The Mustelidae (from Latin mustela, weasel) are a family of carnivorous mammals, including weasels, badgers, otters, martens, mink, and wolverines, among others. Mustelids are diverse and the largest family in the order Carnivora. The internal classification is still disputed, with rival proposals containing between two and eight subfamilies. One study, published in 2008, questions the long-accepted Mustelinae subfamily, and suggests that the Mustelidae consist of four major clades and three much smaller lineages.
Mustelids vary greatly in size and behaviour. The least weasel can be under a foot in length, while the giant otter can measure up to 1.7 m (5 ft 7 in) and sea otters can exceed 45 kg (99 lb) in weight. The wolverine can crush bones as thick as the femur of a moose to get at the marrow, and has been seen attempting to drive bears away from their kills. The sea otter uses rocks to break open shellfish to eat. The marten is largely arboreal, while the badger digs extensive networks of tunnels, called setts. Some mustelids have been domesticated: the ferret and the tayra are kept as pets (although the tayra requires a Dangerous Wild Animals licence in the UK), or as working animals for hunting or vermin control. Others have been important in the fur trade—the mink is often raised for its fur.
As well as being one of the most species-rich families in the order Carnivora, the family Mustelidae is one of the oldest. Mustelid-like forms first appeared about 40 million years ago, roughly coinciding with the appearance of rodents. The direct ancestors of the modern mustelids first appeared about 15 million years ago.
Within a large range of variation, the mustelids exhibit some common characteristics. They are typically small animals with short legs, short, round ears, and thick fur. Most mustelids are solitary, nocturnal animals, and are active year-round.
Most mustelid reproduction involves embryonic diapause. The embryo does not immediately implant in the uterus, but remains dormant for some time. No development takes place as long as the embryo remains unattached to the uterine lining. As a result, the normal gestation period is extended, sometimes up to a year. This allows the young to be born under more favorable environmental conditions. Reproduction has a large energy cost and it is to a female's benefit to have available food and mild weather. The young are more likely to survive if birth occurs after previous offspring have been weaned.
Mustelids are predominantly carnivorous, although some eat vegetable matter at times. While not all mustelids share an identical dentition, they all possess teeth adapted for eating flesh, including the presence of shearing carnassials. With variation between species, the most common dental formula is 22.214.171.124.
Several members of the family are aquatic to varying degrees, ranging from the semiaquatic mink, to the river otters, and to the highly aquatic sea otter. The sea otter is one of the few nonprimate mammals known to use a tool while foraging. It uses "anvil" stones to crack open the shellfish that form a significant part of its diet. It is a "keystone species", keeping its prey populations in balance so some do not outcompete the others and destroy the kelp in which they live.
The black-footed ferret is entirely dependent on another keystone species, the prairie dog. A family of four ferrets eats 250 prairie dogs in a year; this requires a stable population of prairie dogs from an area of some 500 acres (2.0 km2).
The skunks were formerly included as a subfamily of the mustelids, but are now regarded as a separate family (the Mephitidae). The mongoose and the meerkat bear a striking resemblance to many mustelids, but belong to a distinctly different suborder—the Feliformia (all those carnivores sharing more recent origins with the cats) and not the Caniformia (those sharing more recent origins with the dogs). Because the mongooses and the mustelids occupy similar ecological niches, convergent evolution has led to some similarity in form and behavior.
Several mustelids, including the mink, the sable (a type of marten) and the stoat (ermine), boast exquisite and valuable furs, and have been accordingly hunted since prehistoric times. Since the early Middle Ages, the trade in furs was of great economic importance for northern and eastern European nations with large native populations of fur-bearing mustelids, and was a major economic impetus behind Russian expansion into Siberia and French and English expansion in North America. In recent centuries, fur farming, notably of mink, has also become widespread and provides the majority of the fur brought to market.
One species, the sea mink (Neovison macrodon) of New England and Canada, was driven to extinction by fur trappers. Its appearance and habits are almost unknown today because no complete specimens can be found and no systematic contemporary studies were conducted.
The sea otter, which has the densest fur of any animal, narrowly escaped the fate of the sea mink. The discovery of large populations in the North Pacific was the major economic driving force behind Russian expansion into Kamchatka, the Aleutian Islands, and Alaska, as well as a cause for conflict with Japan and foreign hunters in the Kuril Islands. Together with widespread hunting in California and British Columbia, the species was brought to the brink of extinction until an international moratorium came into effect in 1911.
Today, some mustelids are threatened for other reasons. Sea otters are vulnerable to oil spills and the indirect effects of overfishing; the black-footed ferret, a relative of the European polecat, suffers from the loss of American prairie; and wolverine populations are slowly declining because of habitat destruction and persecution. The rare European mink Mustela lutreola is one of the most endangered mustelid species.
One mustelid, the ferret, has been domesticated and is a fairly common pet.
Traditionally the Mustelidae have been divided into two subfamilies: the Lutrinae, comprising the otters, and the Mustelinae, comprising the weasels, martens, badgers and other mustelids. Accordingly, the 57 living species of mustelid have been traditionally classified as follows:
(two subfamilies and 59 living species in 22 genera)
Extinct genera of the family Mustelidae include:
The traditional classification of the Mustelidae family has recently been questioned. A multigene phylogeny constructed by Koepfli et al (2008)  found that the Mustelidae comprise four major clades and three monotypic lineages. This scheme makes the traditional Mustelinae paraphyletic. The revised Mustelinae is a sister clade to the Lutrinae and together these two subfamilies make up the largest clade. The early mustelids appear to have undergone two rapid bursts of diversification in Eurasia, with the resulting species only spreading to other continents later.
Examination of the mitochondrial DNA suggests that the Taxidiinae diverged first, followed by the Melinae. The position of the Helictidinae is unclear because the mitochondrial evidence suggests the subfamily is related to the Lutrinae-Mustelinae clade, while the intron data suggest a relationship to the Martinae.
The following cladogram illustrates the eight subfamily scheme of Koepfli et al (2008). The genetic studies on which the scheme was based did not include the genus Lyncodon, which is therefore unplaced, but it is probably allied with Mustela and Neovison.