Article By Russell Tofts

Other Names

N.B. Most of the names below refer generally to the continental form, the European Common Vole (Microtus arvalis), and are not specific to the Orkney race.

Common Vole, Continental Field Vole, Grass Vole, Meadow Mouse (erroneously), Orkney Field Vole.

Introduction

A subspecies of the Common Vole, this is one of four species of British vole (the others being the Bank Vole Clethrionomys glareolus, Field Vole Microtus agrestis, and Water Vole Arvicola terrestris). The animal has much to commend it, although to describe it, as I have done, as a British species is stretching the truth somewhat because it is found wild only on Guernsey and on various islands in the Orkney archipelago. Its small size means it is unsuitable for young children, but it is friendly and easily tamed, adaptable, active in short bursts during both night and day, easy to keep, and never (in so far that one should never say never) bites.

Stein (1958) was the first to place both the Orkney Vole and the closely allied Guernsey Vole (Microtus arvalis sarnius) as subspecies of the European Common Vole, rather than as separate species, and recent chromosome analysis supports this view, although it remains far from clear when they divided. Its presence in Orkney is an enigma, as the species is not found anywhere else in Britain except here and on the Channel Island of Guernsey, and yet is widespread throughout continental Europe.

Curiously, the voles of Orkney seem to share more affinities with the voles of the Balkans and France than they do with the geographically closer ones of northern Germany. (The part of the Continent nearest to the Orkney Isles is really Norway, but the Common Vole is not found here. The Guernsey Vole, conversely, shows more similarities with the voles of Germany.) Almost certainly it was introduced to Orkney by Neolithic* Man, as evidence found at Skara Brae and other Neolithic sites suggests.

* In Europe the Neolithic period, characterised by primitive crop-growing and stock-rearing and the use of polished stone and flint tools and weapons, lasted from about 4000 to 2400 BC and preceded the Bronze Age.

Size

Head/body length: 114 mm (range 90-135 mm); tail: 38 mm (range 27-45 mm). The tail constitutes about 30% of the overall head/body length. Body weight: 14-67g. The wide disparity in the weight is due to differences in age and sex, with males being approximately 10g. heavier than females.

Similar Species

The Field Vole (Microtus agrestis) has a darker coat, with noticeably hairier ears. At the anatomy level, there is a posterior loop on the second upper molar, and the skull is not as strong as with M. arvalis. The Snow Vole (Microtus lebrunii) is larger and lighter in colour, and the Gunther's Vole (Microtus guentheri) is also considerably larger. Whilst these other European voles are superficially very similar in appearance to the Orkney Vole, none is found wild in Orkney, and consequently there is no danger of confusion arising in the field.

Distribution

The Orkneys comprise a group of over 70 islands off the north-east coast of Scotland, separated from the mainland by the stormy waters of the Pentland Firth, a channel up to 13 km (8 miles) wide and 55-74 m (180-240 ft) deep. Voles are present on all the larger islands except Hoy and Shapinsay. It is possible to tell from which island each animal originates by the colour of its coat. Those from the three southerly islands of Mainland Orkney, South Ronaldsay and Rousay, have a deep brown coat, whereas those hailing from the two northerly islands of Sanday and Westray are paler in colour. Those from the three southerly islands of Mainland Orkney, South Ronaldsay and Rousay, have a deep brown coat, whereas those hailing from the two northerly islands of Sanday and Westray are paler in colour. The Westray voles are also smaller for some reason. It is unlikely that captive specimens are pure-bred but probably have their provenances on several different islands.

Natural Predators

There are no mammalian carnivores found wild in Orkney except for feral cats but a number of avian predators are present, including Buzzards (Buteo buteo), Hen Harriers (Circus cyaneus), Kestrels (Falco tinnunculus), and Short-eared Owls (Asio flammeus), amongst others.

