Weasel Taxonomy: A Weasel by Any Other Name Is Still a Weasel

Those responsible for naming the Mustelidae family picked the right word. They are a weaselly bunch. Weasels, minks, badgers, otters, ferrets, polecats, and martens comprise the 70-plus species in the Weasel family, and the term “weasel”—the common name for these mammals—sometimes refers to the lot of them. Other times, it is solely being applied to the smallest weasel, Mustela nivalis.

The weasel is tough to tie down. It needn’t be, though. So, the first order of business is to unequivocally define our terms and weasels, rather than weasel out of it. Weasels, for this article’s purposes, are any of the 13-16 species in the Mustela genus, and specifically the three that are the focus of this article.

• Mustela erminea: short-tailed weasel, also called a stoat or ermine
• Mustela frenata: long-tailed weasel
• Mustela nivalis: common or least weasel

At a Glance

In their book The Natural History of Weasels and Stoats, Carolyn M. King and Roger A. Powell write,

In North America, “weasel” generally refers to any of the three small, native Mustela species, while in England and New Zealand, “weasel” is reserved for M. nivalis, and M. erminea is called “stoat.” In Europe it might be correct and acceptable to apply the American term “ermine” to all European M. erminea…, but it would not be correct to apply the American name “least weasel” to European M. nivalis except in the far north. … Equally confusing, the common name “ermine” is used by some to mean M. erminea only, and by others to mean a weasel of any species when it is in white winter coat. (12)

Weasels have managed to weasel out of even a simple name! At least there is no question of what constitutes a weasel.

Common or Least Weasel, Mustela nivalis

The least or common weasel is tiny; in fact, it is the smallest mammalian carnivore in the world, but there is nothing small about its appetite. This weasel consumes about 50% of its body weight daily. Its size presents no obstacle to its success as a predator since this it can live in a diverse range of habitats on several continents during changing seasons.

This species of weasel can even take down prey larger than itself, thanks to its 34 teeth and non-retractile claws on each one of its five-toed paws. They store their kill—as do the other species—in a practice known as caching. To keep other hungry critters at bay, they will scent-mark their dens using secretions from their anal glands. They also discharge this foul-smelling secretion when startled or concerned, much like a skunk.

These weasels have long, tubular bodies that are milk chocolate-colored except for a white underbelly and have short legs. Their tails are also short, about one-fourth the length of their bodies. Most notably, their tails are a solid color unlike those of the short- and long-tailed weasels whose tails look as though they were dipped in black ink. They molt their coats seasonally. Around October, their hair begins to grow white. This change takes place in northern populations of this weasel, but not in the southern ones.

Short-tailed Weasel or Ermine, Mustela erminea

The stoat, Eurasian ermine, and ermine are all names that identify Mustela erminea, the short-tailed weasel. As the most widespread weasel species, the ermine is found throughout northern Asia, Europe, and North America. It and the common weasel were introduced to New Zealand to control that country’s rabbit population. Instead, they acquired a taste for its birds, and these weasels are now considered a pest there.

Like other weasel species, the ermine is long and slender with short legs and features a reddish-brown body with a white underside. Its tail is about one-third the length of its body and the bushy tip looks as though it was dipped in black ink. Like the common weasel, the stoat, another of its names, molts its coat before winter. All that remains of its summer coat is the black-tipped end.

The term “ermine” is often used to refer to this species and any other weasel that turns white during the winter months.

Long-tailed weasel, Mustela frenata

The length of a very large carrot, the long-tailed weasel has the widest distribution in the Western hemisphere. While it has much in common with the least and short-tailed weasels—it molts seasonally, has an elongated body that features brown fur and a white-ish underbelly, and feeds on prey both smaller and larger than itself—you won’t find one in Europe or New Zealand.

Most of the confusion with this species is with the short-tailed weasel or ermine, a term that describes two things, per Merriam-Webster: “any of several weasels whose coats become white in winter usually with black on the tip of the tail” and “the white fur of the ermine.” The long-tailed weasel’s tail is the longer version of the short-tailed weasel. They both appear to have been dipped in a black substance. As such, when winter arrives, M. frenata gets the M. erminea treatment. “Look, it’s ermine!” or “Look, it’s an ermine!”

But winter isn’t the only time when the two species are confused. They are so similar in coloring, and if you’re unfamiliar with their size differences, identifying them correctly can be tough but not impossible. A giveaway is that the underside of the long-tailed weasel ranges in color from a white or yellow-white to a rusty orange color.

Keys to Identification

Not all is lost when identifying these three species.

First, these three weasel species come in three sizes: small, medium, and large. The smallest, M. nivalis, is also the most common, hence its common name, the common (or least) weasel. It’s also the easiest to clearly identify since its tail is solid in color regardless of the season.

Second, the intermediate- or medium-sized weasel, M. ermine, is the linguistically wily species since its species name is used both as its common name and to describe all three species when they trade their brown or reddish-brown coats for their winter white ones, but remember this: Ermines always have black fur on the end of their bushy tails. When a weasel crosses your path, be sure to spy the tail as that is often the dead giveaway of the species, or at least the easiest way to distinguish it from its smaller cousin, the least weasel. It’s another thing with the larger weasel, the long-tailed weasel or M. frenata.

Additionally, identification can be difficult between the three species because even their sizes overlap. Males tend to be about twice the size of females, regardless of species. As an example, a female M. erminea (ermine) could be mistaken for a male M. nivalis (least weasel) or a male N. erminea (ermine) could be misidentified as a female M. frenata (long-tailed weasel). It’s a factor King and Powell acknowledge contributes to the confusion people have in correctly identifying these species from each other. “[People] easily become confused by the large difference in size between males and females of the same species, exaggerated by the slow growth rate of young born late in the year, and by imprecise use of common names” (13).