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.

But there are other differences, too, mostly tied to its watery habitat. The tail has a ridge of stiff fur running down it, making it more effective as a rudder, and, while the toes are not webbed, the hind feet are fringed with hairs that perform much the same function. There's also the colour, which is near black on the upper body and sides, but dramatically paler, almost silver, on the underside. While such a counter-shading pattern is common in many animals, it's particularly so in aquatic ones, presumably because, when seen from below by a predator looking upwards, a pale animal is harder to notice against the diffuse glow of the water surface and sky beyond.

And, of course, the water shrew is aquatic... or, more accurately, semi-aquatic, since it doesn't actually sleep in the water. Water shrews live in, or close to, wooded areas with plenty of still pools or shallow streams. They don't seem to mind nearby human activity too much, although polluted rivers are obviously an exception. Unlike some species of water shrew elsewhere in the world, they do sometimes travel a short distance from the water's edge into areas with heavy vegetation cover, especially when they are young, perhaps to limit competition.

The Eurasian water shrew is found throughout most of Europe north of the Mediterranean, with only scattered populations in Italy and the southern Balkans. Those living in Britain are sometimes considered to constitute a distinct subspecies (N. f. bicolor) from those on the continent. Another local subspecies is found only in northern Spain, with the other two being far more widespread - one across Europe and as far east as the River Ob, and the other stretching beyond that to the Pacific coast north of Japan.

The great bulk of the water shrew's diet consists of aquatic invertebrates such as fly and caddisfly larvae and small crustaceans - sowbugs (also known as cress bugs) seem to be a particular favourite.  As with other shrews, they do also eat earthworms, snails, and spiders, which they catch on land, and they also eat tadpoles and fish fry, but these are a minor component of their overall diet. At some times of the year, perhaps when food is most plentiful, they hide caches of uneaten prey amongst dense vegetation, presumably saving it for later when they are hungry - these can include quite large animals, such as frogs.

In common with other shews, water shrews have to eat constantly to stay healthy, and only take short naps, having to stay active both day and night to stave off their hunger. It has been estimated that food takes just three to four hours to pass fully through the digestive tract (it's about 24 hours in humans). They spend a fair proportion of their time below the water in search of food, and may be able to close their nostrils; they can hold their breath for at least two minutes at a time. While they find much of their food on the bottom of shallow ponds and streams, they can dive to 70 cm (28 inches) in search of prey - not bad, considering their small size.

Small animals are more likely to lose body heat while wet than larger ones, and, given shrews' small size and high metabolic rate, we would expect this to be even more of an issue for water shrews. That it isn't is probably partly due to their slightly larger size than other shrews, but they also seem to have a slower metabolism (comparatively speaking) and exceptionally waterproof fur that may be an even better insulator than that of, for example, otters.

Like most other shrews, water shrews are antisocial, and their primary reaction on encountering another of their kind is to fight or chase it - this is particularly true of pregnant females. Except for when males are looking for females during the breeding season, they are highly territorial, but those territories do change during the course of a year, mainly in response to preferred ponds or other water sources drying up or the water-level dropping.

One unusual feature of the European water shrew is its venomous bite. This is much less effective than that of animals such as snakes, but it's still enough to allow them to overpower prey much larger than that which can be taken by other shrews (this explains the frogs in the food caches). The venom is produced in the salivary glands of the lower jaw, although shrews do not have the sort of grooved fangs that snakes have, so the delivery is probably less effective than it is in those animals. Exactly how it works chemically has not been studied in detail, but it is known to be paralytic, rather than lethal, likely allowing the shrews to store still-living prey without risking it starting to rot.

Water shrews breed mainly in the summer, giving birth to a litter of up to eight young after a 20-day pregnancy, and raising it in a grass-lined nest formed from some small hollow or crevice near the waterline. They develop their antisocial habits quickly, with litter-mates starting to fight amongst themselves at 35 days old, and becoming highly intolerant and aggressive by 50 days. Unusually for shrews, they can live for more than one year, sometimes managing a second winter even in the wild, and reaching over three years old in captivity.

While they are absent from Britain, a closely related species, the Mediterranean water shrew (N. anomalus) is found in central and southern Europe - and, in recent years, as far north as Lithuania. Although it is a very close relative of the "Eurasian" species, it is not as efficient a swimmer, feeds more on the land, and only ventures into particularly shallow water. Having said which, aquatic habits seem to have evolved at least three times among shrews, and the other two groups (both of which are Asian) are even better adapted to a watery life than the Eurasian species. The most aquatic of all, the elegant water shrew (Nectogale elegans) has, among other adaptations, webbed feet that can act as suckers to hold onto rocks.

Elsewhere, shrews are found on every continent except Australia and Antarctica, although there are very few south of the equator in South America. They live in all but the harshest of habitats, although they are perhaps most numerous in tropical and subtropical environs such as Central America, Africa, and southern Asia. Most simply live on the ground, but in addition to the water shrews, there are species that spend a lot of time in the trees, and one group that spends so much time underground that they have evolved to look a lot like moles.

Going further out, the shrew family is itself part of a larger assemblage of mostly insect-eating species. These mammals are often thought to be particularly "primitive", despite the fact that all of them possess a number of striking adaptations. In the case of shrews, that's typified by their unusually small size, but some other members of the group are, perhaps, even more distinctive, and I'll be looking at one of those next.

[Photo by "Accipiter", from Wikimedia Commons. Cladogram adapted from Dubey et al. 2007.]