The black-footed ferret (Mustela nigripes), also known as the American polecat^[2] or prairie dog hunter,^[3] is a species of Mustelid native to central North America. It is listed as endangered by the IUCN, because of its very small and restricted populations. First discovered by Audubon and Bachman in 1851, the species declined throughout the 20th century, primarily as a result of decreases in prairie dog populations and sylvatic plague. It was declared extinct in 1979 until Lucille Hogg's dog brought a dead black-footed ferret to her door in Meeteetse, Wyoming in 1981. That remnant population of a few dozen ferrets lasted there until the animals were considered extinct in the wild in 1987. However, a captive breeding program launched by the United States Fish and Wildlife Service resulted in its reintroduction into eight western states and Mexico from 1991–2008. There are now over 1,000 mature, wild-born individuals in the wild in 18 states of the USA, with four self-sustaining populations in South Dakota (two), Arizona and Wyoming.^[1][4]

The black-footed ferret is roughly the size of a mink, and differs from the European polecat by the greater contrast between its dark limbs and pale body and the shorter length of its black tail-tip. In contrast, differences between the black-footed ferret and the steppe polecat of Asia are slight, to the point where the two species were once thought to be conspecific.^[5] The only noticeable differences between the black-footed ferret and the steppe polecat are the former's much shorter and coarser fur, larger ears, and longer postmolar extension of the palate.^[6]

It is largely nocturnal and solitary, except when breeding or raising litters.^[7][8] Up to 91% of its diet is composed of prairie dogs.^[9][10]

Evolution

Like its close cousin, the Asian steppe polecat (with which it was once thought to be conspecific), the black-footed ferret represents a more progressive form than the European polecat in the direction of carnivory.^[2] The black-footed ferret's most likely ancestor was Mustela stromeri (from which the European and steppe polecat are also derived), which originated in Europe during the Middle Pleistocene.^[11] Molecular evidence indicates that the steppe polecat and black-footed ferret diverged from Mustela stromeri sometime between 500,000 and 2,000,000 years ago, perhaps in Beringia. The species appeared in the Great Basin and the Rockies by 750,000 years ago. The oldest recorded fossil find originates from Cathedral Cave, White Pine County, Nevada, and dates back to 750,000–850,000 years ago.^[12] Prairie dog fossils have been found in six sites where ferrets are yielded, thus indicating that the association between the two species is an old one.^[5] Anecdotal observations and 42% of examined fossil records indicated that any substantial colony of medium- to large-sized colonial ground squirrels, such as Richardson's ground squirrels, may provide a sufficient prey base and a source of burrows for black-footed ferrets. This suggests that the black-footed ferret and prairie dogs did not historically have an obligate predator-prey relationship.^[12] The species has likely always been rare, and the modern black-footed ferret represents a relic population. The earliest reported occurrence of the species is from a late Illinoian deposit in Clay County, Nebraska, and is further recorded from Sangamonian deposits in Nebraska and Medicine Hat. Fossils have also been found in Alaska dating from the Pleistocene.^[5][11]

Physical description

Skull, as illustrated in Merriam's Synopsis of the weasels of North America

Black-footed ferret at the Louisville Zoo

The black-footed ferret has a very long body and a blunt head. The forehead is arched and broad, and the muzzle is short. It has few whiskers, and its ears are triangular, short, erect and broad at the base. The neck is long and the legs short and stout. The toes are armed with sharp, very slightly arched claws. The feet on both surfaces are covered in hair, even to the soles, thus concealing the claws.^[13] It combines several physical features common in both members of the subgenus Gale (least, short-tailed and long-tailed weasels) and Putorius (European and steppe polecats). Its skull resembles that of polecats in its size, massiveness and the development of its ridges and depressions, though it is distinguished by the extreme degree of constriction behind the orbits where the width of the cranium is much less than that of the muzzle. Though similar in size to polecats, its attenuate body, long neck, very short legs, slim tail, large orbicular ears and close-set pelage is much closer in conformation to weasels and stoats.^[14] The dentition of the black-footed ferret closely resembles that of the European and steppe polecat, though the back lower molar is vestigial, with a hemispherical crown which is too small and weak to develop the little cusps which are more apparent in polecats.^[14]