Natural Habitat

Owing to shallow soils and the gales that regularly whip the islands, the low-lying Orkneys are virtually treeless. Where inclines do exist, some voles are found at elevations of up to 700 feet, although on the whole they prefer lower ground. Soils here do contain some peat but many are naturally lime-rich and most are continually being enriched from lime-rich seashore sands. Although lying nearly as far north as Greenland, the climate on the islands is surprisingly mild for most of the year on account of the North Atlantic drift current in the ocean. The average annual temperature is a relatively balmy 45° F. and the rainfall averages a mere 35 inches per annum, with most falling as a fine drizzle. There is a long summer photoperiod due to the northern latitude, a considerable assistance to the voles' breeding season.

The vegetation cover in the Orkneys is better than in the Shetland Isles because the local rock, mostly Old Red Sandstone, weathers down easily to form good quality soil, ideal for agriculture which, not surprisingly, has been the mainstay of the islands' economy since prehistory. This species prefers man-made environments. It favours cultivated grasslands such as fields, meadows and pastures, being especially fond of clover fields, but is also found in ditches, embankments, hedgerows, marshes, moors, and gardens. In years of overpopulation the species spreads along man-made arteries, such as paths and railway lines, to colonise new areas.

Social Structure

One of the most peaceable members of the genus Microtus, Orkney Voles are capable of living in large colonies without showing any aggression towards fellow members, even though the species cannot be said to be truly social. I have many years' experience of keeping Orkney Voles, but cannot recall any instance of an animal being attacked by another. In this important respect they differ from many other species of vole which will live in colonies of hundreds of individuals but are strangely unco-operative and aggressive towards each other. In its natural habitat the population density depends on a number of factors, including fecundity, social spacing, amount of vegetation cover, and level of predation.

Behaviour

Active all year-round and does not hibernate. The Orkney Vole is both crepuscular and nocturnal in its habits, but, even in its natural habitat, is occasionally prone to short bursts of activity throughout the day. Conditioned captive specimens are often active on dull days. There is a 2-3 hour cycle of resting and foraging, with longer and more frequent resting periods during daylight hours. Slow-moving most of the time (unlike, say, the Bank Vole Clethrionomys glareolus) and therefore quite easily trapped, it can nonetheless run quickly when the needs arises, its belly almost in contact with the ground, but with many frequent pauses. Stands erect on its hind legs to scan surroundings for possible danger, a behaviour which admittedly I have not seen performed by captive specimens. Hardly ever jumps or climbs but an able swimmer and even capable of short dives. Orkney Voles rely on a network of conspicuous surface runways, especially visible through heather, that connect up a number of burrows just beneath the surface. The tunnels are often quite short, about 6 cm in diameter, terminating in nest-chambers and food larders. In winter the voles build nests of dry grass just below the snow cover. The home range is about 1000 square metres or less.

Vocalisation

Mostly silent but, when upset, angry or frightened, emits a high-pitched squeak, like a bird chirp, usually uttered just once but occasionally repeated several times, even as a continuous vibration and, if extremely agitated, will also gnash its teeth. In an exposed location, it follows this up by "freezing", so as not to draw attention to itself in the hope that the danger will pass, but usually if there is any sort of cover in the vicinity it just flees.

Sexing

The mature adult male can be distinguished by his larger body size and the presence of an obvious scrotal sac. He is heavier than the female and possesses a scent gland on his rump. The ano-genital distance is approximately twice as wide in the male. Females possess eight mammae, but these are not obvious except very occasionally when the female is nursing.

Accommodation

A large glass tank with a secure, well-ventilated lid is best. If the ventilation is inadequate, unacceptable amounts of condensation will collect on the inside of the glass. Since this species spends so much of its time burrowing, it seems unfair to deny it this basic activity, and a deep layer of wood-chips or peat substitute should be provided. In this environment it is usually futile to try to provide "toys" and other furnishings because they soon get submerged, but it is always a good idea to place some plastic or earthenware pipes and cardboard tubes in the tank, as well as a few twigs and branches for gnawing, and a wooden nest box (a budgerigar nest box is ideal) filled with a liberal quantity of dry hay.

