Page last updated: 05/11/2023

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. This is especially the case with the least weasel (M. nivalis), where various subspecies are sometimes mistaken for separate species, perhaps on account of M. nivalis sometimes being erroneously assumed as the smallest weasel by default.

Even though the family Mustelidae is often simply referred to as the “weasel family”, this ambiguous and broad phrase can be misleading, especially given our better understanding of these animals today. For example, many distinctive mustelids such as badgers, otters, and martens are related to weasels, or in some respects weasel-like, but are not true weasels. Scientifically speaking, it is the mustelids belonging to the genera Mustela and Neogale (a.k.a. subfamily: Mustelinae) that are considered true weasels, which includes polecats, stoats, minks, and the ferret.(1)(2)(3)(4) All in all the term weasel has developed such a vague and sometimes arbitrary definition that it is not always possible to know exactly what species is being referred to.

Weasels tend to be 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, some pursue 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.

Appearance and white winter coat

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).(5)(6)

In the English language “ermine” is usually a term used to refer to any of the three species of stoat when in their white winter coat or pelt thereof. Other weasel species that change white are usually not called ermine but experience the same process. Turning ermine is a complex matter that is triggered by several factors, mainly day length and genetics. However, there is much that is still not yet researched or understood about the process, because there can be a lot of variation from location to location, individual to individual, and from year to year.

For instance, most Eurasian stoats in England stay brown all year round, and the few that turn ermine tend to take on a patched, piebald appearance, as if not completing the moult. In Scandinavia, Eurasian stoats start moulting to full ermine in November and keep it for about 4 months or slightly longer. The spring moult starts around March-April. The time it takes for the moult to complete is usually a couple of weeks. Eurasian stoats elsewhere probably follow a similar time cycle, depending on the several trigger factors.

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)(5)

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, the current scientific consensus is that there are approximately 20 distinct species and 150 subspecies of weasels in the world—many of which have their own unique features, size, range, abilities, and vulnerabilities in relation to their respective environments.

Common myths and misconceptions

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”.

It is unlikely that they simply kill for “fun” or out of “spite”

One of the most unsubstantial beliefs about weasels is that they just kill for “fun” or out of “spite”. This belief usually stems from people witnessing or hearing about weasels surplus killing (also known as henhouse syndrome), a behavioural phenomenon where a weasel kills many more prey than it could possibly eat in a single sitting. However, unless we are weasels, how can we possibly know this to make such a bold and oversimplified claim? We gain little understanding of wildlife when we attempt to rationalise their psychology by simply attributing our own dietary needs, intentions, or morals to them. We are not saying they do not enjoy chasing their prey or the taste of their food, but there are several reasons why we would argue it is unlikely they kill for pure malice or out of spite.

Stumbling upon a lot of prey crowded together in the artificial and confined conditions of a chicken coop, for example, is not a situation a weasel would typically experience in the wild. What may seem illogical to us may be a reaction that their instincts have not evolved a “logical” strategy for. The exact reason for why they surplus kill may still be inconclusive, but it is also partly presumed to be an evolutionary response to unpredictable food resources in many carnivorous mammals.(7)(8) This is somewhat similar to how we humans habitually hoard more food than we can eat during times of crisis. If we want to anthropomorphise the intentions of weasels, do we humans hoard during such times just because we find it fun, or is it primarily an instinctual drive for survival?

In their natural environment, weasels kill multiple prey and usually cache the remainder for later consumption.(9) We tend to take more notice when for whatever reason this does not happen. Sometimes many killed chickens in a coop are left behind rather than cached, and this is probably because the weasel was either scared off, or the hole the weasel came though may not have been large enough for the chickens to be taken out. If the hole was large enough, we would argue the coop was poorly constructed or maintained, in which case a number of different predators could have gotten in.

As previously mentioned, while their exact reasons may differ, surplus killing is a behavioural phenomenon found in practically any carnivore under the same or similar circumstances. Domestic cats are known to kill many birds and dogs flocks of sheep,(10)(11)(12)(13) yet many people will empathise with or find excuses to look the other way when it comes to these animals. This in spite of the fact that, unlike wild weasels, many of these pets do not need to hunt because they are well fed by their owners. Like weasels, their actions are likely driven by instincts.

