Mustelidae

Mating systems vary both within and among species. Many species are polygynous and/or promiscuous. Some species are social, while others are solitary. Social organization can vary within species as well. Mustelids require prolonged periods of copulation to induce ovulation of an unfertilzed egg. As a result, copulation may last for several hours before fertilization can be successful.

Mating System: polygynous ; polygynandrous (promiscuous)

Most mustelids breed seasonally, but the length of the reproductive period varies among species. Day length often dictates the onset of the breeding season, which usually lasts 3 to 4 months. Many mustelids undergo delayed implantation, with the fertilized embryo taking up to 10 months (e.g. Meles meles) to implant in the uterus in some species. Environmental conditions such as temperature and day length determine when implantation occurs. Mustelids that live in more seasonal climates are more likely to exhibit delayed implantation. Following implantation, gestation typically lasts 30 to 65 days. Females give birth to a single litter each season, the size of which varies within and among species. For example, sables have an average litter size of 2.2, but can give birth to anywhere from 1 to 7 pups. The mountain weasel averages 8.7 pups per litter, but can have between 3 and 14 young in a single bout of reproduction. Generally, mustelids are altricial, being born small and blind. They reach sexual maturity between 8 months and two years following birth.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous ; delayed implantation

Young are generally born in an alricial state, requiring extenisive care and protection from their mother. Young mustelids typically are able to care for themselves when they are about two months old. Females defend territories in order to acquire enough resources to care for their young and most often nurse and protect them in a burrow or den.

Parental Investment: altricial ; precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)

Taxonomy and Systematics

Mustelidae (weasels and relatives) is the largest family in the order Carnivora. Mustelids are found on every continent except Antarctica and Australia (although Ermine [Mustela erminea] and Least weasels [Mustela nivalis] were introduced by humans to New Zealand). Larivère and Jennings (2009) recognized 57 mustelid species in 22 genera, but noted that future research could result in this number changing.

There has been considerable uncertainty about the position of the mustelids in the carnivoran tree, as well as about phylogenetic relationships within the Mustelidae. With regard to the first question, although the skunks and stink badgers (family Mephitidae) were long considered a subfamily within the Mustelidae, several independent molecular phylogenetic studies have now established that Mephitidae represents a distinct lineage that is basal to a clade that includes the Ailuridae (Red Panda) basally and the Procyonidae (raccoons and relatives) and Mustelidae as sister taxa (Eizirik et al. 2010; Nyakatura and Bininda-Emonds 2012; Sato et al. 2012). Relationships within the Mustelidae are less well resolved, although there have been quite a few molecular genetic studies attempting to infer these relationships. Some results appear to be quite consistent among these studies, such as the position of the American Badger lineage, subfamily Taxidiinae (which includes just one extant species, Taxidea taxus), as the most basal mustelid lineage, and the relationship of the subfamilies Lutrinae (otters) and Mustelinae (weasels) as sister taxa (for review, see Yu et al. 2011 and references therein). Although the precise relationships among them are not entirely resolved, eight mustelid subfamilies are generally recognized (e.g., Koepfli et al. 2008; Larivère and Jennings 2009; Yu et al. 2011), with the following list of included genera and number of species from Larivère and Jennings (2009):

   Taxidiinae:

            American Badger (1 species, Taxidea taxus, found from southern Canada and the midwestern United States south [west of the Mississippi River] to central Mexico). In contrast to the group-living European Badgers, American Badgers are solitary.

   Mellivorinae:

            Honey Badger (1 species, Mellivora capensis, found across much of Africa, with the exception of most of North Africa, and in southwestern Asia).

   Melinae:

            Hog Badger (1 species, Arctonyx collaris, found in Southeast Asia). Helgen et al. (2008) recognized two additional Arctonyx species.

            Eurasian badgers (Meles, 3 species, found in Europe and north-central and southeastern Asia). A 4^thMeles species, formerly treated as a southwest Asian subspecies of the European Badger, was recognized by Del Cerro et al. (2010). All mustelids are solitary except for Eurasian badgers and some otter species.

   Martinae:

            Tayra (1 species, Eira barbara, found from Mexico south through Central America and much of South America north of Patagonia).

            Wolverine (1 species, Gulo gulo, found across northern North America and northern Eurasia).

