Least weasel

The least weasel is, in fact, the world’s smallest predatory mammal and grows no larger than 20 centimetres in length, tail included. 

As with many other mammals, the males are slightly larger than the females and can also weigh slightly more. Even a large male will not weigh more than 100 grams at most, while the females do not get much heavier than around 40 grams. There are also large variations and fluctuations in the weight of weasels, and normal weight can be anything from 40 to 100 grams for males and 20 to 40 grams for females.  

With it’s long, slim body, the weasel is very similar in appearance to the stoat. The weasel has a chalk-white winter coat, while from April/May until October/November it has a light brown summer coat with a white area on the abdomen. There are two different colour variants of the weasel, but we will come back to them later.  

If you are wondering if you saw a least weasel or a small stoat cub, you can tell them apart by the least weasel’s shorter tail and the absence of the stoat’s black tuft on the tip of the tail. If you see a picture of the animal and the tail is not visible, it can actually be impossible to decide whether it was a small stoat or a least weasel.  

Photo: GrahamC57 med CC BY-NC-ND 2.0-lisens. 

The weasel is an animal that thrives in a whole host of habitats. It is found in forests, in the mountains, in cultivated fields in farming areas and pretty much anywhere with small rodents.  

It is a territorial species, which means that every animal has its own territory, which it defends against other weasels. If an intruder or nosy neighbour appears, both screaming and growling may be heard, depending on how threatened the animals feel.  

The males have a larger territory than the females, and the female’s territory may be within the territory defended by the male. 

At centre of the weasel’s territory is a nest constructed in a natural cavity. Sometimes this nest is in a scree or in the passageways of a mouse the weasel has killed, but weasels may also make themselves at home under the floor of a barn, in an old shed or in other man-made structures.  

Photo: big-ashb med CC BY 2.0-lisens. 

The weasel is a true predator and specialises in eating small rodents. Its small size allows the weasel to crawl into narrow holes in the ground made by herbivorous field voles or hunt for other rodents in the gap beneath the snow in winter.  

Once it has caught its prey, it is quick to kill the rodent with a sharp bite to the back of the neck or across the head.  

A field vole can provide enough food for four or five meals for a weasel, which takes it’s prey to it’s den to store for later. Being so small, the weasel has to eat frequently, and it can actually die if it does not eat every single day.  

Given the chance, it will kill several mice and other rodents for later consumption.  

In spring and summer, the weasel will also hunt for both birds and bird eggs if there is a shortage of rodents, but mainly leaves them be if it is a good year for small rodents.  

Photo: fra298 med CC BY-NC-ND 2.0-lisens. 

Females are pregnant for around 40 days after fertilisation and generally give birth to between four and six young, although larger litters can occur if there is a good supply of food.  

There are slightly different times for exactly when the least weasel become pregnant and gives birth. This is probably due to either regional variations or availability of prey. Researchers are still not sure what the precise reason is, but they are working to find out. 

It is nevertheless most usual for the least weasel to give birth in the spring or early summer here in Norway. Instances of some least weasel females having an extra litter in years when rodents are plentiful have also been recorded. In this case, she will fall pregnant in late summer and give birth to the extra litter in early autumn.  

Photo: GrahamC57 med CC BY-NC-ND 2.0-lisens. 

Least weasels are found throughout Norway, even in the far north of the country. They thrive both up in the mountains and in the lowlands, as well as in coastal areas, although they are rarer there. If rodents are readily available, it is a great place for a least weasel to live.  

Like many of the other mustelids in Norway, the least weasel has what is called circumpolar distribution, which means that it is found virtually everywhere in the northern hemisphere. It’s range extends as far south as Morocco in Africa, the Himalayas in Asia and around halfway down the USA.  

One curious fact about least weasels is that the farther north they live, the smaller their body size, which is very unusual in mammals, where the opposite is the norm. So a Norwegian weasel will be slightly smaller and weigh slightly less than a weasel from Spain, for example, even if they are from precisely the same species. The opposite is seen in most other animals, which get slightly larger the further north they live. 

Photo: GrahamC57 med CC BY-NC-ND 2.0-lisens. 

Least weasels are completely reliant on a good supply of small rodents to survive, with these animals controlling it’s distribution and population. You therefore see fewer weasels in a poor year for rodents, but the population booms in good years for rodents and the year after.  

 The weasel is a species with quite a lot of variations within it, and researchers are unable to agree on whether they are different enough to be separate species, whether they should be regarded as subspecies, or whether they are just different colour variants of the same species. Some researchers consider there to be 19 different weasel subspecies around the world, but, fortunately for us Norwegians, all the weasels in Norway are the same subspecies. It is called Mustela nivalis nivalis.  

Another subspecies, Mustela nivalis vulgaris, is found in Southern Sweden and parts of Denmark. It differs from the subspecies found in Norway and the rest of Sweden by there not being a clear separation between the brown and white parts of it’s summer coat. 

Photo: GrahamC57 med CC BY-NC-ND 2.0-lisens. 

You can learn even more about the weasel on the following websites:  

• Weasel at the Norwegian Biodiversity Information Centre.  
• Weasel at Store norske leksikon.  

We have also used them as sources for this information page on the least weasel.  

This article has been written by Nicklas Iversen, a nature guide for Visitor Centre Carnivore Flå.