Date: Thu, 3 Feb 2000 14:48:31 +0100 (MET)

This is partly a translation of a paper that appeared in Ottar no. 224: 3-10, 1999.

Karl Frafjord, Tromso Museum, University of Tromso, 9037 Tromso, Norway

The Shrews of Norway

Norway is a narrow country at the top of Europe, and part of the Scandinavian peninsula. It has a long coast line, and a wide range of habitats with gradients being both coast-inland, south-north, and low-high altitudes. Of the estimated 315 species of shrews in the world, only six are found in Norway. The shrew belong to an old group of mammals and has several "primitive" characters. Shrews are small mammals with short legs, relatively long tail, small ears and small eyes. The Norwegian name is "spissmus"; "spiss" means "pointed" and "mus" means "mouse" ñ drawing attention to the pointed nose of a mouse-like animal. Indeed, most people consider shrews to be just another mouse or vole, not aware of the fact that shrews are insectivores and not rodents. Rodents have powerful "rodent" teeth for eating plant material, the teeth of shrews are completely different and more unspecialised with long incisors pointing forwards. When fully developed, the teeth stop growing, they do not grow continuously as in rodents. At "old" age, the teeth may be heavily worn down, perhaps because of the hard, chitinous prey. The Norwegian species has reddish tooth tips, because the enamel contains iron that may protect against wear. All shrews are carnivores, depending mainly on animal prey (invertebrates), especially insects and earthworms.

This electronic "paper" gives a popular account of the Norwegian shrews.

Description of the Norwegian species

Among the Norwegian species are three common and three rare. The common ones are common shrew, pygmy shrew and water shrew. Masked shrew, dusky shrew and least shrew are all rare and seem to have a limited distribution. The six species belong to two genus: Neomys (1 species) and Sorex (5 species). In northern Norway, all six species exist (with a reservation for the dusky shrew), the masked shrew is absent from southern Norway. In most handbooks, a large variation in size and weight is given. This may be due to the fact that both young, non-mature, and adult, mature animals are included, and it is a confounding way of presenting size dimensions for shrews. Distinguishing between non-mature and mature shrews gives less variation and less overlap between species, and makes species identification more easy. When shrews mature sexually, their body also grow in size (especially females).

Common shrew Sorex araneus

This is probably the most common and numerous mammal of Norway. It can be found in all sorts of habitats, from south to north and from lowland to alpine regions, and is in some years very numerous. It weigh normally 6.5-8 g, mature females 10-12 g. Average body length for non-mature common shrews is 59 mm, tail length 49 mm. The common shrew is the second largest shrew of the genus Sorex, and may dominate the smaller shrews. Consequently, it may exclude smaller species from it's range, and perhaps limit their distribution. Earthworms are a favour food. The common shrew is characterised by a large head, and a tail shorter than the body. It's back is dark brown (but varies), the belly is greyish. Along the body sides it often has a narrow, yellowish band. It's winter fur is longer with an almost black dorsal side, and no yellow along the body sides.

Pygmy shrew Sorex minutus

The pygmy shrew is also widely distributed in Norway, but is rarely found in great numbers. Some places in south Norway it has been found in greater numbers than the common shrew, but normally it is much more rare. The pygmy shrew is perhaps unable to compete with it's larger brother, and may have to subsist in poorer habitats. Going from coast to inland at the west coast of south Norway, a diminishing proportion of pygmy shrews compared to common shrews was found. The pygmy shrew weigh normally 3-5 g, with body and tail lengths for non-mature animals of  46 and 42 mm, respectively. It is characterised by a relatively long head compared to the body, and a tail that is about as long as the head-body length. The fur is short, with a greyish brown dorsal side and a greyish ventral side. The back is somewhat paler than in the common shrew. The fur on the tail is bristling and ends in small tuft of hair. The pygmy shrew eats a variety of small insects and arachnids.

Water shrew Neomys fodiens

The largest Norwegian shrew is easy to identify, with a dark, black back and a white ventral side. A long tail is also an easy character, almost as long as the head-body length. Both tail and feet have a rim of stiff hairs that improves swimming. The water shrew lives along small rivers, but can sometimes be found far away from water. It dives and catches insects, crustacean, and other invertebrates, and may also include a small tadpole or fish in it's diet. It may also hunt on dry land. The water shrew is probably distributed all over Norway, but is rarely seen in great numbers. It's saliva is slightly venomous.

