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Nature Notes: Weasel Confusion

Ed Robinson
March 7, 2023

One of the joys of writing nature books is the opportunity to share my photo presentations with people around Maine. From Kittery to Bar Harbor, Rangeley to Harpswell, folks of all ages enjoy seeing photos of wildlife and telling their own stories about encounters in the outdoors. At times I wonder who is getting more benefit from the presentations, my audiences or me! I also have the privilege of talking about conservation and encouraging people to take care of the natural world around them.

weasel stands on hind legs looking at viewer

A weasel standing (Silfox iStock photo)

While I strive to present gorgeous photos and to share humorous stories, my goal is also to educate audiences about creatures that may be unfamiliar or mysterious. Small birds like the northern saw-whet owl are more often heard than seen, Canada lynx are creatures of the deep woods and black bears are decidedly shy of human interaction. There are numerous misunderstandings about nocturnal white-tailed deer behavior and few people are informed about threatened species like Atlantic puffins. Yet the most persistent challenge I encounter is in talking about weasels.

First, let us consider the broad category of creatures known as “the weasel family.” The Latin word for weasel is “mustela” and at some point, scientists decided to lump many animals together as “mustelids.” The consensus seems to be that around 70 species worldwide fall into this grouping, with a formal family name of Mustelidae (with over 200 additional species thought to be extinct). One problem is that many of the creatures bear only a vague resemblance to the small animals we know as weasels (more on them later). Much larger animals like the badgers, wolverines, sables, sea and river otters are similar to weasels primarily because the mustelids all are equipped with anal scent glands that produce a potent, oily secretion used for territorial marking and protection. The family resemblance of martens, mink, and ferrets to weasels is more obvious to casual observers given their smaller size and streamlined shapes.

Weasel confusion is amplified by the names people in different parts of the world use for various members of the mustelid family. Here in Maine, many people persist in referring to the “fisher cat,” even though the proper name is fisher. American martens are often called “pine martens.” In some parts of the US, folks refer to the long-tailed weasel as the bridled weasel, the big stoat or the masked ermine. In England, when people use the term “ermine” they generally refer only to the diminutive least weasel. It is no wonder that folks come to my presentations more than a little confused as to what creatures I am referring!

In North America, we can narrow down the resident species of mustelids to a couple handfuls: three weasels, fishers, American mink, American badgers, American martens, wolverines, black-footed ferrets, sea and river otters. New England does not currently host American badgers, wolverines, black-footed ferrets nor sea otters (Pacific Ocean only). Now we can focus on those weasels!

When you dig into weasel literature you quickly realize that there is either a decided lack of clarity out there or our scientific colleagues are just as befuddled as we lay people! I have looked at dozens of sources in trying to accurately describe these wonderful creatures and will do my best to cut through the fog. To be fair to our more learned friends, weasels are not the easiest creatures to study due to their size or reclusive habits and they clearly are not a priority species for research funding.

We begin with the largest, the long-tailed weasel, scientific name Mustela frenata. Most sources state the animal’s total length ranges up to 14 inches with a tail that may reach 60 – 70 percent of the body length. Remember that tail length since it is about the only way to judge weasels in the field. A large male (known as a buck, dog, jack or my favorite, a hob) might reach 10 ounces in weight, 40% heavier than females (called a bitch, doe or jill). These weasels have the widest distribution among their North American cousins, from southern Canada to the northern parts of South America.

Next, we meet the short-tailed weasel, scientific name Mustela erminea. Given their scientific name you can guess that many people refer to this creature as an ermine, although some folks refer to them as a stoat. Overall length averages around 12 inches with a tail that is roughly 33 percent of their body length. They are believed to be the most widespread weasel globally, found in northern Asia, Europe and from the Pacific Northwest, Great Lakes and New England all the way up to the Arctic regions.

Finally, we have the least weasel, Mustela nivalis, sometimes known as the little or common weasel just to keep things confusing! Like the short-tailed weasels they are also found around the world in northern habitats but to my knowledge have not been documented in New England. As you would expect they are the smallest weasel with a total length of about eight inches, with a tail that is just 25 percent of their body length. They weigh just three ounces and can slip through tiny openings in search of prey. An interesting point is that the fur of a least weasel will fluoresce under ultraviolet light but that is not much help in field identification for a creature that can vanish in the blink of an eye!

white weasel looks down from tree

A short-tailed weasel in winter (Vershinin M iStock photo)

Please note that all the weasels described here can change from brown to white in the winter months, not just the short-tailed weasel or ermine. But a useful tip to remember is that the least weasel in its winter coat lacks the black tip on the tail of the other two weasels.

Weasels are generally solitary creatures other than during breeding periods, and they are most active at night when their preferred prey species are on the move. Favored sites include stone walls, brush piles, old foundations, and ground burrows. They are territorial in nature, with the males using their scent glands to mark boundaries and generally intolerant of other males within their areas of dominance. Long-tailed and short-tailed weasels breed in mid-summer with the females using delayed implantation of the fertilized eggs to result in birth during April and May. The least weasel has a more traditional breeding cycle and is capable of multiple litters of four to six kits each. All the weasels experience cyclical population swings as the number of prey species vary due to environmental or other causes.

Weasels are among the smallest carnivorous animals in the wild kingdom but they are a force with which to be reckoned. They are aggressive by nature when hunting and eat up to one third their body weight per day. This is driven by a robust metabolism with a heart rate that can exceed 400 beats per minute. Consider that a weasel has a stronger bite relative to its body size than any lion, tiger, or bear! They use their sharp teeth to grab prey by the head or neck, crushing bones and then eating a small portion of their kill and sometimes returning later for another meal. All the weasels are specialists in harvesting small mammals like mice, shrews, and voles but their diet is quite varied. They have been observed eating insects, eggs, amphibians, birds, lizards, snakes, and carrion. Larger animals like rabbits, even young wild turkeys have reason to fear a weasel on the prowl.

Creatures like the fox, fisher, coyote, and large snakes will attack a weasel, as will raptors such as hawks and larger owls. When threatened weasels may run for cover but they have also been observed going into a “weasel war dance.” In this display of fearless intent, the weasel’s hair goes erect, they begin barking and growling, and they leap about while twisting with arched back and stiff limbs. That might give a potential assailant pause!

It is difficult to estimate population numbers for these elusive creatures other than by reviewing harvest data from fur trappers. The International Union for the Conservation of Nature rates the weasels in the “Of Least Concern” category meaning there is no indication they are currently in danger. However, pollution, climate change and habitat loss that affect their prey species obviously will have a knock-on effect on the weasels over time.

If you are somehow still confused about weasels, then shame on me! Thanks for reading.

If you like Ed Robinson’s writing, check out his two Nature Notes books! Click here for more information.