Males measure 500–533 millimetres (20–21.0 in) in body length and 114–127 millimetres (4.5–5.0 in) in tail length, thus constituting 22–25% of its body length. Females are typically 10% smaller than males.^[5] It weighs 650–1,400 grams (1.4–3.1 lb).^[15] Captive-bred ferrets used for the reintroduction projects were found to be smaller than their wild counterparts, though these animals rapidly attained historical body sizes once released.^[16]

The base color is pale yellowish or buffy above and below. The top of the head and sometimes the neck is clouded by dark-tipped hairs. The face is crossed by a broad band of sooty black, which includes the eyes. The feet, lower parts of the legs, the tip of the tail and the preputial region are sooty-black. The area midway between the front and back legs is marked by a large patch of dark umber-brown, which fades into the buffy surrounding parts. A small spot occurs over each eye, with a narrow band behind the black mask. The sides of the head and the ears are dirty-white in color.^[6]

Behavior and ecology

Territorial behavior

Black-footed ferret performing a weasel war dance

The black-footed ferret is solitary, except when breeding or raising litters.^[7][8] It is nocturnal^[7][17] and primarily hunts for sleeping prairie dogs in their burrows.^[18] It is most active above ground from dusk to midnight and 4 a.m. to mid-morning.^[10] Aboveground activity is greatest during late summer and early autumn when juveniles become independent.^[10] Climate generally does not limit black-footed ferret activity,^[8][10] but it may remain inactive inside burrows for up to 6 days at a time during winter.^[19]

Female black-footed ferrets have smaller home ranges than males. Home ranges of males may sometimes include the home ranges of several females.^[8] Adult females usually occupy the same territory every year. A female that was tracked from December to March occupied 39.5 acres (16 ha). Her territory was overlapped by a resident male that occupied 337.5 acres (137 ha) during the same period. The average density of black-footed ferrets near Meeteetse, Wyoming, is estimated at 1 black-footed ferret /99 to 148 acres (60 ha). As of 1985, 40 to 60 black-footed ferrets occupied a total of 6,178 to 7,413 acres (2,500 to 3,000 ha) of white-tailed prairie dog habitat.^[7] From 1982 to 1984, the average year-round movement of 15 black-footed ferrets between white-tailed prairie dog colonies was 1.6 miles/night (2.5 km) (with a spread of 1.1 miles or 1.7 km). Movement of black-footed ferrets between prairie dog colonies is influenced by factors including breeding activity, season, sex, intraspecific territoriality, prey density, and expansion of home ranges with declining population density.^[8][20] Movements of black-footed ferrets have been shown to increase during the breeding season; however, snow-tracking from December to March over a 4-year period near Meeteetse, Wyoming revealed that factors other than breeding were responsible for movement distances.^[8]

Temperature is positively correlated with distance of black-footed ferret movement.^[8] Snow-tracking from December to March over a 4-year period near Meeteetse, Wyoming, revealed that movement distances were shortest during winter and longest between February and April, when black-footed ferrets were breeding and white-tailed prairie dogs emerged from hibernation. Nightly movement distance of 170 black-footed ferrets averaged 0.87 miles (1.40 km) (range 0.001 to 6.91 miles (0.002–11.12 km)). Nightly activity areas of black-footed ferrets ranged from 1 to 337.5 acres (0 to 137 ha)), and were larger from February to March (110.2 acres (45 ha)) than from December to January (33.6 acres (14 ha)).^[8] Adult females establish activity areas based on access to food for rearing young. Males establish activity areas to maximize access to females, resulting in larger activity areas than those of females.^[8]

Prey density may account for movement distances. Black-footed ferrets may travel up to 11 miles (18 km) to seek prey, suggesting that they will interchange freely among white-tailed prairie dog colonies that are less than 11 miles (18 km) apart. In areas of high prey density, black-footed ferret movements were nonlinear in character, probably to avoid predators.^[8] From December to March over a 4-year study period, black-footed ferrets investigated 68 white-tailed prairie dog holes per 1 mile (1.6 km) of travel/night. Distance traveled between white-tailed prairie dog burrows from December to March averaged 74.2 feet (22.6 m) over 149 track routes.^[8]