            Martens and Fisher (Martes, 8 species; various species are found in North America, Europe, and Asia; in the 1950s, the Stone Marten [M. foina], of central and southern Europe and Asia, was accidentally introduced to Wisconsin in the United States, where it is now established at multiple localities). Larivère and Jennings (2009) noted that, in the context of recent data indicating that the genus Martes as currently formulated is paraphyletic unless the Fisher (M. pennanti) is excluded (Koepfli et al. 2008 and references therein), it has been proposed that the Fisher be placed in its own genus, Pekania.

   Helictidinae:

            Ferret-badgers (Melogale, 4 species, found in Southeast Asia).

   Galictidinae:

            Grisons (Galictis, 2 species, found in Central and South America). Bornholdt et al. (2013) confirmed and clarified the species delineations and geographic distributions within this genus.

            Saharan Striped Polecat and Zorilla (Ictonyx, 2 species, found in Africa).

            Marbled Polecat (Vormela peregusna, found in southeastern Europe and Central Asia).

            African Striped Weasel (Poecilogale albinucha, found in southern Africa). Larivère and Jennings (2009) noted that, given recent data indicating that the African Striped Weasel is the sister taxon to the Zorilla (Ictonyx striatus) (Koepfli et al. 2008), it should probably be placed in the genus Ictonyx as well.

   Lutrinae:

            Giant Otter (Pteroneura brasiliensis, found in northern South America).

            New World Otters (Lontra, 4 species, found in North, Central, and South America).

            Sea Otter (Enhydra lutris, found in the coastal North Pacific Ocean).

            Spotted-necked Otter (Hydrictis maculicollis, found across much of sub-Saharan Africa).

            Old World Otters (Lutra, 2 species, found across much of Europe and Asia). Larivère and Jennings (2009) noted that the Japanese Otter is sometimes recognized as a distinct species, L. nippon, although they treated it as a subspecies of the Eurasian Otter (L. lutra).

            African Clawless Otter and Asian Small-clawed Otter (Aonyx, 2 species, found across much of sub-Saharan Africa and Southeast Asia). Larivère and Jennings (2009) note that a subspecies of the African Clawless Otter (A. capensis) is sometimes recognized as a distinct species, the Congo Clawless Otter (A. congicus). Aonyx otters feed on crabs and shellfish.

            Smooth-coated Otter (Lutrogale perspicillata, from Southeast Asia).

   Mustelinae:

            Weasels, European Mink, and polecats (Mustela, 17 species, various species found across much of North America, Europe, and Asia; in Africa, the European Polecat [Mustela putorius] is present in Morocco and the Egyptian Weasel [Mustela subpalmata] is found in Egypt; Ermine [M. erminea] have been introduced to New Zealand and Least Weasels [M. nivalis] have been introduced to New Zealand, Malta, Crete, the Azores, and apparently São Tomé Island). The Domestic Ferret is believed to be descended from the European Polecat and is often referred to as M. putorius furo, but Steppe Polecats (M. eversamanii) may also be among the ancestors of the Domestic Ferret.

            American Mink (Neovison vison, found across most of North America north of Mexico and introduced in China and Japan and widely in Europe). Based on new molecular phylogenetic studies, Harding and Smith (2009) concluded that the American Mink is closely related to several Mustela species and that together with the American Mink these should be segregated out from Mustela in a genus Vison. Although the American Mink and European Mink are morphologically and ecologically similar, molecular phylogenetic analyses indicate that they are not actually closely related within the subfamily Mustelinae.

            Patagonian Weasel (Lyncodon patagonicus, found in western Argentina and central and southern Chile).

General Ecology

Morphology: Most mustelids are small to medium-sized carnivores with long bodies and short limbs (some, such as the Wolverine and badgers, have a stockier body shape). The Least Weasel, which is not only the smallest weasel but also the world's smallest carnivore, may weigh in at as little as 25 g when fully grown. Sexual size dimorphism in mustelids can be striking, with males often twice the size of females. The largest mustelid is the Sea Otter, which can reach 45 kg, and on land, the Wolverine, which can reach 18 kg. Mustelids have five toes on each foot and often strong claws. Their ears can be pointed forward or swiveled to the side, but not folded back.