Feeding

In the wild, Orkney Voles eat the leaves, stems and roots of a wide range of grasses, sedges and herbaceous plants, as well as (when available) berries, bulbs, fungi, lichens, nuts, seeds, tubers (an excellent source of moisture), and even twigs and bark when other foods are scarce. Surplus food is stored for winter use. When the population is high, voles occasionally become a crop pest due to their penchant for grain and hay. They are particularly fond of the thick roots of the Heath Rush (Juncus squarrosus), known locally as Moss Rush or Goose-corn, a common plant of acid moors and barren sandy or peaty heaths and drier situations than most of the Rush family.

Voles in general are strictly vegetarian, but have been known to take small insects occasionally. I know of at least one breeder who regularly offers his voles meal-worms, which they appear to relish. In its natural habitat Orkney Voles have been known to consume their own weight in food each day. Their coarse herbivorous diet comprises cellulosic food which is difficult for any animal smaller than an elephant to digest and yields little of any nutritional value, forcing them to eat a lot in order to derive any benefit from the diet. Grass, though indispensable in the diet, is tough and difficult to digest and the cells are protected by a layer of silica, and consequently it passes through the digestive tract too quickly. In captivity, where the food is more concentrated and the animals are less active, their propensity to overeat can, if one is not vigilant, lead to their becoming overweight.

My colony receives a fairly bland diet of a good quality rodent mixture (one that is low in peanuts, sunflower seeds and other highly fattening items), to which is added fresh grass (complete with roots if possible), meadow hay, and a few pieces of chopped apple or carrots, fresh greens, or other raw vegetables or fruits. They can also be given rodent pellet, canary seed, rolled or whole oats, and canned or dry dog food. Wood gnaws are essential, as they are with all rodents. I feed them just once a day. I always pull the grass from my own garden where I know it to be uncontaminated by chemical sprays, car exhaust fumes, or dog urine. Never collect from the side of the road or other areas where the quality of the grass cannot be guaranteed, and do not use grass trimmings that have passed through a mower because of the danger of the trimmings being tainted with oil. They are the only species of vole I keep to which I regularly give apple, as they have always received it and have suffered no ill effects. Second only to the grass, it is the food item to which they are most partial. Species of vole that I have "taken on" more recently would not have been used to apple and I am concerned their digestive systems might not tolerate this sugar-rich (and, in excess, quite fattening) addition to the diet.

Voles succumb very quickly if deprived of water for even a short while. It is usually no good attempting to provide water in an open dish; because of the voles' constant burrowing activities, they will soon deposit wood shavings or peat into the water, which will leak out due to capillary action and saturate the substrate. This is disastrous because voles cannot tolerate damp conditions for long. Even placing the bowl on a brick to keep it clear of the substrate proves effective only for so long. The best solution is to use a plastic or glass drinking bottle (the latter are superior but are getting increasingly difficult to find).

Reproduction

No mammal is known to be more prolific than the 65 or so species of meadow voles of the genus Microtus, and, not surprisingly, the Orkney Vole breeds freely in captivity. They are polyoestrus. There is a postpartum oestrus. Orkney Voles give birth 3-7 times per year from April to September, although some limited breeding is not unknown as early as February if the weather is mild and food is plentiful. Generally, however, reproduction is governed by day length. The optimum day length appears to be 12-14 hours. In a recent study, females kept under a regime of 12 hours of light and 12 hours of darkness took 42 days to mature, while those kept under 14 hours of light and just 10 hours of darkness matured in just 17 days. Voles kept in dimly lit cages do not seem to breed as prolifically as those kept in well lit cages. Originally I had two separate colonies. The smaller colony, consisting of 4-6 animals, lived in a large glass tank with no additional lighting. In this environment the voles bred, but not regularly and never predictably. The larger colony, on the other hand, consisting of up to 20 individuals, lived in a much smaller enclosure but with a fluorescent tube in the lid that provided bright light for 10½ hours a day, lulling them into thinking it was high summer all year round. This colony bred prolifically and it seemed as if there were almost young animals in the nest. The nest consists of a rounded chamber of grass and roots is built slightly above or a little way below ground level. Litter size varies from 3-6, but in exceptional cases can be as many as 10. Litter size peaks in May. Orkney Voles are very prolific and numbers can increase rapidly under favourable conditions to reach almost plague proportions in some years. Females increase energy consumption by up to 250% when nursing young. Birth weight is 2 g. Eyes open at 9 days, and the young are weaned at 12-21 days. Sexual maturity is attained at around 25 days for females; and 45 days for males.

