The American ermine or American stoat (Mustela richardsonii) is a species of mustelid native to most of North America.

Quick Facts Conservation status, Scientific classification ...

American ermine
Near Beaverhill Lake, Alberta
Conservation status
Secure  (NatureServe)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Family:	Mustelidae
Genus:	Mustela
Species:	M. richardsonii
Binomial name
Mustela richardsonii Bonaparte, 1838
Subspecies
See text
Synonyms
Mustela erminea richardsonii Neogale richardsonii

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It was long considered conspecific with the stoat (M. erminea), but a 2021 study found it to be a distinct species, forming distinct genetic clades from erminea.^[2][3][4] The finding has been accepted by the American Society of Mammalogists.^[5] The Haida ermine (M. haidarum) is thought to be a hybrid species originating from ancient hybridization between M. erminea and M. richardsonii.^[2]

The specific epithet refers to Arctic explorer and naturalist John Richardson.

The species is found throughout most of North America aside from most of Alaska (although it is found on some islands in southeastern Alaska), eastern Yukon, most of Arctic Canada, and Greenland, where it is replaced by M. erminea. It reaches the northern extent of its range in Baffin Island and a portion of eastern mainland Nunavut and ranges from here to cover almost all of western North America south to northern New Mexico, and eastern North America south to northern Virginia. It is absent from most of the Southeastern United States and the Great Plains.^[2]

In North America, where the ecological niche for rat- and rabbit-sized prey is taken by the larger long-tailed weasel (Neogale frenata), the American ermine preys on mice, voles, shrews, young cottontails,^[6] chipmunks, deer mice, jumping mice, and house mice. Usually the ermine kills by biting at base of skull. Small birds, frogs, small fish, and earthworms are other types of prey for ermines.^[7]

Ermines live and find cover from predators in hollow spaces from logs, burrows and man made structures. Ermines sometimes den within their prey's nest and use their skin and fur as a lining for their den.^[7]

They breed in dense parts of the forest. The season for breeding is late springtime to the summer from July to August. The males mature in a year while the females only take three to four weeks to mature. The females carry a litter of four to seven babies for 255 days then gives birth.^[1]

Some of the larger wild predators of ermines are minks, martens, fishers, bobcats, coyotes, and large owls and hawks. Occasionally a domesticated cat or dog may kill an ermine. Their small agile bodies help them evade these predators, while also allow them to compete with their predators for food in more barren months.^[7]

About 13 subspecies are known:

The fur of ermine was valued by the Tlingit and other indigenous peoples of the Pacific Northwest Coast. They could be attached to traditional regalia and cedar bark hats as status symbols or made into shirts.^[10]

• American ermine surplus killing a family of chipmunks, as illustrated by Ernest Thompson Seton
• Skulls of a long-tailed weasel (top), an American ermine (bottom left), and least weasel (bottom right), as illustrated in Merriam's Synopsis of the Weasels of North America
• Winter coat