The Ermine, Least Weasel, and Long-tailed Weasel (Mustela frenata) completely change their coats between summer and winter, with a dramatic change from brown to white in the northern parts of the range and at high elevations (in other areas, the summer coat is dark). Interestingly, Ermine and Long-tailed Weasels retain the black tail tip in the winter, which experiments have suggested may focus the attention of potential predators away from the body as a weasel makes its escape. Japanese Weasels (Mustela itatsi) replace their dark brown summer coat with a much paler yellowish one in the winter.

Habitat: Mustelids can be found in a great diversity of habitats, including marine (Marine Otter [Lontra felina] and Sea Otter), rivers (North American River Otter [Lontra canadensis] and Spotted-necked Otter), temperate forest (European Pine Marten [Martes martes] and American Marten [Martes americana]), dry open woodlands (African Striped Weasel), and grasslands (Black-footed Ferret [Mustela nigripes]). Some mustelids have very broad habitat preferences. For example, Ermine and Least Weasels can be found from grasslands to woodlands and Honey Badgers from forests to deserts.

Although most mustelids are terrestrial, their lifestyles are highly variable, ranging from fossorial (badgers--an American Badger can dig itself out of sight in just a few minutes), semi-fossorial (weasels and polecats), or semi-arboreal (martens) to semi-aquatic  (mink and otters) or aquatic (Sea Otter and Marine Otter). River otters must actively swim to stay on the surface, but Sea Otters are so thoroughly aquatic they can drink seawater and can float passively on the surface thanks to their dense fur, which traps air and provides extra buoyancy.

Diet: Many mustelids have diverse diets, but animal prey is generally a major component of the diet—and mustelids must eat a lot to maintain their basal metabolism. If they are able to kill more than they need immediately, they may stash the surplus for later use in a rock crevice or burrow. In addition to killing their own food, some mustelids, such as Wolverines and Fishers, may also feed on carcasses of animals killed by other predators, such as wolves. Across much of their range, European Badgers rely heavily on earthworms, which can make up nearly two thirds of their diet, but at certain times of year and in certain places, fruit can make up the bulk of their diet—and they also feed on small vertebrates, acorns, roots, tubers, grass, and other foods.

During the 18^th and 19th centuries Sea Otters were heavily hunted for their fur. Because Sea Otters prey on sea urchins, which in turn feed on algae, such as Giant Kelp, eliminating Sea Otters led to local increases in sea urchin populations and this in turn resulted in the loss of kelp beds as they were consumed by sea urchins. Thus, Sea Otters are a classic example of a "keystone species", i.e., a species whose impact on an ecological community is disproportionately large given its numbers and/or biomass.

Reproduction: Many mustelid species appear to be induced ovulators, with females shedding eggs from their ovaries only when stimulated by copulation. Some authors have speculated that the large baculum in the penis of the males of many mustelid species may serve to ensure sufficient stimulation of the cervix, triggering the release of a sequence of hormones that results in ovulation. Least weasels, the smallest mustelids, reach sexual maturity at three months; larger species such as Sea Otters, Giant Otters, and Wolverines reach sexual maturity at two years. Mustelid lifespans range from one to two years for the small weasels to up to 20 years for the largest mustelids.

Conservation Status

A close relative of the American Mink, the Sea Mink (Neovison macrodon), reportedly lived among rocks along ocean shores in the northeastern United States and southeastern Canada, feeding mainly on fish, but little is known about this species since it was hunted heavily for its fur and was apparently extinct by the late 1800s, leaving no complete specimens behind. This is the only mustelid known to have been driven to extinction by humans, but a number of others are in trouble.

At the time of the review of the family Mustelidae by Larivère and Jennings (2009), the IUCN listed seven mustelid species as Endangered (Giant Otter, Marine Otter, Southern River Otter [Lontra provocax], Sea Otter, Hairy-nosed Otter [Lutra sumatrana], European Mink, and Black-footed Ferret); five as Vulnerable (Nilgiri Marten [Martes gwatkinsii], Marbled Polecat, Asian Small-clawed Otter [Aonyx cinereus], Smooth-coated Otter, and Colombian Weasel [Mustela felipei]); and four as Near Threatened (Hog Badger, Wolverine, Eurasian Otter, and Mountain Weasel [Mustela altaica]). Of the remaining species, 35 were listed as Least Concern and six as Data Deficient. In fact, so little is known about most mustelid species that reliable status assessments are difficult. The Colombian Weasel (Mustela felipei) was described only in 1978 (Izor and de la Torre 1978) and may be the rarest carnivore in South America (Ramírez-Chaves et al. 2012).

