Small British Mammals: Water Shrews

Two of the three species of shrew found in Britain are very similar to one another in appearance and habits. This is hardly surprising, given how closely related they are. The odd one out, however, is not quite such a close relative, and is rather more distinctive.

This is the Eurasian water shrew (Neomys fodiens), which unsurprisingly, is simply called the "water shrew" in Britain. Despite its differences, even at first glance, it's pretty obvious that it is a shrew: it's a small, long-tailed animal with short fur, tiny ears, small eyes, and a narrow, pointed, snout filled with sharp teeth. However, by the standards of shrews, it's unusually large. Fully grown adults can reach as much as 10cm (4 inches) in length, and weigh up to 25g (0.9 oz.), closer in size to a typical mouse than to other native shrews.

The Laziness of Venomous Shrews

While there are some bats that could challenge them for the title, by most measures, shrews are the smallest of all mammalian species. They typically weigh no more than about 20 grams (two-thirds of an ounce), and are often much smaller than that. Being so small poses a number of challenges, but there's one in particular that affects mammals of this size that would not affect, say, beetles.

This is because mammals are warm-blooded, and need to generate internal heat in order to keep functioning (unless they're hibernating). The smaller you are, however, the more rapidly you lose heat through your body surface, which means that shrews need a fierce metabolism to keep burning enough calories to keep themselves warm. This means that they have to eat almost constantly, but it also means that they want to exert themselves as little as possible while doing so, so as to waste the minimum amount of energy in the process.