Family Mustelidae
badgers, otters, weasels, and relatives

Mustelidae is the largest family within Carnivora and is comprised of 56 species in 22 genera. Members of this family include weasels, stoats, polecats, mink, marten, fishers, wolverines, otters, badgers and others. While many authors have traditionally considered skunks a subfamily within Mustelidae, recent molecular evidence indicates that skunks do not lie within the mustelid group and instead are recognized as a single family, Mephitidae, a systematic understanding which is accepted here (Dragoo and Honeycutt, 1997; Flynn et al., 2005; Marmi et. al., 2004; Sato et. al., 2003; Sato et. al., 2004). (Dragoo and Honeycutt, 1997; Flynn et al., 2005; Marmi, Lopez-Giraldez, and Domingo-Roura, 2004; Nowak, 1991; Sato et al., 2003; Sato et al., 2004; Vaughan, Ryan, and Czaplewski, 2000; Whitaker and Hamilton, 1998)

Mustelids inhabit all continents except Australia and Antarctica, and do not occur on Madagascar or oceanic islands. Members of this group can be found in diverse habitats, which include both terrestrial, aquatic and marine environments. Mustelids are mainly carnivorous, with various members of the family exploiting a great diversity of both vertebrate and invertebrate prey. Mustelids are generally proficient hunters; some weasels can take prey larger than themselves. Members of this family often hunt in burrows and crevices, and some species have evolved to become adept at climbing trees (e.g., marten) or swimming (e.g., sea otters, mink) in search of prey. (Nowak, 1991; Sato et al., 2003; Vaughan, Ryan, and Czaplewski, 2000; Whitaker and Hamilton, 1998)

Generally, mustelids have elongate bodies with short legs and a short rostrum, as typified by weasels, ferrets, mink, and otters. Wolverines and badgers have broader bodies. An order of magnitude difference in size exists between the smallest and largest mustelid species. The smallest species is the least weasel (Mustela nivalis), weighing between 35 and 250 grams. Wolverines (Gulo gulo) and sea otters (Enhydra lutris) reach 32 kg and 45 kg, respectively. All mustelids have well developed anal scent glands, which serve various functions, including territorial marking and defense. (Hutchings and White, 2000; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Members of the family Mustelidae inhabit all continents except Antarctica and Australia. They do not occur on Madagascar or oceanic islands, but have been introduced to New Zealand. (Dragoo and Honeycutt, 1997; Sato et al., 2003; Vaughan, Ryan, and Czaplewski, 2000)

Mustelidae are distributed from the arctic to the tropics and occupy nearly all terrestrial habitats. Several species are semi- or nearly fully aquatic and inhabit freshwater rivers and streams, as well as coastal marine waters. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

There has been much debate regarding the relationships among various groups groups within Mustelidae. Between four and seven subfamilies have been recognized in the past. Currently up to six subfamilies are generally accepted: Old World badgers (Mustelinae), badgers (Mustelinae), honey badgers (Mustelinae), otters (Lutrinae), skunks (Mephitinae), and all other weasel-like mustelids (Mustelinae). Recent molecular evidence has shown that skunks do not lie within Mustelidae, and thus skunks have been elevated to the family Mephitidae. The position of other taxa within Mustelidae have also been subject of recent debate (e.g., Gulo). Furthermore, the systematics of red pandas (Ailurus fulgens) has been controversial, with this species being placed alternately within Ursidae or Procyonidae. Recent comprehensive molecular studies of Carnivora, however, indicate a relatively close relationships between Ailurus and stink badgers (Mydaus) (Flynn et al., 2005). (Dragoo and Honeycutt, 1997; Flynn et al., 2005; Sato et al., 2003; Wilson and Reeder, 1993)

The morphological characters considered to unite all mustelids are enlarged anal scent glands, the loss of the second upper molar, and the loss of the carnassial notch on the fourth upper premolar. Mustelids share these characters with skunks, the main reason for their historical inclusion in the family. These characters may be the result of convergent evolution, however. For example, all carnivores have enlarged scent glands, and those of skunks are much more enlarged than in Mustelidae. Furthermore, the anal scent gland in skunks is associated with a nipple, rather than a duct as in Mustelidae. (Dragoo and Honeycutt, 1997; Flynn et al., 2005; Koepfli and Wayne, 2003; Marmi, Lopez-Giraldez, and Domingo-Roura, 2004; Sato et al., 2003; Sato et al., 2004)

