What defines a weasel can be very confusing since evolution has lead to a variety of species sharing a similar anatomy. The taxonomy also constantly changes, since there is a lot of disagreement over which weasel species are distinct and which are subspecies.

Even though the family Mustelidae is often simply referred to as the “weasel family”, this broad phrase can be misleading in the English language, especially given our better understanding of genetics today. For example, many distinctive mustelids such as badgers, otters, and martens are related to weasels, or in some respects weasel-like, but are arguably not true weasels. Scientifically speaking, it is the mustelids belonging to the genera Mustela and Neogale that are true weasels.(1)(2)(3) All in all the term weasel has developed such a vague and sometimes arbitrary definition that it is not consistently applied or understood.

The weasels are usually considered the tiniest mustelids. They have long and slender bodies adapted to pursuing prey into their burrows, losing little speed when their normal gait transitions to a crouching position. Despite being the smallest mustelids, some attack and kill prey up to ten times their size—a risky but rewarding ability when the need for protein is so great. Weasels vary in body length from approximately 13 to 52 cm (5 to 20 in), with females being smaller than the males. Their tails may be from 1.2 to 29.4 cm (0.5 to 11.5 in) long.

When it comes to fur colour, most weasels have reddish or brown upper coats and white bellies. During the colder months, only five mustelids are known for changing to their white winter coats depending on the subspecies and their native region, and they are all weasels—the least weasel (M. nivalis), long-tailed weasel (N. frenata), Eurasian stoat (M. erminea), Haida stoat (M. haidarum), and North American stoat (M. richardsonii).(4)(5)

Common myths and misconceptions

How “weasel” can be a confusing common name

Usually in England and New Zealand, “weasel” is a vague common name used to refer to the least weasel (Mustela nivalis), while in North America the term often applies to any of the four small native Mustela and Neogale species.(3)(4)

Given the dominance of Western media, this has led some people to assume that there is only one or a few weasel species in existence, or that these species are somehow more weasel-like compared to others within the subfamily Mustelinae. In truth there are numerous different species and subspecies of weasels in the world—many of which have their own unique features, size, and range, including abilities and vulnerabilities in relation to their respective environments.

A family misunderstood

Centuries of human-centric myths, superstations, and preconceived notions have contributed to weasels being some of the most stigmatised mammals on Earth. The common expression “weasel out of” and the misnomer term “weasel word” also have not helped their image. To this day, society continues to habitually attribute human traits or intentions to these animals—believing they are simply bloodthirsty and egg-sucking creatures that kill for pleasure.

Like most wild predators, weasels hunt and do what they must to survive. Small predators in particular live on the edge with threats around every corner, including their own high metabolic rate and short digestive tract. It is a constant race around the clock to stay warm and alive,(6) so they have to be more intense than most to make it through the day, and unless we know the reasons for why they evolved to behave as they do, it is easy to misinterpret their intentions. We are not saying they do not enjoy chasing their prey or the taste of their food, but it is not simply malice for fun.

Surplus killing

Some would argue the reason weasels are thought to kill for pleasure is due to the act of surplus killing—especially in regard to domestic fowl. When presented with an abundance of food, multiple prey are sometimes killed so the weasel may return to cache the remainder for later consumption.(7) According to the Hankensbüttel Otter Centre, species like the European polecat have natural hunting instincts which are triggered by fleeing animals.(8) Under the artificial and confined conditions of a chicken coop, an intruding European polecat will sometimes attempt to kill more chickens than they can eat. While the exact reason for this behavioural phenomenon may still be inconclusive, is presumed to be an evolutionary response to unpredictable food resources in many carnivorous mammals,(9)(10) a bit similar to how we humans habitually hoard more food than we can eat during uncertain times.

A tunnel hunter has to stay as slim as possible, which is not without costs—the high metabolic rate and little storage of body fat will cause them to perish if they do not eat within a few hours throughout the day.(11) Weasels sometimes kill more than they can eat, but it is also possible the weasel is unable to return to eat or cache the remainder of its kill, due to someone scaring it off or removing the carcass in its absence.

They do not live in packs

There is a misconception that weasels live in packs, based on observations of females with well-grown young. A mother weasel takes her kits on excursions around the neighbourhood in a characteristic formation with the kits following her, which could be contributing to the impression. The little family eventually splits up at the end of the summer when the kits are independent.

They do not suck eggs

Despite the claim in William Shakespeare’s “Henry V.”,(12) weasels do not suck eggs. In relation, the misnomer term “weasel word” may have first appeared in Stewart Chaplin’s 1900 short story “The Stained-Glass Political Platform”, in which weasel words were described as “words that suck the life out of the words next to them, just as a weasel sucks the egg and leaves the shell”.(13)

The truth is weasels (more specifically stoats) have evolved a particular way to consume eggs. They bite a small hole in the egg, turn it so it lays in a horizontal position, then lick the liquid content when it starts flowing.(14) So essentially they drain eggs, not suck them. Seeing empty eggshells left behind may have given rise to the myth that weasels suck eggs, which—due to lacking the necessary jaw muscles—they are not physically capable of doing.(15)

They do not suck or “drink” blood

Some people are a little too influenced by vampiric folklore, since there is a common rumour that weasels will suck the blood of their prey. As with eggs, weasels do not—and physically cannot—suck blood.(4)(16) Some argue that weasels are capable of sucking blood or eggs because they can draw milk from their mother when unweaned. However, this is called “suckling”, which is a completely different reflex from “sucking”. Following an act of surplus killing, seeing uneaten animals with teeth marks in their necks may have given rise to the unsubstantiated myth that their blood was “sucked out” by the weasel, when the more likely explanation is that they latched onto their necks to simply kill them.

