Most people are generally familiar with the weasel: a small, sleek predator with a mean reputation. But very few people actually get to observe weasels in the wild. And when encounters do occur, they are often brief, and glimpses are fleeting. Weasels are primarily nocturnal, and can be secretive in nature. But they can also be bold and curious at times, leading to interesting encounters. The last weasel that I observed in the wild was just as curious to see me as I was to see it!

Unsurprisingly, weasels are members of the weasel (Mustelidae) family. Generally referred to as ‘mustelids,’ members of the weasel family all evolved from a common ancestor and radiated into many specialized forms. Other mustelids include otters, martens, badgers, and the wolverine. Most mustelids are carnivorous, and many are known for their ability to capture prey items much larger than their own body. The weasels are highly-skilled predators, specializing in capturing rodents and small mammals that are often underground or beneath the snow. Weasels generally have a fearsome reputation, which is likely undeserved. While they are sophisticated predators, they are not aggressive toward humans and can actually be quite inquisitive when approached.

A grainy cell phone picture captures a brief encounter I had with a curious long-tailed weasel (Mustela frenata) in the mountains of northcentral Pennsylvania (Photo: Jim Kauffman).

While weasels may be hard to see, they are even more difficult to identify. This is because the Chesapeake Region is actually home to three different species of weasels. Superficially, these species appear very similar. But each species has evolved unique characteristics that allow them to exploit unique ecological niches and geographic regions. Long-tailed weasels (Mustela frenata), short-tailed weasels (Mustela erminea), and least weasels (Mustela nivalis) all have ranges that include most of the Chesapeake Bay watershed. Long-tailed weasels are the largest, followed by the smaller short-tailed weasel (also known as an ermine or stoat), with the least weasel being the smallest. The least weasel is actually the smallest carnivoran mammal on earth! Their similar morphology and range overlap often makes identification difficult, especially because weasels never seem to stop moving (unless they are asleep).

A least weasel (Mustela nivalis) in summer pelage (Photo:

Winter Whitening

One adaptation in particular has allowed weasels to become extremely successful as a group. As winter approaches, many weasels shed their brown summer ‘pelage’ (fur or hair of a mammal) and develop an almost pure-white winter coat. As both predators and prey, this adaptation has served them well. It might seem like this adaptation would have developed to help weasels remain undetected by their prey. But as I mentioned earlier, weasels never stop moving. They are not ambush predators, so they hunt their prey by moving quickly throughout the forest floor while surveying woody debris, rocks, and subterranean voids. So having this camouflage wouldn’t benefit a predator that is constantly on the move. As it turns out, it’s more likely that this is an adaptation to prevent weasels from becoming prey themselves!

Both aerial predators (hawks and owls) and mammalian carnivores (foxes, bobcats, and coyotes) will prey on weasels. By changing coats during the winter, weasels have developed an adaptation that is advantageous for a predator their size. While weasels are actively hunting their own prey, their small size means that they are often being targeted by larger predators. Camouflage coloration that matches their surroundings helps to break up their silhouettes against the forest floor, making it more difficult for predators to capture them.

But not all weasels develop white winter pelts. While all three weasel species of the Chesapeake Bay Watershed have the ability to change their coats, not every individual does so. In fact, winter whitening is often dictated by latitude. Generally speaking, weasel populations in northern latitudes turn white during the winter. Weasels in mid-latitudes sometimes turn white, but not always. And weasels within southern populations almost always remain brown during winter. Here in the Chesapeake region, we lie within a transition zone. This means that some of our weasels turn white, some remain brown, and others have a mottled winter coat. This gradient of coat coloration is likely a response to natural selection. Individuals within a population are more likely to survive and reproduce if they retain advantageous characteristics that are passed on to the next generation. And a coat coloration that matches the seasonal landscape seems to be advantageous for these weasel species!

If you look closely, you will notice that long-tailed and short-tailed weasels do not turn completely white during winter. These species both retain a small black patch at the tip of their tails. This is another adaptation to prevent predation by aerial predators. When a hawk or owl targets a weasel as prey, it often focuses on the black tail tip instead of the weasel’s vital areas. This creates a scenario where aerial predators are more likely to ‘miss’ a capture attempt when targeting a weasel. But if this is the case, then why don’t least weasels have black tail tips? Because least weasels are so small, this trait would likely not benefit them. This means that their tails do not extend far enough from their body for this ‘trick’ to work. It has also been suggested that least weasels do not need this adaptation, as they tend to spend more of their time under snow cover, within an area known as the ‘subnivian’ zone (hence the scientific name M. nivalis).

Climate Change and Coat Coloration

As climate change causes shifts in historic climate patterns, wildlife species are forced to adapt or disappear. If snowfall and snowpack regimes change significantly, weasel populations will have to follow suit in their capacity to change (or not change) color. Weasel species now show considerable variation in their ability to adapt to regional climate conditions, so it may be possible for populations to successfully adapt to changing winter conditions.

An ermine emerges from its subnivian burrow on a winter morning (Photo

Providing Weasel Habitat

Creating or managing habitat for weasels is fairly simple. Weasels can occupy a wide range of habitat types, but tend to avoid open areas that make them prone to predation by larger predators. So promoting forested habitats will provide weasels with the resources needed for survival and reproduction. While contiguous forested habitats are preferred, weasels will also use forest-adjacent edge and brushy habitats. Forested habitats with coarse woody debris on the forest floor, rocks, and brush piles are highly sought-after by weasels. Weasels rely on small mammals for prey, so habitats that provide cover for small mammal species are especially important. Maintaining forest habitats, creating additional forest habitat, and reducing mowing will benefit weasels. The reforestation and riparian forest buffer programs administered by The Alliance create forested habitats that benefit weasels and many other wildlife species.

A white winter weasel carries its vole dinner across the snow (Photo

Weasels and the Fur Trade

Weasels are included in a group of mammals generally known as ‘furbearers.’ These are mammals that are harvested primarily for their pelts and hides. Furbearers are generally not consumed by humans, but have soft, warm furs that can be utilized for garments and other products. Throughout human history, weasel pelts have served as status symbols, often adorning robes, ceremonial dress, and garments worn by upper-class nobility. Weasels were once a significant part of the fur trade during European colonization of North America, and thousands of skins were processed annually to support this industry. Weasel species are still harvested by some trappers and hunters throughout the Chesapeake Region today.

Weasel pelts adorn a Native American headdress. Raven Blanket Nez Perce (Photo by Edward S. Curtis.

In addition to these three weasel species, there are a few other mammals that turn white during the winter. Can you name them?