It is rare that the hand-rearing of offspring needs to be contemplated, as the mother is usually very good at rearing the young by herself. If, however, a nursing female should die before the young are weaned, it is worth remembering that a different but related species, the American Meadow Vole (Microtus pennsylvanicus), was successfully reared using Esbilac®.

Parasites

Wild voles are hosts to a wide range and often large numbers of ectoparasites, including fleas, ticks, lice and mites, but, with the exception of ticks, which are relatively large anyway, are not always obvious except in sick or debilitated specimens, unless the parasite burden is exceptionally heavy.

Conservation

Although numbers appear to have declined in recent years, it is still very numerous (estimated to be in excess of one million individuals at the onset of the breeding season). Given its extremely restricted range, it probably cannot increase to any great extent. The slight decline noted recently is of little immediate concern as several other species of voles experience similar, often quite dramatic, fluctuations in numbers every few years. But even before the recent decline, it was surprising how seldom one saw a vole in the Orkneys. A colleague of mine, who has a house on Rousay to which he retreats for about ten weeks each year, claims to have seen the species only twice in ten years, and one of those was dead in a cat's jaws!

Life Span

On average about 1 – 1.5 years, although under optimum conditions in captivity they do occasionally live longer. One captive specimen died in 1994 at the age of 2¾ years.

The Enigmatic Voles of Orkney

A biological enigma which continues to defy a rational explanation is the presence in Orkney of a species of vole found nowhere else in Britain apart from another small isolated population on the British Channel Island of Guernsey. Archaeological excavations in caves at La Cotte de St. Brelade on Jersey reveal that an almost identical form had once inhabited this island as well but was now extinct, possibly because of competition from the Jersey Bank Vole (Clethrionomys glareolus caesarius).

The species is widespread and familiar elsewhere in Europe from Denmark and the Baltic coast south to northern Spain and eastwards to the Ural Mountains, being absent only from the Mediterranean peninsulas, southern France, Scandinavia and Brittany, as well as mainland Britain and Ireland. Possible artificial introductions are found on the islands of Yeu (France) and Spitsbergen.

In Orkney, although it has been decreasing slightly in recent years, it remains a common animal estimated to number over one million individuals at the onset of the breeding season.

Bones recovered at the locality of Quanterness show that the animal has been resident in Orkney for at least 5,500 years and differed little in size from voles living there today.

As long ago as 1805 the Reverend George Barry, in his pioneering work "History of the Orkney Islands", described in detail an animal known locally as the Vole Mouse. In a passage containing a respectable amount – much of it surprising accurate – of zoological information (a remarkable feat when one considers it was penned some two hundred years ago at a time when most people were not interested in small, rather obscure, Hebridean rodents), he wrote that the animal 'is very often found in marshy grounds that are covered with moss and short heath, in which it makes roads or tracks of about three inches in breadth, and sometimes miles in length, much worn by continual treading, and warped into a thousand different directions.'

The people of Orkney were, of course, well aware of the existence of this animal, but this was the first time (as far as is known) that the Orkney Vole had been recorded in literature. Like everybody else, however, the Reverend Barry failed to recognise its uniqueness. The realisation that it was a different species to other British voles did not strike home for over 80 years, and was eventually provided by the son of a Pre-Raphaelite painter.

J.G. Millais was a British naturalist, big-game hunter, and soldier. Returning home one evening in August 1886 from a fishing expedition in Orkney, he noticed what he thought at first was a Water Vole (Arvicola terrestris) running along a sheep track, a species he was pretty certain did not occur on the island. Intrigued, he set about trapping some specimens for closer examination, eventually declaring the animal was unlike any other British vole and pronounced it to be a "new" species, naming it "Microtus orcadensis", the Orkney Vole.

