Mustelids are an extremely diverse group of carnivorous mammals belonging to the family Mustelidae (within the order Carnivora), which includes weasels, badgers, otters, martens, polecats, wolverines, and grisons, among others.
Characterized by their slender bodies, small heads, thick necks, rounded ears, and short legs, most mustelids also have sharp claws adapted for hunting or digging. A distinctive feature of many species is their anal scent glands, which release a strong-smelling secretion used for communication among individuals.
There are currently 66 recognized living species of mustelids, making it the largest family in the suborder Caniformia within the order Carnivora. They are found on every continent except Antarctica and Australia, thriving in a range of habitats, from terrestrial to aquatic environments. Primarily carnivorous, they feed on small mammals, fish, and aquatic invertebrates such as crustaceans, though some species occasionally consume plant matter, including fruits and vegetables.
They are fairly small, though the largest species, the sea otter (Enhydra lutris), is around 3.3 ft (1 m) long and weighs about 55 to 99 lb (25 to 45 kg). In contrast, the smallest species, the least weasel (Mustela nivalis), is barely 4 to 10 in (11 to 26 cm) long and weighs only 0.9 oz (25 gm).
Mustelids exhibit sexual dimorphism in size, with the males being larger than females.
Most mustelids have slender, elongated bodies with small heads, thick necks, short legs, and rounded ears. They have five toes on each foot and move either in a plantigrade (on the sole) or digitigrade (on the digits) gait.
Their bodies are covered by a dense fur that helps camouflage against the background. This fur often undergoes seasonal changes in color and texture to help mustelids adapt to their altering surroundings.
Except for sea otters, all mustelids bear small scent glands near the anus (anal sacs), which produce a pungent secretion helpful in marking their territories and during sexual signaling. Males have a bifurcated penis and a baculum (penile bone).
Although dentition varies among different mustelids, the teeth of all species are specially adapted for a carnivorous or flesh-eating lifestyle. They possess a characteristic upper-back molar that rotates 90° toward the inside of the mouth while feeding. Additionally, these animals are equipped with pointed upper and lower carnassial teeth, which help shear meat easily.
The general dental formula for this group is 3.1.2-4.1/3.1.2-4.1-2 = 28-38.
Currently, the 66 living species of mustelids are classified under 8 subfamilies and 22 genera.
Skunks were once classified as a subfamily within Mustelidae, but DNA studies have shown that they are genetically distinct. As a result, they have been reclassified into their own family, Mephitidae. Similarly, while mongooses share a superficial resemblance to mustelids, they belong to a separate family, Herpestidae.
Fossil evidence of mustelids spans from the Oligocene to the early Miocene Epoch.
Mustelids range from the Arctic to the tropics and are found on all continents except Antarctica and Australia. They are also not found in Madagascar or other oceanic islands but have been introduced to New Zealand by humans.
As a large group, they practically occupy every terrestrial habitat, as well as aquatic and semi-aquatic environments (both marine and freshwater). Weasels, badgers, and wolverines are primarily land-dwelling and live in forests, grasslands, wetlands, and meadows. While the fisher, tayra, and martens are partially arboreal (tree-dwelling), all badgers are fossorial (burrow-dwelling). In contrast, sea otters are fully aquatic, while river otters and minks are semi-aquatic.
The only domesticated mustelid is the ferret, which is commonly kept as a pet.
These mammals are primarily carnivorous and feed on a wide variety of prey, including small mammals (like rodents), fish, and aquatic invertebrates, like crustaceans. However, they also consume plant matter, like vegetables and fruits, when necessary.
Depending on the species, mustelids are both diurnal and nocturnal, though most are active at night. Their movement also depends on body shape. Species with narrow bodies are agile and quick, darting through their environment, while those with broader bodies lumber around their surroundings.
As a broad group, their lifespans range between 5 to 20 years in the wild.
Most mustelids are seasonal breeders, with the reproductive period lasting for about 3 to 4 months (depending on the day length). Both polygynous (in minks and pine martens) and promiscuous (in sea otters) mating systems are common in this group.
These mammals require prolonged periods (several hours) of copulation to induce ovulation in females (inductive ovulation). After fertilization, the embryo takes a considerably long time (up to 10 months in the European badger) to implant in the uterus, a phenomenon called delayed implantation or embryonic diapause. Following a gestation period of about 30 to 65 days, the females give birth to a single litter every season. While sables have an average litter size of 2.2, mountain weasels have that of 8.7.
At birth, the young are altricial, meaning they are small, blind, and entirely dependent on their mothers for nursing. They begin to care for themselves around 2 months of age and are weaned shortly after. Sexual maturity is reached between 8 months and 2 years, depending on the species.
They are preyed upon by canids, like dogs, wolves, jackals, and foxes. Some large snakes and birds, like raptors and owls, also kill them.
Around 38% of all mustelid species are considered threatened, owing to several factors, like habitat loss and fragmentation, as well as hunting. Since the Early Middle Ages, mustelids have been hunted for their fur, particularly in northern and eastern Europe. Today, mink farming remains a major part of the commercial fur industry.
Currently, four species of otters, the giant, sea, marine, and Southern river otters, are listed as Endangered (EN) in the IUCN Red List. The European mink is Critically Endangered (CR), while the Colombian weasel is Vulnerable (VU).
The sea mink and Japanese otter are now extinct due to excessive hunting for their fur.
Sea otters possess the densest fur of any animal, with an astonishing 600,000 to 1,000,000 hair follicles per square inch of its body.
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