The importance of small mammals

Peter Brittonin Conservation & Ecology

When discussing the UK's small mammals, it is difficult to know where to stop but, for the purposes of this article, I will concentrate on voles, mice and shrews; those above ground species that play an important role in the middle of the food chain.

Put in its simplest terms, the food chain is: grass > invertebrates > small mammals > carnivores and raptors, with the latter decomposing on death, in the process returning nutrients to the grass. It is this third rung on the ladder that I will concentrate on in this article.

Small mammals include mice, voles, shrews, moles, bats, hedgehogs and rats, with some of these having already featured in their own right within the pages of our magazine and online. A quick search of the Pitchcare website will find them easily enough.

So, I am going to focus on the smaller species that scurry about our wild areas and, on occasions, through domestic environs.

In a domestic environment, they can cause damage to property and spread disease. For example, a house mouse will happily chew through electric cables, whilst pooing on a basis that is well beyond 'regular'!

Compared to mainland Europe, the number of species resident in the UK is considerably less, comprising those that made it to our lands ahead of the creation of the English Channel (native), plus a handful of introduced species, such as the Edible dormouse and black and brown rats.

The rodents, i.e. those with prolonged front incisors, are the harvest mouse (the UK's smallest species), wood mouse or long-tailed field mouse, yellow-knecked mouse, house mouse, dormouse, bank vole, field vole and water vole. Two sub-species of vole live on the islands of Orkney and Guernsey. The three introduced species complete the small rodent line-up.

Although its external appearance is generally that of a long-nosed mouse, shrews are not rodents. They are, in fact, more closely related to moles and hedgehogs, being related to rodents only in that both belong to the Boreoeutheria Magnorder which comprises all placental mammals, including humans.

Shrews have sharp, spike-like teeth, not the familiar gnawing front incisor teeth of rodents.


Leaving aside the house mouse which, as its name implies, lives almost exclusively in buildings feeding on scraps, insects and spiders, the dietary requirements of mice is varied.

Mice are primarily seed eaters, particularly those of oak, beech, ash, lime, hawthorn and sycamore. They also eat small invertebrates such as snails and insects, particularly in late spring and early summer when seeds are less plentiful. They also consume berries, fruits, and roots and, if seeds or stones are plentiful on the ground, they carry them back to their nests/burrows for storage.

The gestation period of mice is around twenty to twenty-six days, and they give birth to a litter of up to fourteen young (average six to eight). One female can have up to ten litters per year, with the young often becoming pregnant once weaned. Females can remate whilst still feeding the previous litter, so it is clear that the mouse population can increase very quickly. Breeding occurs throughout the year for the wood mouse and between February and October in the Yellow-knecked mouse.

These mice do not hibernate, but can go into a state of torpor should the winter prove excessively harsh.
The dormouse is a different animal altogether. Not only is it strictly nocturnal, it is the only UK mouse that hibernates, giving rise to the sleepy dormouse of Alice in Wonderland fame.

Harvest mouse (left) and Field mouse

The average dormouse litter size is four and these are typically born in July or August, but litters may be born as early as late May or early June. Young dormice are weaned after about one month, but remain with the mother as juveniles before they become independent and disperse. They must reach a weight of between 15-18g to survive the winter hibernation. Dormice usually just have a single litter, but those that breed in early May be able to have a second.

The UK population is unknown, but there has been a long term decline in both number and range; recently there is an indication that the decline is slowing and, as part of an ongoing dormouse reintroduction programme, the current range is slowly being extended. The current dormouse range is Southern England, South Wales and along the border between England and Wales. Even where dormice are considered present, their distribution is patchy.

During the winter, they hibernate and are not normally active again until April or May. Thus, dormice may spend three-quarters of their year "asleep".


Two of the UK voles - bank and field - maintain above-ground runways, which expand like a railway-system through their entire home range. Voles are seldom seen outside these runways, which allow for a faster and safer journeys to and from the nest. Their climbing ability is very poor.

Underground nests are dug from anywhere from just under the surface to, in the case of the field vole, 30 to 40cm deep into the ground. Nests are used for food storage, offspring raising and as a place for rest and sleep. Nests can be shared and defended by up to five females, with juveniles that are related in most cases.

Vole runways and Common vole

Females are territorial and an overlap of occupied areas does not occur. As voles have a polygynous mating system, the males do not maintain territories and move as so-called 'floaters' between several females' territories in order to mate as often as possible. They can show overlap in territories. Males predominantly conduct dispersal, being most often caused by the competition for mates.

