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Dec. 20, 2011

The Life of the Least Weasel

by Kara Rogers

Click to enlarge images

Photo credit: Kevin Law.

With a black button nose, large round eyes, and fuzzy knobs of ears, the least weasel is undoubtedly adorable. And it is made all the more so by its small size, with the tiniest individuals weighing just 25 grams and measuring a mere four inches in length. But beneath the fur and the lanky little profile lies a fierce meat-hungry predator, a bantamweight killer that carries a reputation as the world's smallest carnivore*.

The least weasel (Mustela nivalis) has an appetite for rodents and other small mammals but will also prey on lizards, birds' eggs, chicks, and small amphibians, such as frogs and salamanders. In some instances, aided by their deftness and determination, they will attack rabbits and other mammals that are relatively large. The type of prey taken by least weasels depends mainly on seasonal and geographical factors, which determine the abundance of prey animals, particularly voles, lemmings, mice, and shrews.

Least weasels enjoy a wide distribution, inhabiting much of Europe, North Africa, Asia, and the northern region of North America. They are also found on New Zealand, on a small handful of Japanese islands, on São Tomé off central Africa, on the Portuguese archipelago of the Azores, and on several Mediterranean islands. The species, however, is not native to the majority of these islands.

Within its geographical range, the least weasel occupies a variety of habitats. For instance, while it often lives in boreal forests, where it benefits from dense tree cover and understory that conceals it from its predators, it is also found in meadows and prairies and in scrubby and even semi-desert areas. In each of these habitats, it commonly makes its home in burrows or dens abandoned by other small animals.

In winter, the least weasel’s fur is transformed from a rich brown with white underbelly and feet to completely white with a few black hairs adorning the tip of its tail. This conversion is most thoroughly effected in the northern reaches of its distribution, where fading into the pale backdrop of snow-covered landscapes is not simply an art but a skill for survival. Indeed, its white coat conceals it from prey and predator alike, enabling it to blend in with the snow as it tunnels close to its quarry before a kill and to hide from its greatest adversaries—birds of prey circling in the air or perched in trees high above. Least weasels’ survival in winter is aided further by their habit of creating food stores, which can see them through even the harshest of freezing weather.

The reproductive behavior of the least weasel differs from that of many mustelids (the common name given to members of the family Mustelidae, which includes weasels). Indeed, some mustelids, including badgers, martens, and wolverines, exhibit delayed implantation, in which fertilized embryos produced from a mating that occurred in late summer or fall do not attach to the uterine lining and begin to develop until spring. Delayed implantation helps ensure that offspring will be born under favorable conditions, such as when food resources are plentiful and the weather is warm. It also limits the number of litters produced per year to just one. In contrast, least weasels do not experience delayed implantation, and they have a brief gestation period (34 to 37 days). Hence they are able to produce two litters of offspring in a single year.

The least weasel’s robust reproductive activity has helped it maintain apparently viable populations, despite threats such as loss of habitat to logging and agriculture and exposure to poisonous substances, including rodenticides. But it will not be able to endure such pressures for very long. Its short life span, just 1 to 2 years in the wild, leaves the species susceptible to swift declines. Populations in Europe are already deteriorating, and it is now only rarely seen in certain other areas of its native range.

*The northern short-tailed shrew (Blarina brevicauda), which weighs about 20 to 22 grams and measures three to four inches in length, also eats small animals; however, it also eats seeds and other plant plants and therefore is not strictly carnivorous.
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Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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