The Mustelidae from Latin mustela (weasel), are a family of carnivorous mammals, including weasels, badgers, otters, ferrets, martens, and minks, among others. Mustelids are a diverse group and form the largest family in the order Carnivora, comprises about 56–60 species across eight subfamilies.

In Britain we have a small group of seven native species which represent just four of the sub-families.

Subfamily Mustelinae. Genus Mustela

Weasel (Mustela nivalis)
The weasel is our smallest carnivore. Weasels flanks are tawny brown back and the dividing line between that and the pale belly is patchy and irregular. They also have brown patches on each side of their throat.
The tail is entirely brown; very short and often hard to see. It does not go white in winter, in Britain and remains a rich, tawny brown all year. In Europe, some do turn white
May be found along hedgerows, in woods and on rough grassy banks. Its food mainly consists of voles and mice, some of which may be caught in their burrows.

Stoat or ermin (Mustela erminea)
A male stoat is noticeably larger than a female. The flanks are uniformly brown; forming a straight, contrasting line with pale belly. It has no brown patches on its throat and the most obvious feature is its long tail. The rearmost third always black.
Stoats inhabit open country nesting in drystone walls and old burrows. They hunt in daylight, sometimes stalking prey in the open and are capable of bringing down a rabbit, ten times its size.
In winter white forms are common in northern Britain, its fur is then called ermine. But the tail tip stays black. This does not normally happen in the south.

European polecat (Mustela putorius)
The polecat has a shorter, more compact body; a more powerfully built skull and dentition; is less agile; and it is well known for having the characteristic ability to secrete a particularly foul-smelling liquid to mark its territory.
The tail is short, about a third of its body length. The eyes are small, with dark brown irises. The hind toes are long and partially webbed, with nonretractable claws. The front claws are strongly curved, partially retractable and more robust than in other members of the genus.
The European polecat feeds on small rodents, birds, amphibians and reptiles,
It had become extinct in most of England and survived only in Wales, where the population has spread over the border. The species was reintroduced into the Cumberland and Westmorland, Argyll and on Speyside during the 1970s and 1980s. A survey in 2015 found that the polecat had spread into areas such as East Anglia and South Yorkshire, where they had not been seen for 100 years.
The European polecat is the sole ancestor of the ferret, which was domesticated more than 2000 years ago for the purpose of hunting vermin.


Subfamily Melinae. Genus Arctonyx

Eurasian badger, (Meles meles)
The Eurasian badger is the largest and probably best loved member of this family. It is a powerfully built black, white, brown and grey animal with a small head, a stocky body, small black eyes and short tail. It is nocturnal and is a social, burrowing animal that sleeps during the day in one of several setts in its territorial range. These burrows have multiple chambers and entrances and are extensive systems of underground passages. If undisturbed several badger families that use these setts for decades.


Subfamily Lutrinae. Genus Lutra

Eurasian otter (Lutra lutra)
The otter species are all semiaquatic, aquatic or marine, with diets based on fish and invertebrates.
Otters have long, slim bodies and relatively short limbs. Their most striking anatomical features are the powerful webbed feet used to swim, and their seal-like abilities holding breath underwater.
Otters are active hunters, chasing prey in the water or searching the beds of rivers, lakes or the seas. Most species live beside water, but river otters usually enter it only to hunt or travel, otherwise spending much of their time on land.
In the British Isles, they were common as recently as the 1950s, but became rare in many areas due to the use of chlorinated hydrocarbon pesticides, habitat loss and water pollution (they remained relatively common in parts of Scotland and Ireland). Population levels reached a low point in the 1980s, but are now recovering strongly.


Subfamily Guloninae. Genus Martes

European pine marten (Martes martes)
European pine marten’s fur is usually light to dark brown and grows longer and silkier during the winter, while being short and coarse in the summer. It has a cream- to yellow-coloured “bib” marking on its throat. Martens are the only mustelids with semiretractable claws, which enables them to lead a more arboreal lifestyle. They are mainly active at night and dusk. They have small, rounded, highly sensitive ears and sharp teeth adapted for eating small mammals, birds, insects, frogs, and carrion.

In Great Britain, the species was only found in northwestern Scotland but they have now spread from their Highlands stronghold. In England, pine martens are extremely rare, and were long considered probably extinct. The first confirmed sighting of a pine marten in England for over a century was recorded in woodland in Shropshire, in 2015. A camera trap in the North York Moors in Yorkshire recorded one in 2017, and in 2018 the first ever footage of a pine marten in Northumberland was captured. A small population of pine martens also exists in Wales, in Cwm Rheidol forest.


Family Mustelidae. Genus Neovison

American mink (Neovison vison)
American mink first arrived in Britain in 1929, but only in commercial fur farms. They were first reported to be breeding wild in the UK in 1956, as a result of escapees and deliberate releases. Today it is virtually impossible to estimate the number of mink living in our waterways.
Feral mink are naturally a chocolate-brown colour but farm-bred animals can vary in colour from white or grey through to black. Limbs are short and tails are about one third of their body length

Unless some areas are kept free or relatively free of mink, it is considered that the water vole will become extinct in much of Britain within a few years.