Reproduction and development

Black-footed ferret kits

The reproductive physiology of the black-footed ferret is similar to that of the European polecat and the steppe polecat. It is probably polygynous, based on data collected from home range sizes, skewed sex ratios, and sexual dimorphism.^[8][20] Mating occurs in February and March.^[8][19] When a male and female in estrus encounter each other, the male sniffs the genital region of the female, but does not mount her until after a few hours have elapsed, which is contrast to the more violent behavior displayed by the male European polecat. During copulation, the male grasps the female by the nape of the neck, with the copulatory tie lasting from 1.5–3 hours.^[5] Unlike other mustelids, the black-footed ferret is a habitat specialist with low reproductive rates.^[20] In captivity, gestation of black-footed ferrets lasts 42–45 days. Litter size ranges from 1–5 kits.^[17] Kits are born in May and June^[21] in prairie dog burrows.^[7] Kits are altricial and are raised by their mother for several months after birth. Kits first emerge above ground in July, at 6 weeks old.^[10][20][21] They are then separated into individual prairie dog burrows around their mother's burrow.^[10] Kits reach adult weight and become independent several months following birth, from late August to October.^[10][20] Sexual maturity occurs at one year of age.^[10]

Intercolony dispersal of juvenile black-footed ferrets occurs several months after birth, from early September to early November. Dispersal distances may be short or long. Near Meeteetse, Wyoming, 9 juvenile males and 3 juvenile females dispersed 1 to 4 miles (1–7 km) following litter breakup. Four juvenile females dispersed a short distance (<0.2 miles (0.3 km)) but remained on their natal area.^[20]

Diet

Black-footed ferret with a prairie dog carcass, as illustrated in Ferret Facts and Fancies (1915)

Up to 91% of the black-footed ferret's diet is composed of prairie dogs.^[9][10] The diet of the black-footed ferret varies depending on geographic location. In western Colorado, Utah, Wyoming, and Montana, black-footed ferrets historically associated with white-tailed prairie dogs and were forced to find alternate prey when white-tailed prairie dogs entered their four month hibernation cycle.^[17] In Wyoming, alternate prey items consumed during white-tailed prairie dog hibernation included voles (Microtus spp.) and mice (Peromyscus spp. and Mus spp.) found near streams. In South Dakota, black-footed ferrets associate with black-tailed prairie dogs. Because black-tailed prairie dogs do not hibernate, little seasonal change in black-footed ferret diet is necessary.^[8][17]

In Mellette County, South Dakota, black-tailed prairie dog remains occurred in 91% of 82 black-footed ferret scats. Mouse remains occurred in 26% of scats. Mouse remains could not be identified to species; however, deer mice, northern grasshopper mice, and house mice were captured in snap-trap surveys. Potential prey items included thirteen-lined ground squirrels, plains pocket gophers, mountain cottontails, upland sandpipers, horned larks, and western meadowlarks.^[10]

Based on 86 black-footed ferret scats found near Meeteetse, Wyoming, 87% of black-footed ferret diet was composed of white-tailed prairie dogs. Other food items included deer mice, sagebrush voles, meadow voles, mountain cottontails, and white-tailed jackrabbits. Water is obtained through consumption of prey.^[7]

One adult female black-footed ferret and her litter require approximately 474 to 1,421 black-tailed prairie dogs per year or 412 to 1,236 white-tailed prairie dogs per year for sustenance. These figures assume that each adult black-footed ferret occupies 1 prairie dog colony, each young black-footed ferret will disperse to a new colony when mature, and prairie dogs are the only prey species available. This dietary requirement would require protection of 91 to 235 acres (37–95 ha) of black-tailed prairie dog habitat or 413 to 877 acres (167–355 ha) of white-tailed prairie dog habitat for each female black-footed ferret with a litter.^[22]

Distribution and habitat

The historical range of the black-footed ferret was closely correlated with, but not restricted to, the range of prairie dogs (Cynomys spp.). Its range extended from southern Alberta and southern Saskatchewan south to Texas, New Mexico, and Arizona.^[5] As of 2007, the only known wild black-footed ferret population was located on approximately 6,000 acres (2,428 ha) in the western Big Horn Basin near Meeteetse, Wyoming.^{[7][8][9][19][20]} Other populations might exist but remain undetected.^[5] Since 1990, black-footed ferrets have been reintroduced to the following sites: Shirley Basin, Wyoming; UL Bend National Wildlife Refuge and Fort Belknap Indian Reservation, Montana; Conata Basin/Badlands, Buffalo Gap National Grassland, and the Cheyenne River Sioux Reservation in South Dakota; Aubrey Valley, Arizona; Wolf Creek, Colorado; Coyote Basin, straddling Colorado and Utah; and northern Chihuahua, Mexico.^[21]

