The stoat (Mustela erminea) is a small predator belonging to the Mustelidae family, which includes weasels and badgers. This mammal is recognized by its highly elongated and flexible body, a shape adapted for hunting prey in confined spaces. Its appearance is defined by a sleek, low-slung profile and a coat that transforms depending on the season and location. The stoat is an agile hunter found across Eurasia and North America.
General Body Profile and Dimensions
The stoat possesses a slender, cylindrical body supported by short legs, a morphology adapted for following rodents into their burrows. Its head is small and triangular, resting on a long neck that aids in quick strikes on prey. The overall body length, excluding the tail, typically ranges from 170 to 325 millimeters, placing it among the smaller carnivores.
A notable feature is the pronounced sexual dimorphism in size, with males being significantly larger than females. On average, males weigh approximately 258 grams, while females weigh less than 180 grams, making the male nearly twice as heavy. The male’s body length can be up to 25% greater than the female’s, a size difference that influences their respective hunting strategies.
Seasonal Color Change
The stoat’s coat color changes with the seasons in its northern range, a phenomenon known as molting. The summer coat consists of uniform chestnut or sandy-brown fur across the upper body, head, and legs. This darker coloration contrasts sharply with the creamy white or yellowish fur covering its throat, chest, and belly.
As winter approaches in colder, snow-covered regions, the stoat replaces its brown fur with a dense, white coat for camouflage. This white winter phase is historically known as ermine. The color change is triggered primarily by the reduction in daylight hours (photoperiod), an adaptation that allows the stoat to blend into snowy environments.
This change to ermine does not occur in all populations; stoats in milder climates or southern latitudes often retain their brown summer coats year-round. In transitional zones, individuals may exhibit a patchy or piebald mix of brown and white fur during the winter months. The winter coat also becomes noticeably thicker, providing insulation against low temperatures.
The Distinctive Black Tail Tip
Regardless of the season, the stoat possesses one constant identifying mark: a black tip on its tail. This feature remains dark even when the animal is in its white ermine phase. The tail is relatively long, measuring between 65 and 120 millimeters, which is about one-third of the animal’s head and body length.
The black tail tip is a reliable field mark for identification. It is theorized that the contrasting color serves as a deflection target for pursuing predators, particularly birds of prey. The predator may strike at the visible black tip, allowing the stoat to escape injury to its vulnerable body.
Stoat Versus Weasel
Distinguishing the stoat from its smaller relative, the least weasel (Mustela nivalis), often comes down to three physical differences. The most immediate distinction is the presence of the black tail tip on the stoat, which the least weasel lacks, having a uniformly brown tail. Stoats also have a relatively longer tail compared to their body size than weasels.
Size is another differentiating factor, as the stoat is a larger and more robust animal, measuring up to 40 centimeters in total length, while the least weasel rarely exceeds 27 centimeters. The dividing line between the brown upper fur and the white underparts also differs between the species. The stoat has a clean, straight demarcation line, whereas the color boundary on the least weasel appears jagged and irregular.