Water voles have been discussed in a previous issue of Pitchcare.


Shrews belong to the family Soricidae in the order Insectivora. They have a very high metabolic rate and seem to be on 'fast forward' all the time! They have an insatiable appetite for insects and worm and can devour their own body weight in a day. They will die within twelve hours if deprived of food.

The Water shrew is the largest of the three species with a distinctive dark grey coat and white belly. They are capable of delivering venom to their prey via grooves in their teeth. The poison is used to paralyse their prey and not as a form of defence.

Outside the breeding season, both male and female water shrews maintain a territory but, during the breeding season, only the females do so. At this time, the males wander about visiting various female territories which indicates a promiscuous mating system without pair bonding. On the whole, they are solitary animals that seem to mutually avoid each other and there is no social hierarchy.

Three to four litters of between four to eight young are produced each year.

Common shrew (left) and Pygmy shrew

The common and pygmy shrews look, on casual inspection, to be very similar, but the chestnut brown of the larger common shrew sets it apart from the smaller grey coated pygmy shrew. However, the latter's longer tail gives them an appearance of being of similar size to the common shrew.

Active by day and night, they are very territorial and aggressive for their size and can sometimes be heard fighting, their high pitched squeaks particularly noticeable during the summer months.

The breeding season lasts from April to September, but peaks during the summer months. After a gestation period of about twenty-four days, the female gives birth to a litter of five to seven babies. A female rears two to four litters each year. The young are weaned and independent within twenty-five days.

Young shrews often form a caravan behind their mother, each carrying the tail of its sibling in front with its mouth.

These shrews carnivorous and insectivorous diets consist of insects, slugs, spiders, worms, amphibians and, in the case of the common shrew, small rodents.

Conservation requirement

The importance of these little critters to the natural balance of wildlife should not be underestimated. As well as preying on many pests, they are natural spreaders of seeds with their propensity for pooing at an alarming rate. As they say, what goes in must come out!

Additionally, with their high reproduction rate, they offer a vital food source for such carnivores as stoats, weasels, pine martens, falcons, hawks, owls, foxes and the like.

Red fox (left and Pine Marten

In truth, most of the species discussed here require very little conservation effort, with the dormouse and water voles being exceptions. Whilst most species will make their home anywhere, it is helpful to leave fallen branches, provide log piles and leaf cover, not just for their habitats but also their prey.

Care should be taken to avoid cutting rough areas between April and September when most species will have young in the nest. Whilst adults have the speed and dexterity to avoid a rotary triple, their young do not.

Leaving wide field margins beside hedgerows provides cover and food, which will encourage and maintain populations. Long grass on roadside verges is also important. A varied woodland area will encourage small mammals and groups of branches should be left when clearing patches of ground.

In the case of the dormouse, coppice management of woodlands can create such perfect conditions; but cleared areas and wide rides may interfere with their movement, because they live almost exclusively in the trees. Surveys show dormice have declined in Britain this century. Loss and fragmentation of ancient woodlands, climatic difficulties and suspension of coppicing are all probably connected with this. Nest boxes, put up with the entrance facing a tree trunk, are attractive to dormice and help survival and breeding success.

Re-introducing the dormouse is often suggested, but this require suitable areas of woodland habitat and long periods of supplementary feeding.

What's the difference between a mouse, a vole and a shrew?

Mouse: very large eyes, long tail, very large ears, pointed snout.
Vole: small eyes, short tail, small ears, rounded snout.
Shrew: small eyes, short tail, small ears, pointed snout.

Harvest mouse (Micromys minutus)

The harvest mouse is the smallest rodent in Europe, weighing just 6g, but up to 35g pre-hibernation

It can be identified by its blunt nose, short, rounded hairy ears and golden-brown fur. Its tail is almost as long as its body.

It lives in long, tussocky grassland, reedbeds, hedgerows and around woodland edges.

Nests are spherical and made of tightly woven grass and are elevated from the ground in tall grasses.

It is mainly vegetarian, eating seeds and fruits, but will also eat invertebrates.

House mouse (Musculus musculus or Musculus domesticus)

The house mouse is one of the most successful mammals in the world and is found almost everywhere.

It has been domesticated for pets and also as laboratory mice.

It has dull greyish-brown fur, a pointed snout, rounded ears and a long naked or almost hairless tail.