Masked shrew Sorex caecutiens

The masked shrew appears to be relatively common in the most northeastern part of Norway (Finnmark county), where most specimens have been found in Pasvikdalen. Further south there exist a few records in Troms and Nordland counties, but the exact distribution of the species is unknown. It is a medium sized species, normal weight is 4-6 g, with a brown dorsal side and pale (whitish) ventral side. Ventrally, and on feet and tail, some parts can be somewhat creamy white or yellowish. It has a tuft of hairs at the end of the tail, larger than in the pygmy shrew. The masked shrew is foremost an eastern taiga inhabitant, but may also be able to live in the alpine region. In some parts of Siberia, it may be more common than the common shrew. It eat mainly insects, but appears also to eat some seeds.

Dusky shrew Sorex isodon

Few dusky shrews have been found in Norway, most in the southeastern part. Only one record (three captures) of dusky shrews from north Norway (Troms county) exist, but this population could not be verified later. The dusky shrew is the largest of the genus Sorex, and is common on the Russian taiga. It is very similar to the common shrew in appearance, but slightly larger with a darker ventral side. The dusky shrew has bigger feet than the common shrew, the hind limb measure is more than 12 mm (13-15 mm). Subfossil records of the species have been made in caves in Nordland county (7000-2000 years old).

Least shrew Sorex minutissimus

The least shrew is the smallest mammal in Norway and among the smallest in the world, weighing 1.5-3 g. Has been recorded only about 13 times in Norway (some of these records may be questioned), and only once in north Norway (Troms county). All over it's distribution in northern Eurasia, it appears to exist in very small numbers. Newly recorded in North America, this population could be a different species. It's small numbers may be due to a poor trapability, making it difficult to record. The least shrew is characterised by a rounded head, a short but broad snout, and a short tail. Hind-limb length is shorter than 9 mm. Colouration on the dorsal side is similar to the common shrew, but the ventral side is paler. It has been found in a variety of habitats in south Norway, and may possibly prefer more humid habitats. Very little is known about the least shrew and it's whereabouts.

High metabolic rate and short life

All shrews have generally the same basic architecture, and a characteristic appearance that make them easy to recognise from other mammals. Still, a large number of adaptations exist, and many species are quite capable of environmental adaptation. Shrews are plantigrade, running on their whole hands and feet. Despite short legs, they can run at quite a speed. The head and skull is narrow and the zygomatic arch is missing. The snout and mandibles are long and narrow, working almost as forceps to pick up small prey. The brain is small, but has comparatively large frontal lobes associated with a good sense of smell. Small eyes indicate short-sightedness, the sense of smell is probably more important than sight in locating prey. Most shrews live on or under the ground, searching through the vegetation for food. They can use the passages of small rodents.

Vocalisations are not important to most shrews, and are mostly heard during fights. High-frequency sounds barely detectable to the human ear is produced, and may also work as a primitive form of echo-orientation. Norwegian shrews do not smell very well, they have (to us) bad-smelling glands along the body sides and in other places on the body. Secretions and the sense of smell is probably important in social interactions, but these secretions may also be distasteful to some carnivores. Few mammals actually eat shrews, although they may kill them as the domestic cat often do (often presenting them to it's human room mate). But some birds (especially owls) may capture and eat shrews.

Shrews are always hungry. They have a very high metabolic rate, and many species must eat their own weight daily to survive. They are constantly looking for food, and may survive no more than 2-3 hours without food. Shrews are active in bouts throughout the day, both summer and winter. It is amazing that they are able to find enough to eat and to survive the long and cold northern winter, even if they are protected by the snow. None of the Norwegian shrews are known to be able to enter torpor to conserve energy during resting periods. In the subnivean layer, shrews may hunt dormant invertebrates, in addition to a few species that may also be active in winter. They also readily eat dead animals, including their own conspecifics. Shrews live a short, but busy, life, the heart rate can be 1000 beats/minute. Few live longer than 1.5 year. "Old" shrews are recognised from their worn pelage, with almost naked tail and feet, and from their worn teeth. In the common shrew, a reduction in the size of the mandible has been found in northern compared to southern Norway. A reduced size may reduce metabolic needs under worse climatic regimes.

