British Wildlife of the Week

Weasel

The weasel is small. Really small. Much smaller than most people realise. It is not only the smallest mustelid, it’s also the smallest carnivore in the world. Growing to between 13 and 26 cm in length and weighing as little as 25 grams in some cases – about the same as an AA battery – it is a mere 0.0025% of the weight of the planet’s biggest terrestrial carnivore, the polar bear.  

The weasel is often confused with its close relative, the stoat. Both are long, slender, fast-moving predators with a chesnut-brown head and upperparts, and creamy underparts. But the two species differ in two main ways. The first is the tail. A stoat’s tail is around half the length of its body and ends in a bushy black tip, whereas a weasel’s tail is usually quite short and inconspicuous.

The second difference is size. As already mentioned, the weasel is minute, significantly smaller than the stoat on average. But size can be unreliable – not only because their relative size is hard to judge when viewing a lone animal, but also because weasels are extremely variable. Although some weasels are small enough to squeeze through a large wedding ring, a really big one can rival a small stoat in dimensions. In fact, this species varies in size more than almost any other mammal – between the sexes (females are roughly half the size of males), between populations, and from one individual to another.

Being so slim and flexible gives the weasel a huge advantage when hunting mice, voles and other small mammals because it can follow them into their burrows and trap them. But small size and an energetic hunting style mean it rapidly burns through energy reserves, and it must consume about a third of its body weight every day to survive. Perhaps this is the reason why, of the seven mustelids found in the UK, the hyperactive little weasel has the shortest lifespan – it rarely lives beyond 12-18 months.

The weasel is also one of the fastest breeders in its family. In years with plenty of voles, this species may squeeze two generations into a single breeding season. Females are sexually mature by just three or four months old, so those born early in the year, in April and May, can produce a litter of their own by August. In fact, this time of year, towards the end of summer, is great for seeing weasels in Britain because the population has been swelled by large numbers of new adventurous youngsters. Weasels can turn up almost anywhere, including – if you are lucky – your own garden, but it’s usually best to head to some woods, moorland or lowland pastures to catch a glimpse of this diminutive predator.

Next time on British Wildlife of the Week, we’ll be looking at a lizard that looks like a snake – the legless slowworm.