Wildlife watch: the common shrew

The commonest mammal in the UK is the field vole with an estimated population of 75 million. Setting aside us humans for a moment, the second most common mammal is the aptly-named ‘common’ shrew with a population of about 40 million. It might seem surprising that we don’t see them more often but they are small and secretive.

Common shrews belong to the Order Insectivora, one of the oldest surviving mammal lineages in the world. The very first mammals to run around on the earth’s surface, along with the dinosaurs, were very much like modern-day shrews.

With a weight of about 9 grams (similar to a £1 coin) common shrews are tiny. Their body length is about 8cm and tail about 4cm. They have small eyes and quite poor vision, but a very good sense of smell because they have a long, pointed nose and a larger than normal part of their brain dedicated to olfaction. Their nose and whiskers also provide them with a good sense of touch so they can detect any movement of prey close-by.

Having a long lineage means that some aspects of their biology are quite primitive. As well as a small brain and poor eye sight, they have a cloaca, the term used for a common exit to their digestive, urinary and reproductive tracts. But given the fact that creatures similar to shrews have been in existence for about 135 million years they also have some adaptations which make them very successful.

Along with other shrews and bats, common shrews can use echo-location to locate their burrows and detect prey including insects, slugs, snails and earthworms. It is reckoned that shrews can detect earthworms hidden up to 12cm beneath the ground and they are prepared to dig deep to get them. Shrews secrete a toxin from their salivary glands so a single bite is enough to paralyze small prey but their reputation for killing large animals with poison is unfounded. Shrews also produce a foul-tasting secretion through their skin which deters some predators and is the reason why cats often kill but decline to eat shrews.

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If seen well, shrews are quite easy to distinguish from other small mammals because of their long snout and very small overall size. In mainland Britain we have three species of shrew: the water shrew; common shrew and pygmy shrew. Water shrews are the largest of the group with very dark pelage on their backs, they are commonly found in ditches, ponds etc but also stray well away from water at various times.

Distinguishing between common and pygmy shrews is very difficult. Common shrews are larger than pygmy shrews (which have a body length of about 6cm, tail length of 4cm and weight of 6 grams), but size is difficult to estimate in the wild. Tail length is one factor to look for: the tail of common shrew is about a half of its body length, in pygmy shrews the tail is proportionately larger at about two-thirds of its body length. One other factor to spot is the pelage. Common shrews have chestnut flanks with a darker back and pale underside whereas pygmy shrews have just two tones, pale underneath and uniform dark brown on flanks and back.

The term ‘shrewish’ is one way in which the shrew has entered the English language. Meaning bad-tempered or aggressive this is probably a reference to the shrew’s tendency to bite and its sheer level of activity. Being such a small animal, it is forced to hunt for food almost continually. A shrew must eat about 90% of its body-weight in food every day and for a lactating female shrew this increases to at least 150%.

Common shrews are found in most habitats including gardens. They like hedgerows and tall grassy patches where they can hide effectively. Shrews are highly territorial coming together only to mate, they occupy burrows where three or four litters can be reared each summer. One of the most appealing images of a shrew, and one that I have witnessed only once in my lifetime, is that of a ‘caravan’ of shrews. That is a mother leading her family of youngsters from one place to another, each young shrew holding onto the tail of a sibling in front with its teeth!

To support common shrews in your garden:

• Leave areas of unkempt grass, particularly near scrub/hedgerows
• Don’t use chemicals to kill insects
• Encourage ground-living insects by creating habitat piles of logs and stones

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