Weasels may lap up blood if it trickles from a wound, or to get moisture and extra nutrients with their food in a practical sense, but there is no significant proof that they actually indulge in it. They are not hematophagous animals and do not consume blood in the exaggerated “bloodthirsty” context that we are sometimes given.

They are not the primary regulators of rodent control

Predator versus prey reproductive rates, density of prey, and food resources for prey in an area are factors to consider to know if weasels are capable of controlling populations of certain small pests.(17) Introducing weasels to new environments in the hope of controlling rodent or rabbit populations in the past have rarely been successful, and have at times catastrophically backfired in countries like New Zealand.(4) When not irresponsibly introduced by humans, weasels merely play their small part in keeping the ecosystem balanced.

Sometimes, it is not a weasel that killed the chicken

There are many animals that will kill chickens: dogs, foxes, skunks, and even owls. If chickens that are in a well-fortified coop are mysteriously killed with damage to their vent area (the anus), it is common to suspect only something small like a weasel could have gotten into the coop. While weasels are common predators of chickens, they usually go for the neck to kill their prey, not the vent. If the killed chickens have damage to their vent area but the neck and head were left untouched, than a vent pecking chicken within the flock might be the culprit.(18)


  1. Koepfli, Klaus-Peter, Kerry A. Deere, Graham J. Slater, Colleen Begg, Keith Begg, Lon Grassman, Mauro Lucherini, Geraldine Veron, and Robert K. Wayne. Multigene phylogeny of the Mustelidae: resolving relationships, tempo and biogeographic history of a mammalian adaptive radiation“. BMC biology 6.1 (2008): 10.
  2. Koepfli, Klaus-Peter, Jerry W. Dragoo, and Xiaoming Wang. The evolutionary history and molecular systematics of the Musteloidea. Biology and Conservation of Musteloids. Oxford University Press, Oxford (2017): 75-91. p. 80.
  3. Patterson, Bruce D., et al. On the nomenclature of the American clade of weasels (Carnivora: Mustelidae)“. Journal of Animal Diversity 3.2 (2021): 1-8.
  4. King, Carolyn M., and Roger A. Powell. The Natural History of Weasels and Stoats: Ecology, Behavior, and Management. Oxford University Press, 2006.
  5. Colella, Jocelyn P., et al. Extrinsically reinforced hybrid speciation within Holarctic ermine (Mustela spp.) produces an insular endemic. Diversity and Distributions 27.4 (2021): 747-762.
  6. Brown, James H., and Robert C. Lasiewski. “Metabolism of weasels: the cost of being long and thin.” Ecology 53.5 (1972): 939-943.
  7. Mills, L. Scott. “Conservation of wildlife populations: demography, genetics, and management (2nd ed.)”. Hoboken, NJ: Wiley-Blackwell. p. 148
  8. Alles über den Iltis. The Hankensbüttel Otter Centre. Accessed 18 April 2020.
  9. Does surplus killing represent a waste of energy for foxes?. Wildlife Online. Accessed 22 July, 2020.
  10. Kruuk, Hans. “Surplus killing by carnivores”. Journal of Zoology 166.2 (1972): 233-244.
  11. King, Carolyn M. The Advantages and Disadvantages of Small Size to Weasels, Mustela Species”. Carnivore behavior, ecology, and evolution. Springer, Boston, MA, 1989. 302-334.
  12. Shrank, Cathy, and Raphael Lyne. The complete poems of Shakespeare. Routledge, 2017. p. 140 (307).
  13. Chaplin, Stewart. The Stained-Glass Political Platform.. The Century Magazine (1900): 304-308.
  14. Harris, Steve. How to identify bird egg thieves. BBC Wildlife magazine. 30 April, 2020.
  15. Rising, Gerry. Weasels. March 15, 1999.
  16. Heidt, Gary A. “Anatomical and behavioral aspects of killing and feeding by the least weasel, Mustela nivalis L.”. Journal of the Arkansas Academy of Science 26.1 (1972): 53-54.
  17. Hanski, Ilkka, et al. Small‐rodent dynamics and predation“. Ecology 82.6 (2001): 1505-1520.
  18. Pötzsch, C. J., et al. “A cross-sectional study of the prevalence of vent pecking in laying hens in alternative systems and its associations with feather pecking, management and disease”. Applied Animal Behaviour Science 74.4 (2001): 259-272.

Old World Weasels

Weasel species of Africa, Asia, and Europe. This page includes a few species found in the Americas.

New World Weasels

Weasel species of the Americas that diverged from Old World weasels between 11.8 – 13.4 million years ago. This page includes the North American mink.


So-called “polecats” are weasel species with a heavier and bulkier build than most of the others. This page includes the ferret and European mink.

What Are Mustelids?

Badgers | Ferret-Badgers | Fisher | Grisons | Martens | Otters | Tayra | Weasels | Wolverine