Life history

A shrew is born during the summer, survives it's first winter if lucky, reproduces the second summer, and dies during the next autumn. A general life history may look like this:

It is born during the summer, leaves the nest and the mother fully grown-up about three weeks old, and lives alone in it's own territory during the autumn and winter. During the next spring it grows in size and becomes sexually mature. The male leaves it's territory searching for females. The female gives birth to 1-3 litters (in north Norway perhaps no more than 1-2 litters) during the summer. During the autumn or winter it (the adult generation) dies.

This story is not absolutely correct. Some shrews may breed in their first summer, and some may survive two winters. But all in all, there is relatively small overlap in time between different generations. In southern areas the first litters are born in May, but this is most likely later in northern Norway. The female may mate again shortly after giving birth, and can be both pregnant and lactating at the same time. Pregnancy last about 25 days in the common shrew. The female builds a nest from leaves and mosses where the young are born. Normal litter size is 5-7 young. The young grows quickly, and common shrews are weaned and leave their nest 22-25 days old when they are grown-up. If disturbed, the female may move it's litter to another nest. Caravanning behaviour has never been witnessed in Norway, although the common shrew may be capable of doing this.

Shrews are vulnerable to changes in environmental conditions, and are sometimes exposed to high mortality. During the summer they may proliferate and increase rapidly in numbers. Consequently, populations may fluctuate much in numbers, both within and between years. It is not clear whether, or how, these fluctuations are related to the similar fluctuations in small rodents at these latitudes. By late summer, or autumn, a large number of young shrews (esp. common shrews) may be roaming about searching for an empty territory. The pygmy shrew may have a larger territory than the common shrew, perhaps because it is being displaced from the richer habitats by it's larger brute. In poor habitats, a larger territory may be needed to sustain an animal. In Norway, shrews are particularly common in rich and varied forested habitats (not spruce plantations).

Shrews and man

Shrews are widely distributed and may occur in large numbers. Because they feed almost exclusively on animal prey, they are higher up in the food chain than small rodents that feed mostly on plant material. This expose them more to potentially dangerous contaminants, that accumulate through the food chain. Pesticides and metals from prey animals may accumulate in the shrew's body, and could affect it negatively even if not mortal. Growth, reproduction, or survival might be reduced. For the same reason, shrews can possibly be used as indicator species for certain anthropogene chemicals in nature. Changes in the number and distribution of shrews, as well as the proportion of species, may also be used as indications for changes in their environment.

In Norway, shrews are rarely found in buildings, and they are neither harmful to us nor the building. To the farmer they should be useful
animals, because they may eat insects that are harmful to the crop. However, as most animals, shrews have difficulties on intensively farmed land, and can only survive along it's fringes. Likewise, tree plantations are not good shrew habitat. A large part of the richest habitats in Norway are occupied by humans, either for farmland, tree plantations, roads, cities, etc. Corridors of natural habitat in farmlands and more variation in the landscape would make it easier for shrews to survive and disperse. The water shrew is particularly affected by human use and misuse of waterways, in addition to pollution and acidification.

Shrew conservation in Norway is mostly related to conserving habitats and to stop pollution. None of the six species can be classified as endangered or threatened by extinction, although lack of knowledge is a problem. The three common species (common, pygmy and water shrews) are probably in no direct danger, although the water shrew is likely to suffer along many streams. Little is known about the three rare shrews in Norway (masked, dusky and least). They may be more widely distributed and more common than the few records indicate, but no conservation measures can be initiated until more knowledge exist.  These three species belong to the "Siberian fauna element", and Norway is only at the fringe of their world distribution. The very small least shrew is an intriguing species, but appears to be very difficult to study.

Karl Frafjord                      Tel. (+47) 77 64 50 00
Tromsø Museum                      Fax  (+47) 77 64 55 20
University of Tromsø
N-9037 Tromsø, Norway     e-mail: karlf@imv.uit.no
http://www.imv.uit.no/ommuseet/enheter/zoo/karlf/index.html
 

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