Historical habitats of the black-footed ferret included shortgrass prairie, mixed-grass prairie, desert grassland, shrub steppe, sagebrush steppe,^[20] mountain grassland, and semi-arid grassland.^[5] Black-footed ferrets use prairie dog burrows for raising young, avoiding predators, and thermal cover.^[7][10] Six black-footed ferret nests found near Mellette County, South Dakota, were lined with buffalo grass, prairie threeawn, sixweeks grass, and cheatgrass. High densities of prairie dog burrows provide the greatest amount of cover for black-footed ferrets.^[7][8] Black-tailed prairie dog colonies contain a greater burrow density per acre than white-tailed prairie dog colonies, and may be more suitable for the recovery of black-footed ferrets.^[7] The type of prairie dog burrow may be important for occupancy by black-footed ferrets. Black-footed ferret litters near Meeteetse, Wyoming, were associated with mounded white-tailed prairie dog burrows, which are less common than non-mounded burrows. Mounded burrows contain multiple entrances and probably have a deep and extensive burrow system that protects kits.^[7] However, black-footed ferrets used non-mounded prairie dog burrows (64%) more often than mounded burrows (30%) near Meeteetse, Wyoming.^[8]

Mortality

Primary causes of mortality include habitat loss, human-introduced diseases, and indirect poisoning from prairie dog control.^{[10][17][19][21]} Annual mortality of juvenile and adult black-footed ferrets over a 4-year period ranged from 59% to 83% (128 individuals) near Meeteetse, Wyoming.^[20] During fall and winter, 50% to 70% of juveniles and older animals perish.^[20] Average lifespan in the wild is probably only 1 year but may be up to 5 years. Males have higher rates of mortality than females because of longer dispersal distances when they are most vulnerable to predators.^[20]

Given an obligate-dependence of black-footed ferrets on prairie dogs, black-footed ferrets are extremely vulnerable to prairie dog habitat loss. Habitat loss results from agriculture, livestock use, and other development.^[21]

Black-footed ferrets are susceptible to numerous diseases. They are fatally susceptible to canine distemper (Morbillivirus),^[5][20] introduced by striped skunks, common raccoons, red foxes, coyotes, and American badgers.^[19] A short-term vaccine for canine distemper is available for captive black-footed ferrets, but no protection is available for young born in the wild. Other diseases that black-footed ferrets are susceptible to include rabies, tularemia, and human influenza. Sylvatic plague (Yersinia pestis) probably does not directly affect black-footed ferrets, but epidemics in prairie dog towns may completely destroy the black-footed ferrets' prey base.^[20]

Predators of black-footed ferrets include golden eagles, great horned owls, coyotes, American badgers, bobcats, prairie falcons, ferruginous hawks, and prairie rattlesnakes.^[10][19][20]

Oil and natural gas exploration and extraction can have detrimental impacts on prairie dogs and black-footed ferrets. Seismic activity collapses prairie dog burrows. Other problems include potential leakages and spills, increased roads and fences, increased vehicle traffic and human presence, and an increased number of raptor perching sites on power poles. Traps set for coyotes, American mink, and other animals may harm black-footed ferrets.^[9]

History

Native American tribes, including the Crow, Blackfoot, Sioux, Cheyenne, and Pawnee, used black-footed ferrets for religious rites and for food.^[17] The species was not encountered during the Lewis and Clark Expedition, nor was it seen by Nuttall or Townsend, and it was not until it was first described in Audubon and Bachman's Viviparous Quadrupeds of North America in 1851 that it became known to the scientific world.^[23]

It is with great pleasure that we introduce this handsome new species ; ... [it] inhabits the wooded parts of the country to the Rocky Mountains, and perhaps is found beyond that range... When we consider the very rapid manner in which every expedition that has crossed the Rocky Mountains, has been pushed forward, we cannot wonder that many species have been entirely overlooked... The habits of this species resemble, as far as we have learned, those of [the European polecat]. It feeds on birds, small reptiles and animals, eggs, and various insects, and is a bold and cunning foe to the rabbits, hares, grouse, and other game of our western regions.

