Field Trip: Weasels

by Dan Schneider and Peter Pautler

Sought after for their luxurious furs and entrenched in popular myth as killers and gluttons, weasels (of the family Mustelidae) are solitary creatures well adapted to catching small mammals, birds and fish.

Of the 11 species of weasels widespread across temperate regions of North America, nine live in Ontario: American marten, fisher, least weasel, short-tailed weasel (or ermine), long-tailed weasel, mink, American badger, wolverine and river otter. Small to moderate-sized animals, weasels have elongated, low-slung torsos and short legs. For most species, the tail is long relative to their body length.

Mustelids possess fewer, but more specialized, teeth than do other carnivores. Extended incisors, with needle-like points, allow mustelids to dispatch prey quickly with a fatal bite to the back of the neck near the base of the skull. The wolverine, which often feeds on carcasses other predators have abandoned, uses the carnassial teeth – modified molars located in the upper and lower jaws – to cut through frozen flesh and crush bones to get at the nutritious marrow within.

The American marten and fisher pursue squirrels and birds through the tree canopy. The American badger uses its formidable front claws to rapidly dig out any prey, such as mice, voles, ground hogs and cottontail rabbits that mistakenly seek the safety of an underground burrow.

Another remarkable adaptation of animals in the weasel family is their reproductive cycle. All mustelids, except for the least weasel, experience delayed implantation. After copulation and fertilization, which usually take place shortly after a female gives birth in spring, the rapidly growing embryo, known as a blastocyst at this stage, ceases development and remains “suspended” in the uterus. In mid to late winter, the blastocyst implants in the uterine wall and the embryo resumes normal development. The birth of kits follows in 25 to 65 days, depending on the species. Delayed implantation provides several advantages. It ensures the birth of offspring at a time of year when environmental conditions are favourable and food supplies are adequate; mustelid mothers are able to meet the energy requirements of nursing and weaning offspring without the extra demands of growing fetuses; and young mustelid kits have sufficient time to grow and become expert hunters before the arrival of the next winter.

Because fewer than 200 American badgers remain in Ontario, the Committee on the Status of Endangered Wildlife in Canada and the Ontario Ministry of Natural Resources (MNR) have designated the animal as endangered. The two biggest threats to the survival of American badgers are the ongoing loss of grassland habitat and their propensity for being struck by cars.

The wolverine is believed extirpated from the easternmost part of its historical range in Quebec and Labrador. MNR has designated the wolverine as threatened. Development, logging and roads threaten its long-term existence in Ontario. The remaining mustelid species are considered secure in Ontario.

Long-tailed weasel (mustela frenata)

Description: brown above, buff-coloured below and no white on feet in summer; white in winter, always with black tip at end of long tail nearly half length of body; head and body 20–26 cm, tail 8–15 cm, weight 90–340 g
Ontario range: throughout southern Ontario to north of lake superior
Habitat: open brushy or grassy areas near water; small woodlots, croplands, meadows, suburban residential areas
Predators: fox, coyote, wolf, bobcat, lynx, hawks and owls
Prey: mainly rodents; some birds, snakes and insects; occasionally fruits and berries
Who knew? The black tail tip of this species and the short-tailed weasel is thought to divert predators’ attention away from the animal’s head and thus allow it to escape.

American marten (martes americana)

Description: yellowish brown on body, dark brown on tail and legs, pale buff-coloured patch on breast and throat; bushy tail; head and body 35–43 cm, tail 18–23 cm, weight 700–1,200 g
Ontario range: northward from edge of Canadian shield and Manitoulin Island
Habitat: mature coniferous and mixed forest, cedar swamps
Predators: fisher, bobcat, wolf, lynx, coyote, great horned owl, trappers
Prey: mainly rodents and other small mammals; also birds, amphibians, reptiles, fish; occasionally insects and fruit
Who knew? The most arboreal of Ontario’s weasels, the marten has semi-retractable claws that are extended to aid in tree climbing and in pursuing squirrels and finding nests in the trees.

Short-tailed weasel or ermine (mustela erminea)

Description: brown with white underparts and feet in summer, white in winter; tail, always black tipped, about one-quarter of body length; head and body 13–23 cm, tail 5–10 cm, weight 30–170 g (males much larger than females)
Ontario range: throughout the province
Habitat: from tundra to coniferous and mixed forest; borders of open areas
Predators: marten, wolverine, fisher, badger, cats, hawks, owls, coyote and foxes
Prey: mostly small rodents; also some fish, birds, eggs, insects, carrion, reptiles and amphibians
Who knew? White and luxuriant ermine fur was a popular lining for aristocrats’ clothing during the middle ages.

