The ermine (Mustela erminea), also called the stoat in North America, is a small mustelid belonging to the family Mustelidae. It has a circumpolar distribution, occurring throughout the boreal and sub‑arctic zones of Europe, Asia, and North America, and extending into alpine regions farther south. Ermines inhabit a wide range of habitats, including coniferous forests, tundra, heathland, and open grasslands, where dense cover provides shelter for hunting and denning. In summer the coat is brown with a distinctive black tip on the tail; in winter, individuals in snowy regions molt to a pure white pelage, a classic example of seasonal camouflage. Adults measure 17–32 cm in body length, with a 7–12 cm tail, and weigh 0.1–0.3 kg, males being slightly larger than females. The species is a solitary predator, highly territorial, and capable of taking prey up to its own size. Its keen sense of smell and acute hearing allow it to locate voles, lemmings, and other small mammals beneath snow or leaf litter. The ermine’s flexible spine and retractable claws enable rapid pursuit and capture of agile prey. Seasonal coat changes are hormonally regulated and have been studied as a model for mammalian photoperiodic responses. The species has been historically hunted for its luxurious winter fur, known as ‘ermine’, which contributed to its cultural symbolism in royalty and heraldry. Current threats include habitat fragmentation from logging and infrastructure development, climate‑induced reductions in snow cover that diminish its winter camouflage, and occasional poisoning from rodent control programs. Conservation monitoring focuses on population trends in the Arctic and sub‑arctic, where the ermine remains abundant but vulnerable to rapid environmental change.

Ermines are primarily solitary, with each adult maintaining an exclusive home range that may span 2–10 km² depending on prey density. They are most active at dawn and dusk (crepuscular), though individuals in high‑latitude regions can exhibit continuous activity during the brief summer daylight. Territory is defended through scent marking using anal glands and urine, and occasional visual displays such as tail flicking. During the breeding season, males increase their patrol distances to locate receptive females, while females become temporarily less territorial while caring for kits. In winter, the white coat provides camouflage that allows stealthy ambush of prey under snow, and the species may use sub‑nivean tunnels to travel unseen. Juveniles disperse from the natal den at 8–10 weeks, establishing their own territories and contributing to gene flow across populations.

The ermine is a strict carnivore, relying almost exclusively on animal tissue for nutrition. Its diet is dominated by small mammals such as voles, lemmings, mice, and shrews, which can constitute up to 80 % of stomach contents in tundra populations. Birds, eggs, and occasionally amphibians and insects supplement the diet, especially during the breeding season when protein demand is high. An adult may consume 10–20 % of its body weight per day, equivalent to 10–30 g of prey, depending on prey size and availability. Hunting strategy involves a rapid chase aided by a flexible spine and retractable claws; the ermine can follow prey into burrows and use its elongated body to flush out hidden animals. In winter, it employs a “snow stalking” technique, using its whiskers to detect vibrations beneath the snowpack. Seasonal shifts in prey abundance drive opportunistic feeding, and in some regions ermines have been observed scavenging carrion or preying on fish in shallow streams.

Sexual maturity is reached at 10–12 months for females and 12–14 months for males, though first breeding may be delayed by harsh winter conditions. The breeding season in the northern range extends from March to May, with a peak in April. After mating, the female experiences delayed implantation; the fertilized embryo remains dormant for 2–3 months before implantation, resulting in a total gestation of about 250 days. Litter size varies from 4 to 12 kits, with an average of 7, and is influenced by the mother’s age and prey availability. Kits are born altricial, blind and hairless, weighing approximately 2 g. They remain in the den for 3–4 weeks, during which the mother provides all nourishment. By six weeks, kits begin to explore the surrounding area, learning hunting techniques by observing the mother. Weaning is completed by 8–10 weeks, after which juveniles disperse to establish independent territories.

The IUCN classifies the ermine as Least Concern, reflecting its broad distribution and large global population, estimated at several million individuals. Nevertheless, localized threats can cause declines. Habitat loss from commercial logging, mining, and infrastructure development fragments the forest and tundra mosaics essential for hunting and denning. Climate change poses a significant risk by reducing the duration and depth of snow cover, which compromises the winter white camouflage that protects ermines from predators and enhances hunting efficiency. Reduced snow also alters the abundance of key prey species such as lemmings, leading to cascading effects on ermine reproductive success. Historical over‑exploitation for fur, though largely ceased, left residual population impacts in some European regions. Accidental poisoning from rodent control baits and secondary exposure to anticoagulant rodenticides have been documented in North America, causing mortality in non‑target predators. Conservation actions focus on monitoring population trends, protecting critical habitats, and regulating pesticide use to mitigate indirect poisoning.

• The ermine’s winter white coat was historically prized for royal garments and is the origin of the phrase “the ermine robe.”
• It exhibits one of the fastest metabolic rates among mammals, requiring frequent feeding to maintain body temperature.
• Ermines can delay implantation of the embryo for up to three months, synchronizing birth with spring prey abundance.
• In some Arctic cultures, the ermine is considered a spirit animal symbolizing purity and agility.
• The species can run up to 13 km/h (8 mph) and leap three times its body length when pursuing prey.