—Audubon and Bachman (1851)^[23]

Decline

For a time, the black-footed ferret was harvested for the fur trade, with the American Fur Company having received 86 ferret skins from Pratt, Chouteau, and Company of St. Louis in the late 1830s. During the early years of predator control, black-footed ferret carcasses were likely discarded, as their fur was of low value. This likely continued after the passing of the Endangered Species Act of 1973, for fear of reprisals. The large drop in black-footed ferret numbers began during the 1800s through to the 1900s, as prairie dog numbers declined because of control programs and the conversion of prairies to croplands. Sylvatic plague, a disease introduced into North America, also contributed to the prairie dog die-off, though ferret numbers declined proportionately more than their prey, thus indicating other factors may have been responsible. Disease and inbreeding depression may have also contributed, as studies on black-footed ferrets from Meeteetse revealed low levels of genetic variation. Canine distemper devastated the Meeteetse population in 1985. A live virus vaccine originally made for domestic ferrets killed large numbers of black-footed ferrets, thus indicating that the species is especially susceptible to distemper.^[15]

Reintroduction and conservation

Ferret in the wild, July 2008

US federal and state agencies in cooperation with private landowners, conservation groups, Native Americans, and North American zoos, have been actively reintroducing ferrets back into the wild since 1991. Beginning in Wyoming, reintroduction efforts have since expanded to sites in Montana, South Dakota, Arizona, Utah, Colorado, and Chihuahua, Mexico. The Toronto Zoo has bred hundreds, most of which were released into the wild.^[24] Several episodes of Zoo Diaries show aspects of the tightly controlled breeding. Proposed reintroduction sites have been identified in Canada. However, in May 2000, the Canadian Species at Risk Act listed the black-footed ferret as being an extirpated species in Canada.^[25] A population of 34 animals was released into Grasslands National Park in southern Saskatchewan on October 2, 2009,^[26] and a litter of newborn kits was observed in July 2010.^[27]

As of 2007, the total wild population of black-footed ferrets was well over 650 individuals (plus 250 in captivity) in the US. In 2008, the IUCN classified the species as globally endangered, a substantial improvement since the 1996-assessment when it was considered extinct in the wild, since at that time the species was indeed only surviving in captivity. The black-footed ferret is listed as "Endangered" under the Endangered Species Act since September 20, 2005. An April 2006 report in The New York Times puts South Dakota's Conata Basin population at around 250. Arizona's Aubrey Valley population is well over 100 and they have started a second reintroduction site using around 50 animals. An August 2007 report by Wyoming researchers in the journal Science counted a population of 223 in one area of the state (the original number of reintroduced ferrets, most of which died, was 228), and an annual growth rate of 35% from 2003–2006 was estimated.^[28][29] This rate of recovery is much faster than for many endangered species, and the ferret seems to have prevailed over the previous problems of disease and prey shortage that hampered its improvement.^[29]

See also

• European polecat
• Steppe polecat

References

Notes

 This article incorporates public domain material from the United States Department of Agriculture document "Mustela nigripes".

Bibliography

• Audubon, John James; Bachman, John (1851). The quadrupeds of North America, Vol. 2. New York, V.G. Audubon. http://www.archive.org/details/quadrupedsofnort02audu 
• Coues, Elliott (1877). Fur-bearing Animals: A Monograph of North American Mustelidae. Government Printing Office. http://www.archive.org/details/furbearinganima00couegoog 
• Feldhamer, George A.; Thompson, Bruce Carlyle; Chapman, Joseph A. (2003). Wild mammals of North America: biology, management, and conservation. JHU Press. ISBN 0-8018-7416-5 
• Kurtén, Björn (1980). Pleistocene mammals of North America. Columbia University Press. ISBN 0-231-03733-3 
• Merriam, Clinton Hart (1896). Synopsis of the weasels of North America. Washington : Govt. Print. Off.. http://www.archive.org/details/synopsisofweasel00merriala