Later collecting trips revealed similar, yet slightly different, voles occurring on several other of the Orkney Isles. These were all accorded subspecific status depending on the island where they were found, even though the phenotypic differences between the different populations were not as pronounced as first thought. The subtle distinctions between each population are not thought sufficient, by modern standards, to warrant splitting them into so many different subspecies. The five races recognised at the time (but not anymore) were: M.o. orcadensis (from Mainland Orkney); M.o. ronaldshaiensis (South Ronaldsay); M.o. rousaiensis (Rousay); M.o. sandayensis (Sanday); and M.o. westrae (Westray).

Although it was clear that all the voles bore a close resemblance to the European Common Vole, only as recently as 1959 were cross-breeding experiments conducted that finally confirmed what was already suspected: that the Orkney (and Guernsey) Voles were not distinct species as had once been thought, but merely geographical varieties of the Common Vole. As had been predicted, voles from German and Orkney readily interbred to produce fertile offspring. Today all voles found in the Orkneys, irrespective of island, are lumped together under one broad banner as Microtus arvalis orcadensis, and the Guernsey Vole (originally Microtus sarnius) has been downgraded to Microtus arvalis sarnius.

Initially it was thought that this vole must have been found all over Britain at one time, but had probably been unable to compete with the more feisty Field Vole (Microtus agrestis) and had vanished and now survived only on such islands that this latter species had not penetrated. The popular view was that the ancestral voles had reached Orkney by means of a land bridge, long since submerged by the sea, that had once connected the islands to the Scottish mainland. Ignoring the fact that there is no fossil evidence to suggest that the species has ever occurred on the British mainland, Martin Hinton, one-time rodent taxonomist at the British Museum (Natural History), agreed that the vole was a relic from the Pleistocene era, and suggested that it had existed in Orkney since pre-glaciation times. This was the view endorsed by Dr Harrison Matthews who hypothesised that Microtus corneri, extinct in Britain since the late Pleistocene, was ancestral to M. orcadensis.

The last ice age finally came to an end 10,000 years ago. But did the animal really inhabit these islands so long ago? What Martin Hinton failed to appreciate was that there had been more than one glaciation (the exact number is still being debated, but it is known for certain that there have been at least three, and possibly as many as seven or eight, separate ice ages, with some authorities asseverating that up to fifty is not an unreasonable assertion, but much depends on the exact definition of what makes an ice age). The first ice age in Scotland occurred 600,000 years ago. Since then there have been at least two more. The most recent had two maxima of glaciation, the first of which occurred 55,000 years ago. After that there were warmer periods but, 25,000 years later, the ice began to advance once more, reaching maximum glaciation between 20,000 and 17,000 years ago. Warmer summers followed and the ice sheets gradually retreated again, although there were three re-advances, the last occurring about 11,000 years ago (practically yesterday in geological terms).

Hinton also underestimated the climatic severity of an ice age. It would have been almost impossible for such small animals as voles to survive prolonged periods of ice or to find enough to eat during such times. On the Continent its range does not extend into the more northerly countries, suggesting it is not as hardy as the Field Vole. Much more likely is that it arrived some time after the last age had ended.

This raises the question of how? as the land bridge theory has recently been discredited since it is now known that, following the retreat of the glaciers, the sea level around the top of Scotland was much higher than it is today, making the existence of a land bridge in post-glacial times an impossibility. Furthermore, there is no fossil evidence that the species was ever found on the British mainland.

So how did it arrive on those remote islands, and from where, if not from mainland Scotland? The current theory is that voles were introduced by Neolithic farmers. The windswept Orkney Isles, then as now, had only limited natural resources, necessitating frequent imports of essential commodities, often from Continental Europe, and this would have provided ample opportunity for animal hitchhikers to colonise the islands, although historically voles were much less likely to stowaway on boats than rats or mice.

But whatever the provenance of this vole, there is one thing on which all zoologists agree – it has the strangest distribution of any British mammal.