Fisher (martes pennanti)

Description: very dark brown, white-tipped hairs give a frosted appearance; bushy tail; blunter face than marten; head and body 50–60 cm, tail 33–38 cm, weight 1.4–5.4 kg
Ontario range: northward from Manitoulin island, Bruce Peninsula, Kingston area
Habitat: large mixed forests, especially near water
Predators: mainly trappers, but possibly also wolf
Prey: any animal it can overpower – small to medium-sized mammals including porcupine (see sidebar), birds, carrion, fish, snakes, amphibians and insects; some plant material including berries, seeds and fern tips
Who knew? Reintroduction of fishers to original range areas on Manitoulin Island and the Bruce Peninsula and in the Ottawa valley has extended the species’ range into southern Ontario.

Mink (mustela vison)

Description: dark brown thick fur, white chin patch; partially webbed feet; head and body 30–43 cm, tail 13–23 cm, weight 600–1,300 g
Ontario range: throughout the province
Habitat: along streams, rivers and lakes
Predators: trappers, great horned owl, bobcat, fox, coyote, wolf, black bear, dogs
Prey: fish, frogs, crayfish, small mammals, birds and worms, as well as some plants
Who knew? Another species of mink, the sea mink (mustela macrodon), used to live on Canada’s Atlantic coast but was driven to extinction by trapping. The last sea mink was trapped in New Brunswick on Campobello Island in 1894.

American badger (taxidea taxus)

Description: yellowish grey fur; white stripe on forehead, white cheeks with black spot in front of each ear; black feet with extremely long front claws; head and body 46–56 cm, tail 10–15 cm, weight 6–11 kg
Ontario range: two small populations, one along the north shore of Lake Erie in the extreme south, the other in the rainy river area west of lake superior
Habitat: open grasslands and farmland
Predators: humans, coyote, golden eagles
Prey: mainly groundhogs and cottontail rabbits in Ontario; also ground squirrels and other small mammals, birds and eggs, reptiles, invertebrates and carrion
Who knew? Like other weasels, badgers pursue their prey into burrows but differ from other mustelids by digging out their prey. They literally eat their victim out of house and home, and enlarge the burrow for their own use.

River otter (lontra canadensis)

Description: upperparts dark brown, underparts paler and silvery; sleek appearance due to short oily fur; non-bushy tail thick at base; broad, flattened head with small ears; short, powerful legs with fully webbed toes; head and body 66–76 cm, tail 30–43 cm, weight 4.5–11 kg
Ontario range: northward from Bruce peninsula, southern Georgian Bay and north shore of lake Ontario
Habitat: along streams, rivers and lakes; also marine estuaries
Predators: bobcat, lynx, coyote, wolf, trappers
Prey: mostly fish; also mammals, birds, amphibians and aquatic invertebrates
Who knew? The otter fishes well beneath thick ice, often using unfrozen beaver dam outlets to reach the frigid water. Otters are famous for travelling over snow by alternating belly slides with a few powerful, loping bounds.

Wolverine (gulo gulo)

Description: dark brown, pale patch on cheeks and forehead, broad buffy stripes from shoulders join at rump; oversize paws allow buoyancy in snow; resembles a small bear except for large bushy tail; head and body 74–81 cm, tail 18–23 cm, weight 16–27 kg
Ontario range: extreme northwestern Ontario near Manitoba border; a few records north of lake superior
Habitat: tundra and boreal forest
Predators: humans, wolf packs, large bears
Prey: primarily a scavenger; preys on any animal it can overpower, mostly small game but occasionally larger mammals such as deer or caribou; also eats eggs of groundnesting birds, roots and berries
Who knew? The wolverine’s reputation for raiding traplines and food caches of trappers gave the species its scientific name, which means “glutton.” the wolverine is the largest member of the weasel family in Ontario.

Least Weasel (Mustela nivalis)

The least weasel is the smallest Canadian true carnivore. So elusive is the little critter, ON Nature was unable to find a photograph of it. And so small, the wee predator must elevate its metabolism 400 percent higher than normal to stay warm in winter, and eat up to half of its body weight each day.
Description: Summer coat brown on back and white beneath, winter coat white; sometimes a few black hairs but no black tip at the end of short tail; head and body 14 – 16 cm, tail 25 – 35 mm, weight 38 – 63 g (only twice the weight of a deer mouse)
Ontario range: Northward from edge of Canadian Shield
Habitat: Forest and meadows
Predators: Long-tailed weasel, hawks, owls, cats and foxes
Prey: Mostly small rodents; some insects and amphibians


Comprehensive text: Mammals of North America: Temperate and Arctic Regions, Adrian Forsyth, Fire fly Books (1999).
Field Guide: Mammals of Ontario, Tamara Eder, Lone Pine Publishing (2002).
Website: Hinterland Who’s Who, www. index_ e.asp

Dan Schneider and Peter Pautler are resource interpreters with the Grand River Conservation Authority.


