The stoat (Mustela erminea) is a small, agile mammal belonging to the weasel family, Mustelidae. It is widely distributed across the northern hemisphere and is recognized for its slender, elongated body shape. While commonly known as the stoat, the animal is also referred to as the short-tailed weasel in North America. When its coat turns pure white in winter, it is known by the name of ermine.
The Baseline Weight of a Stoat
An adult stoat’s head and body length typically ranges from 17 to 32.5 centimeters (7 to 13 inches), not including the tail. The tail adds another 7 to 12 centimeters to the total length. This long, low-slung body allows the stoat to navigate the burrows and tunnels of its prey.
The typical weight range for an adult stoat is between 140 and 455 grams (5 to 16 ounces). This broad range accounts for the significant biological differences between the sexes and individual variation. On average, a male stoat weighs about 258 grams (9.1 ounces), while a female typically weighs less than 180 grams (6.3 ounces).
Factors Causing Weight Variation
The most significant factor influencing a stoat’s body mass is sexual dimorphism, as males are substantially larger than females. Males can be 1.5 to 2 times heavier than females and are generally about 25% longer in body size. This size difference likely aids in competition for mates.
Geographical location is another variable affecting size, often due to the existence of multiple subspecies across the globe. While many species follow Bergmann’s Rule by growing larger in colder climates, the stoat’s pattern is more nuanced. Stoats from certain northern populations have been observed to be smaller than those found further south, indicating that local environmental pressures and subspecies genetics play a greater role.
A stoat’s weight can also fluctuate with the changing seasons. Individuals may accumulate fat reserves in the autumn to help them survive the colder winter months. This seasonal weight gain is often accompanied by the growth of a much denser coat, particularly in colder regions where the fur turns white for camouflage.
Distinguishing Stoats by Size
The stoat’s size helps differentiate it from its closest relatives, the weasels. The Least Weasel (Mustela nivalis) is notably smaller, earning it the title of the world’s smallest carnivore, with males typically weighing only 106 to 131 grams. The least weasel is more diminutive than the stoat and lacks the characteristic black tip on its tail.
The Long-Tailed Weasel (Neogale frenata) is generally a larger animal, with a total length that can reach up to 56 centimeters, making it visibly longer than the stoat. While weight ranges can sometimes overlap, the long-tailed weasel’s most distinguishing feature is its tail, which is proportionally much longer, often exceeding half of its head and body length. The stoat’s tail, by comparison, is shorter, usually constituting only about one-third of its total body length.