Adult mustelids range in size from 114 mm and 25 g (least weasel) to over 1 m and 45 kg (sea otters). These animals are generally long-bodied with short legs. Most species have slender bodies, but some, like badgers (Mustelinae, Mustelinae) and wolverines have much broader bodies. The skull is elongate with a relatively short rostrum. Adult males are generally about 25 percent larger than females of the same species. The ears are short, as are the legs, each of which bears five digits. The claws do not retract and, in digging species, are especially robust. Mustelids are digitigrade or plantigrade. The dental formula varies among species: 3/3, 1/1, 2-4/2-4, 1/1-2 = 28-38. The canines are long, and the carnassials are well-developed. The upper molars are often narrow in the middle, giving them an hourglass shape. Mustelids have a powerful bite; in many species, the large postglenoid process locks the lower jaw into the upper, causing the lower jaw to only move in the vertical plane, without any rotary motion. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000; Whitaker and Hamilton, 1998)

Mustelids typically live between 5 and 20 years in the wild. (Nowak, 1991)

Members of the family Mustelidae are either diurnal or nocturnal. Many of the long, narrow-bodied species are quick and agile, and move in a bounding, scampering fashion. The broader-bodied forms have a more lumbering gait. Some species are adept climbers, while others are excellent swimmers. Many species spend a great deal of time on the ground, searching for food in crevices, burrows, or under cover. Many species shelter in burrows. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Social behavior varies both within and among species, and may vary in relation to local environmental conditions such as food availability. For example, European badgers are known to form groups with several males and females that are all reproductively active within the group. Yet in other parts of their range, European badgers may live solitarily or in pairs.Many species are territorial for at least part of the year, with individuals competing over hunting areas or access to mates (e.g., Mustela erminea). (Johnson, MacDonald, and Dickman, 2000)

Vision and hearing are important in Mustelidae, but olfaction is particularly well developed. In addition to using scent cues to find food, scent-marking is the main form of communication in this family. Secretions from well-developed scent glands function in territorial interactions, indicate reproductive state, and are used in other social contexts. The degree and function of scent marking varies among species, and according to social and environmental conditions within species. (Hutchings and White, 2000; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Mustelids are primarily carnivorous, but some species may at times eat plant material. A wide range of animal taxa are preyed upon by various members of this family; many mustelids are opportunistic feeders rather than specialists. However, many mustelids are especially adept at capturing small, mammalian prey. Weasels, for example, are capable of chasing and capturing rodents in their burrows. Otters are well-adapted to chasing and capturing aquatic prey, including fish, crustaceans, and other aquatic invertebrates. Mustelids hunt in a variety of terrestrial, aquatic, and arboreal habitats. Some species regularly prey on animals larger than themselves. Some species have been known to store food (e.g., Mustela, Gulo). (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Mustelids are generally small carnivores, and are therefore subject to predation by larger carnivores such as canids with which they co-occur. They may also fall prey to large snakes (Serpentes), raptors (Falconiformes), and owls (Strigiformes). Some mustelids secrete noxious chemicals to discourage predators. In some of these species, aposematic color patterns can help ward off predators. (Bright, 2000; Nowak, 1991)

Mustelids mainly impact their communities through their effects on prey populations. Many species limit rodent and bird populations. In some cases, mustelids limit rodents that are considered pests, in other cases, mustelids threaten rare bird species. Some species, such as sea otters (Enhydra lutra) are keystone predators, enhancing the diversity of their community by keeping highly competitive prey species in check. Honey badgers (or ratels, Mellivora capensis) have developed commensal relationships with both humans and honey guides (Indicator indicator), using both to aid in the location of bee colonies. (Bright, 2000; Macdonald and King, 2000; Nowak, 1991)

Some mustelids are considered pests, either for harming poultry livestock, for threatening other species in the wild, or for transmitting diseases. European badgers have been implicated in the transmission of bovine tuberculosis. Cattle may become infected from grazing on land where badgers have defecated. Up to 20% of badgers carry the disease in areas where bovine tuberculosis is a problem. Since 1975, badgers have been culled in the United Kingdom, but there is no conlcusive evidence that it has helped control bovine TB. As mammalian carnivores, mustelids can also be infected by, and transmit, rabies. (Bright, 2000; Gough and Rushton, 2000; Nowak, 1991)

Many mustelids help control rodent populations that are considered to be pests. In addition, many are hunted and/or raised for their pelts, which are often considered highly valuable (e.g., the pelts of mink and sable). Some species have been domesticated and are traded as pets (e.g., ferrets). (Bright, 2000; Nowak, 1991)

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. (Bright, 2000; Gough and Rushton, 2000)

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. (Bright, 2000)