Nature 2011 June

The Untamed Shrew, the Acrobatic Mouse and the Gardening Vole

All around us an almost hidden community of small mammals live out their brief lives at a frenetic pace. Despite their diminutive size shrews, harvest mice and voles are as much a part of the natural history of the Chilterns today as their larger and more obvious cousins. They were also among the first mammals to occupy this ancient landscape.

Although abundant, shrews remain almost invisible even though they are often but a few yards away from us in our gardens, in hedgerows, fields and woodland. Meanwhile, harvest mice may have a high-rise lifestyle but remain largely unseen camouflaged within the hedgerow or arable field. Voles engineer their concealment travelling through meadows and hedgerows within well-trodden high-speed grass tunnels. The high energy lifestyle of all three demands round the clock foraging and, though hard to hear, it is the sounds of their high-pitched calls which might betray them. By remaining silent and still, we have the best opportunity to locate and observe them.

We have three species of shrew in our midst: common, pigmy and water. All are identified by their long pointed nose, small ears, pinhead eyes, red teeth and dense velvety fur with distinctive dark and light brown patches and whitish underneath. Shrews are the most short-lived of the small mammals, living maybe only for three months. Common and pigmy shrews live amongst leaf litter in hedgerows or woodland. Water shrews enjoy a semi-aquatic lifestyle near clean water streams and ponds.

It is estimated there are over 41 million common shrews in England but, unlike the Victorian ideal of small children, their piercing cry ensures they are rarely seen but more likely heard. Occupying small burrows deserted by larger mammals, they scurry beneath the leaf litter, along well-trodden paths. Diet comprises insects, earthworms, slugs and snails. Water shrews favour shrimps, frogs, newts and even small fish. Shrews mate in April and, being promiscuous, females give birth in September to a brood sired by several males. If very lucky the young may be seen keeping line astern to their mother forming a ‘caravan’, each youngster grasping in their teeth the tail of their predecessor.

The shrew is an important source of food for kestrels, tawny and barn owls, weasels, foxes and stoats. Water shrews also have to contend with pike. Domestic cats will catch but not eat shrews: instead they present their prize to their owners, not as a gift but because of the unpleasant taste.

In contrast to the shrew’s athletic speed, the harvest mouse is the acrobat of the small mammal community. Weighing in at just four grams, it is the smallest rodent in Britain. The prehensile tail – the only one outside ‘New World’ mammals – provides anchorage and balance and is essential given its aerial lifestyle. Harvest mice may live up to three feet off the ground in a nest of hay suspended precariously across a cleft stem or stout reed stalk.

Uniquely, outside the primates, they have opposable toes on their broadened hind feet which allow them to climb up narrow stalks and forage upside down using their forelegs to gather food. They are adorned in yellow and russet coats to match their surroundings, with blunt faces and largely hairless ears and tips to their tails. These mice are slightly longer-lived than shrews: spanning two seasons. With up to three litters of around six young a year between late Spring and Autumn, harvest mice don’t have time to offer much parental care. The young are born blind and hairless and grow very quickly, exploring outside the nest within two weeks, and are abandoned to fend for themselves by the end of the third week. As with shrews, their pace of life comes at a cost, demanding total commitment to foraging for high energy food: from nectar to berries, as well as grain, moss, fungi, roots and even insects.

Bank, field and water voles have made something of a comeback in the Chilterns. Voles are prolific breeders with four or five broods a year and their population fluctuates across the seasons, rising tenfold over short periods. However, they remain sensitive to the extremes of weather, the seasonal scarcity of food and, in the longer term, pollution and landscape change. In contrast to shrews and harvest mice, voles have rounded faces and spiky fur. In an identity parade the ones with furry ears are voles.

Voles use their large eyes to forage for food but deploy their smart scent detection to identify friend from foe. Though mainly consuming berries and seeds, soft fleshy fruits and the leaves of herbs, shrubs and trees, they are opportunists and will not refuse snails or insects. If we see a vole in our garden it will most likely be a bank vole. The field vole likes meadow grasses, specialising on the bents and fescues. The water vole prefers the chalk streams to our hilltops.

Some events in nature remain unchanged it seems. Like the Swiss clock with its eponymous name the first cuckoo started calling here again this year on 17 April. Meanwhile, for the first time in several years, the rabbit population seems to be on the rise again. Returning late at night I’ve started to see them, frozen in the headlights. They are also appearing brazenly in some gardens during daylight.

Another unwanted invasion which might be steadily gaining hold at the moment is from Japanese knotweed, which has established itself in a few places around the villages, possibly where there has been recent ground clearance or perhaps where garden waste has been dumped on the roadside. The crossroads at Heath End and behind the Hill Fort close to the site of the old brickfields are two such examples. Once it takes hold it will quickly advance, suffocating any other plants in its reach and eradication can become complex and expensive. It’s as well to strike back sooner rather than later while one has a chance to conquer it.