Weasels are no more aggressive than most other carnivores. They likely get their “bloodthirsty” reputation because 1) they are better able to fit through holes in poorly-built coops compared to larger surplus killing-prone predators, and 2) they have incredibly active metabolisms and little storage of body fat, thus need to hunt more frequently compared to carnivores with lower metabolic rates.(14)(15)(16)

They do not suck eggs

Despite the claim in William Shakespeare’s Henry V.,(17) 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”.(18)

The truth is weasels (more specifically stoats) will 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.(19) 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.(20) The fact that weasels also do not have protruding lips for sucking on a smooth surface like an egg, would be another, more straightforward argument against the claim.

They do not suck or “drink” blood

Vampiric folklore has also made its way into mustelid myths, since there is a misconception that weasels will suck the blood of their prey. As with eggs, weasels do not—and physically cannot—suck blood.(5)(21)(22) Some argue that weasels are capable of sucking blood 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.(23) 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.(5) When not irresponsibly introduced by humans, weasels merely play their small part in managing these animals.

Sometimes, it is not a weasel that killed the chicken

There are many animals that will kill chickens: dogs, foxes, striped skunks, raccoons, and even some birds of prey. If chickens that are in a secured coop are mysteriously killed with damage to their vent area (the cloaca), 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.(24)

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.


  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. Merriam-Webster.com. Dictionary, Merriam-Webster. Weasel. Accessed 22 September, 2022.
  5. King, Carolyn M., and Roger A. Powell. The Natural History of Weasels and Stoats: Ecology, Behavior, and Management. Oxford University Press, 2006.
  6. 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.
  7. Does surplus killing represent a waste of energy for foxes?. Wildlife Online. Accessed 22 July, 2020.
  8. Kruuk, Hans. “Surplus killing by carnivores”. Journal of Zoology 166.2 (1972): 233-244.
  9. Mills, L. Scott. Conservation of wildlife populations: demography, genetics, and management (2nd ed.). Hoboken, NJ: Wiley-Blackwell. p. 168.
  10. Loss, Scott R., Tom Will, and Peter P. Marra. The impact of free-ranging domestic cats on wildlife of the United States. Nature communications 4.1 (2013): 1-8.
  11. Crowley, Sarah L., Martina Cecchetti, and Robbie A. McDonald. Hunting behaviour in domestic cats: An exploratory study of risk and responsibility among cat owners. People and Nature 1.1 (2019): 18-30.
  12. Irons, Athwenna. DevonLive. 28 March, 2023. UK dog attacks on sheep ‘spiralling out of control‘”. Accessed 06 April, 2023.
  13. Gareth, Lewis and Bethan, James. BBC. 27 November, 2021. Farming: Dog attacks on sheep could lead to bigger fines. Accessed 06 April, 2023.
  14. Brown, James H., and Robert C. Lasiewski. “Metabolism of weasels: the cost of being long and thin.” Ecology 53.5 (1972): 939-943.
  15. 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.
  16. Turbak, Gary. 01 February, 1992. Living Hungry. The National Wildlife Federation. Accessed 05 January, 2023.
  17. Shrank, Cathy, and Raphael Lyne. The complete poems of Shakespeare. Routledge, 2017. p. 140 (307).
  18. Chaplin, Stewart. The Stained-Glass Political Platform.. The Century Magazine (1900): 304-308.
  19. Harris, Steve. How to identify bird egg thieves. BBC Wildlife magazine. 30 April, 2020.
  20. Rising, Gerry. Weasels. 15 March, 1999.
  21. 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.
  22. Linn, Ian. “Weasels”. Sunday Times Publications, 1962.
  23. Hanski, Ilkka, et al. Small‐rodent dynamics and predation“. Ecology 82.6 (2001): 1505-1520.
  24. 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 stoats and 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 European mink.


A ferret is a type of “polecat” and the only domesticated mustelid not commercially bred for their fur. They are the domesticated form of the European polecat.

What Are Mustelids?

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