  1. June 8, 2012    

    While driving along Sand Lake Rd in Rideau Lakes Twp. I saw a dead small animal, (length of a chipmunk) beside the road. I stopped and noted it had pale brown fur above, a shortish brown tail with a black tip and a sulphur yellow underbelly. I called the Queens University Biological St. and left a message. I brought the creature home, in a plastic bag and put in my freezer. The web indicates weasels of this colouration are limitedb to parts of Asia (yellow-bellied weasel) If this is a rarity I believe I should report it.
    TAG! You’re it!

    David Boyd

  2. Ron Ron
    July 14, 2013    

    Just returned from a daily walk, today along naturally wooded Sawmill Creek Trail (yes, mountain bike damage here too starting) near U of T Erindale campus (Sun. July 14 2013), then crossing Mississauga Road up from historic St. Peter’s Anglican Church to descend into the Credit River Valley. On a sandy bank clearing on said river, this observer noted a soaking wet weasel exploring gaps among cut carbonate dimension stone at water’s edge. The weasel’s fur seemed solidly black, an elongated lean body, ground hugging movements, guess 20cm long, large black eyes. Bicolour descriptions of weasel fur keep this observer guessing as to the species. At this collapsed dam site on Credit River just north of Dundas St. West, its ecosystem was shared with insect snaring Cedar Waxwings hovering above the river water and schools of tiny minnow sized fish in the modest river current. A little nature news morsel from a Mississauga conservation area.

  3. July 16, 2013    

    Thanks for sharing Ron.

  4. Rob Evans Rob Evans
    May 18, 2014    

    Where are the photos on these mammals?

  5. Ernie Trach Ernie Trach
    July 20, 2014    

    We live in northern Manitoba just shy of 55th
    Have a little weasel under our deck and backyard as we are at the edge of bush and swamp
    Wife says as long as it feasts on mice it can stay ,but start chasing our rabbits or
    Her prized dog it will be a different story

  6. Mike Chilibeck Mike Chilibeck
    December 2, 2014    

    I had a least weasel living in the wood pile at my cottage in Lac Ste-Marie Quebec last year. While he was there, the mouse population at the cottage dramatically declined. Since he has gone (an owl might have gotten him) the mouse population has really rebounded. I would do anything to get him back. He lived in my wood pile. Is there anything I could be doing to improve my chances of attracting another?

  7. December 9, 2014    

    Hi Mike,

    ON’s Tanya Pulfer says:

    “If it’s good habitat it will return. Plus weasels are actually hard to see (unless they are accustom to us). So you might still actually have some nearby.

    However, mice/small mammals have bust and boom years. This may be a boom year for them. Which is really great for increasing the chance of seeing other carnivores who really rely on small mammals.

    So – in short – if it is good habitat, they will come.”

    ON Noah

  8. Bruce Bruce
    February 6, 2015    

    I live in Grimsby,ON and was watching geese and ducks at the mouth of 40 Mile Creek which empties into Lake Ontario. At this time of year,following very cold & windy weather, lake ice builds up into mounds twenty feet high. At about 4:45 pm Feb. 06/15, I saw this black animal running over the ice mounds and thought it was a black squirrel. What’s a squirrel doing out there? With my binoculars,I discovered that it was a black weasel type animal. It disappeared over the edge of the mound and never reappeared while I was there. It may have gone into the water.

  9. susana susana
    May 23, 2017    

    hello bruce – I live in virgil and this morning I saw a small black animal which I too thought was a black squirrel scampering along the rocks/logs of four mile creek. I watched it dart into the creek and tumble around a bit and came out with a mouthful, so I thought it was a fish! unfortunately the back legs were kicking so it must have caught a small rat swimming across the creek… i was about 30ft away without binos, so that’s my best guess; so intriguing!

  10. Randy Sheik Randy Sheik
    June 21, 2017    

    Yesterday, we had a visitor to our backyard (Bells Corners area of Ottawa), which I think was a least weasel.
    It appears to have been cleaning out a chipmunk den. It was seen carrying one chipmunk out under the fence, but when it returned for another, it was spooked by our large dog. This caused it to drop the chipmunk while falling into the swimming pool. It climbed out without any problem and disappeared under the fence. I retrieved the dead chipmunk and noted that there appeared to be bite marks to the back of the head. It returned a short while later and was surprised when it saw me and again disappeared under the fence. The day before yesterday there were 3 or 4 young chipmunks and their mom playing near one end of the pool, now there are none.

  11. June 27, 2017    

    Thanks for this anecdote Randy!

    Great to hear you have the chance to see those charismatic least weasels – perhaps they are taking care of local mice in the area too!

    -ON Noah

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