The Black-footed Ferret is a mustelid that came extremely close to extinction. Black-footed Ferrets were once common on the North American Great Plains, where they fed almost exclusively on prairie dogs. Unfortunately, the widespread destruction of prairie dog colonies led to a dramatic Black-footed Ferret decline to the point that the species was believed to be extinct, although unconfirmed sightings persisted. A tiny remnant population was rediscovered in Wyoming in 1981 when a dead individual was brought back by a dog. All wild individuals were brought into captivity for breeding and the species was listed as Extinct in the Wild for many years. Fortunately, intensive captive breeding and reintroduction efforts over many years have met with some limited success. Although the status of the species remains very precarious, the reintroductions have been sufficiently successful that in 2008 the IUCN changed the status of the species from Extinct in the Wild to Endangered.

Intensive trapping has seriously impacted populations of some mustelid species, such as the Fisher and American Marten, whose populations were severely reduced in many parts of their range in North America by the early 20th century. Fur trapping brought the Sea Otter to the brink of extinction by the early 20th century, but conservation efforts resulted in significant population rebounds in the latter part of the century.

Some other mustelids, such as the Sable (Martes zibellina) and American Mink, are still highly valued for their fur commercially and African Striped Weasel skins are used in traditional ceremonies in Africa. Although American Mink are still trapped in the wild, many millions are also bred on farms in the United States, Europe, and, more recently, China. The escape of American Mink from fur farms has resulted in the establishment of wild populations in Europe and Asia, which are believed to have negative consequences for competitors, such as European Mink, as well as for native prey, such as Water Voles in Britain. In New Zealand, Ermine and Least Weasels were introduced deliberately in the 1880s in an effort to reduce populations of introduced European Rabbits. Ermine have thrived, not only feeding on rabbits but also devastating local fauna, especially flightless birds. Today, Ermine are present in virtually all forested areas in New Zealand, although Least weasels are rare, presumably due to the scarcity of the smaller prey on which they specialize (voles are absent and feral House Mice are the only rodents smaller than 50 g).

Mustelids are hunted for food in some parts of the world, although they are more often taken incidentally (antelopes, deer, pigs, and primates being the most sought after targets). In Guinea, for example, where there is significant sale and consumption of bushmeat, Honey Badger and Spotted-necked Otter are among the species sold. Some mustelids, such as otters, are captured for the illegal international pet trade or for body parts for use in traditional medicine in China and elsewhere.

As with most species, the greatest threat faced by mustelids is generally habitat degradation, fragmentation, and destruction, although hunting poses a major threat to some species, such as the Neotropical Otter (Lontra longicaudis), which is heavily hunted for its pelts, and other threats may be important for particular species.

(Larivère and Jennings 2009 and references therein)

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Some mustelid species are considered highly threatened by the IUCN, while other species are so abundant that they are considered pests. Approximately 38% of all species of Mustelidae are considered threatened,which is a much higher proportion than mammals in general (15%). Habitat destruction is a serious risk to species with restricted habitat requirements such as otters and martens. Smaller carnivores that are restricted to small habitat fragments may also be at risk to predation by larger carnivores that can more easily move among fragments. Hunting has been a problem for some species, while others, particularly tropical mustelids, do not seem to be declining as a result. Endangered mustelids include: Colombian weasels (Mustela felipei), European mink (Mustela lutreola), Indonesian mountain weasels (Mustela lutreolina), marine otters (Lontra felina), southern river otters (Lontra provocax), sea otters (Enhydra lutris), and giant Brazilian otters (Pteronura brasiliensis). Sea mink (Neovison macrodon) became extinct in recent times.

Numerous re-introduction programs for various mustelid species have met with mixed success. Generally, "soft" re-introductions, those that allow the animals to acclimate to their new surroundings while in a temporary enclosure, are more successful than "hard" re-introductions, in which captive-bred animals are released directly into the wild. Black-footed ferrets (Mustela nigripes) are considered extinct in the wild, although several re-introduction programs are underway.