly because the footprints seemed a bit too big and furry for a mink, and partly because no sensible, hungry mink should be so far from the delicacies along the shore.

Trudging through the brush can be easier in winter than in summer. Snow presses down many of the blueberry and menziesia branches, and the two humans on snowshoes could stomp over the bent branches. Our canine companion was less fortunate; her snowshoe-less feet sometimes plunged through the brush piles, to the full length of all four legs, leaving her to wallow her way out.

Even though our broad feet helped us through and over the bushes, we still emerged with our knit caps full of lichens and twigs. And every so often a snow-laden arch of branches would give way, depositing us unceremoniously into a hole. We think this is fun, apparently, because we keep doing it.

Along the way, we noticed an area with a spectacular display of beard lichen festooned on almost every branch. Some of the strands were easily over six feet long. We wondered how it is that there are localized ‘hot spots’ for this lichen. Environmental conditions for good growth, including light and lack of aerial pollutants, must be part of the explanation. But it seems likely that dispersal patterns also contribute to the patchiness of strong lichen colonies: Spores and fragments of lichens are carried on the wind, so the direction, speed, and timing of winds would probably deposit them in semi-predictable patterns. Here’s a complex research problem awaiting a clever young scientist.