Its tail is the same length as its body but it's thicker and scalier than the tails of other species of mice.

Wood or Field mouse (Apodemus sylvaticus)

Also known as the field mouse, the wood mouse is the most common rodent in the UK. It thrives in woodland, rough grassland and gardens.

Its fur is brown with a reddish tinge and a white or greyish underside. Its tail is roughly the same length as its head and body.

It may be distinguished from the similar yellow-necked mouse (see below) as it lacks a yellow collar on the chest.

It stores berries and seeds in the autumn in underground burrows or sometimes in old birds' nests.

Yellow-necked mouse (Apodemus flavicollis)

Yellow-necked mice live in extensive burrows which are also used to store food.

They can be easily confused with the more common wood mouse, and the two were only identified as separate species in 1834.

It can be distinguished from the wood mouse by its collar of yellowish fur, which forms a bib on the chest that can be quite difficult to see. The yellow-necked mouse may also be larger in general and lighter in colour.

It feeds on buds, seeds or small insects.

Wood mouse (left) and Dormouse

Dormouse (Muscardinus avellanarius)

Dormice occur mainly in southern counties, especially in Devon, Somerset, Sussex and Kent. There are few recorded localities north of the Midlands, though they are present in parts of the Lake District and in scattered Welsh localities.

The dormouse is a strictly nocturnal species, found in deciduous woodland and overgrown hedgerows. It spends most of its time climbing among tree branches in search of food and rarely comes to the ground. During the day it sleeps in a nest, often in a hollow tree branch or a deserted bird nest or nest box

Dormice feed on flowers, pollen, fruits, insects and nuts.

Bank vole (Myodes glareolus)

Bank voles are active day and night. It is the smallest UK vole with a reddish-chestnut coat and an off-white underside.

Like all voles, it has a blunt snout, small eyes and ears. Its tail is short; just half the length of its body.

At first sight, they can be confused with field voles, which are greyer with a shorter tail, or wood mice, which have a longer tail and move much more quickly.

They can climb bushes to feed on fruit, nuts and small insects.

Field vole (Microtus agrestis)

Also known as the short-tailed vole, the field vole is a very common species in grassland, heathland and moorland.

It is different from the bank vole in having a much shorter tail, shaggier fur and furry ears. It is usually greyish-or yellowish-brown with a pale grey underside.

This species is less likely to be seen than the bank vole as it spends more of its time in runs and burrows and can be aggressive to other trespassing voles.

It eats grass, seeds, roots and leaves

Water vole (Arvicola amphibious)

The water vole has suffered a serious decline in the UK with numbers dropping by 90%.

Sometimes known as the water rat, it is the largest species of vole in the UK and is sometimes mistaken for the brown rat. It lives around water: rivers, streams, ditches and ponds. When it enters the water, it makes a distinctive 'plop' sound.

Look for its glossy brown or black fur and blunt muzzle with small, black eyes. Its ears are rounded and almost hidden, and it has a dark, slightly furry tail.

Common shrew (Sorex araneus)

Common shrews have a short tail that is only half the length of its body. Its body is mainly dark brown with chestnut-coloured sides and grey or silver undersides.
It's a very active and fast-moving species and needs to eat every two to three hours. It scurries though the undergrowth in woodland and grassland searching for insects, worms, slugs, spiders and larvae.

Shrews don't hibernate, but they do become less active in winter, living in burrows that may have been made by other species.

Water shrew (left) and Common shrew

Pygmy shrew (Sorex minutus)

One of Britain's smallest mammals (alongside pipistrelle bats), pygmy shrew can be distinguished from the common shrew by its paler, grey-brown fur and a long, slightly hairy tail.

Like common shrews, they're quick and active and forage grass, roots, fruit, seeds and invertebrates.

It's a territorial species and is quite aggressive. It is known to swipe its tail from side to side if it encounters another pygmy shrew.

Water shrew (Neomys fodiens)

This is an elusive species and is rarely seen, but it is the largest species of shrew in the UK.

Its fur is dense and silky and is dark grey or black with a whitish underside and tufts of white around the eyes and on the ears. It has large hind feet and is the only shrew likely to be seen in water.

Unlike other species in Britain, water shrews have venomous saliva that is capable of paralysing prey such small fish and frogs.

With thanks to:
Woodland Trust
The Mammal Society
The Wildlife Trusts

Article Tags:
Conservation & ecology

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