Sunday 19 May 2019

Small British Mammals: Common and Field Voles

Field vole
In many respects, voles are extremely similar to mice. In many languages, the local word for 'mouse' also describes voles, and this was even the case in English prior to around the 19th century, when the term "vole-mouse" was borrowed from a now-extinct Scandinavian tongue, and shortly thereafter abbreviated to simply "vole". In its original language, it literally meant "field", and a similar etymology can be seen in languages such as German (Feldmaus) and French (campagnol) among many others.

Scientifically, voles have also been mixed in with the mice at different times. While they have recognised as a distinct group with their own scientific name since 1821, for much of the 20th century they were considered a subgroup within the wider mouse family. That's really only changed in the last 20 years or so, as genetic and biochemical evidence has shown that the two groups, while related, have distinct evolutionary histories.

Nonetheless, there are differences between voles and mice that don't require a close examination of their genetic makeup to determine. However, they do tend to be general, and a little vague - voles typically have proportionately smaller eyes and ears than mice, shorter and hairier tails, and a more rounded body form. A more reliable indicator is to take a good look at their teeth, since voles typically have only two cusps on each of their molars, rather than three as mice do. But, even leaving aside the fact that this isn't exactly an easy thing to determine for most people, it isn't 100% reliable for all species, anyway.

Even so, voles are not mice (as currently defined) and there are, in fact, more species of them in Europe than there are members of the mouse family. Britain, being separated from the mainland, has rather less, with four native species of vole, compared with six native 'mice' (including the two rats).

Of these, probably the most common is the field vole (Microtus agrestis). Field voles live across almost the whole of central and northern Europe and a broad swathe of Russia, roughly as far as the Lena River in eastern Siberia. They are found throughout Great Britain, but not in Ireland, the Irish Sea evidently having proved one challenge too many.

In the last few years, some genetic evidence has suggested that the more southerly European populations actually belong to two separate species - one in Catalonia, southern France and the northern Balkans, and the other in Galicia and northern Portugal. These seem to have diverged during the last glaciation around 70,000 years ago, but if so, they look essentially identical today, and they are still commonly treated as subspecies.

Field voles are somewhat larger than house mice, being about 10 to 13 cm (4 to 5 inches) in length, with a stockier build and a much shorter tail. Their fur is typically brownish, but can vary considerably, with white, black, and even piebald individuals known. The word "field" here refers to open country, rather than agricultural land, and their preferred habitat seems to be fallow grand or grassy countryside close to sources of water. Having said which, they live in a wide variety of places, from moorland and bog to woodland. While they tend to avoid more intensively farmed fields, they can be found on the margins and orchards don't seem to be a problem for them, either.

The vast majority of a field vole's diet consists of grasses, with a marked preference for softer, more low-lying grasses; in Britain they have been noted to avoid the tough, tall stems of cocksfoot grass, even where it's the most available food source. Mosses and low-lying herbs such as dandelions are another major dietary component, but they can eat other foods too, including a few flies and other insects.

Field voles can be either nocturnal or diurnal, depending on the circumstances, and this may even change depending on the season. They follow an ultradian rhythm - which is to say, they have more than one sleep/wake cycle per 24 hour period, being active for around two to four hours at a time before needing to sleep. Most of this time is spent feeding, since they have to eat around 70% of their own weight in food every day. Since they don't hibernate, winter can be a difficult time for them, resulting in a significant weight loss over the season, with only the fittest individuals surviving.

Unlike mice, field voles aren't great at digging burrows, and those they do dig are relatively simple, and no more than 30 cm (1 foot) in length - barely twice their own body length. Most prefer to dig a simple depression in the ground, and construct a spherical grassy nest about 20 cm (8 inches) across on top of it. These can prove particularly useful in winter, allowing the animal to keep warm and save on food intake.

They don't tend to travel far from home, at least once they have left their mother's nest, rarely needing to move further away than around 12 metres (40 feet) and often much less. Although they're pretty tolerant of one another through the winter, if hardly sociable, this all changes once the weather is warm enough for breeding, with both males and females becoming territorial. Despite which, they seem pretty promiscuous, and it is not unusual for a female to be pregnant by two different fathers simultaneously.

Voles breed, if anything, even more rapidly than mice - at least when the latter are not in some warm and food-rich habitat such as a barn or house. Pregnancy lasts just three weeks, and the mother comes into heat again almost immediately afterwards. With weaning also taking three weeks, it's perfectly possible for a single vole to give birth to four litters during the lengthy breeding season, and voles born around April will have children of their own before breeding stops for the winter some time around October. (This naturally varies with the local climate). With litters having three to five young on average, and a maximum of about eight, that's an awful lot of new voles per year, if there's sufficient food to support them.

In reality, populations experience a 'boom and bust' cycle on the order of a few years, influenced as much by the fluctuating population of their predators as by the amount of grass in the neighbourhood. It's also relevant that, while they can live up to three years in captivity, it's a rare field vole that survives for more than one winter in the wild.

Common vole
Far less common in Britain is the... um... common vole (Microtus arvalis). In many respects, the two species are extremely similar, and the behavioural and physical differences between them are subtle. On average, they tend to have shorter, lighter-coloured fur, and less hairy ears, but given the variation in coat colour of field voles, the former means little in practice.

Common voles are found through much of Europe, but tend to live further south than field voles. For instance, they do not live in Scandinavia outside of Denmark and one small region of south-eastern Finland, but they do live in central Spain, northern Italy, and the central Balkans. In the east, they reach as far as the Altai Mountains of Central Asia, although the subspecies living beyond the Urals is sometimes considered to be a full species in its own right. They are also found on a number of small islands off the European coast including, for example, Guernsey.

Since they live in the same wide range of habitats and eat the same foods as field voles, it's unsurprising that they too are a common species across most of their range. Hence the name. In Britain, however, it's a very different matter, because they aren't found on the British mainland at all. Instead, they live only on the Orkney Islands, which lie ten miles off the northeast coast of Scotland.

Genetic studies have shown that these voles are most closely related to those found in western, rather than eastern Europe, and particularly to those living in northern France and Belgium. The best explanation we have for their presence is that Neolithic humans brought them to the Orkneys around 3,000 BC. Since the voles on the islands don't particularly suffer from any signs of inbreeding, quite a large number of them must have been carried across to establish the initial population. Quite why is a mystery; they may have been accidentally transported in grain fodder, but a deliberate act is also a possibility.

Over the 5,000 years since that arrival, however, Orkney voles have developed in isolation. While they are generally not considered distinct enough to count even as a subspecies, they are, on average, slightly larger than their continental relatives, and give birth to smaller litters. It's apparently even possible to distinguish voles on the northern islands of the group from those further south by the precise colour of their fur.

Common voles are more prone to digging burrows than are field voles, constructing tunnels up to 150 cm (5 feet) in length terminating in shallow nest chambers 15 cm  (6 inches) below the ground. Different burrows are connected by runways through the grass, from which the voles are generally reluctant to stray. Although females are territorial, they sometimes share burrow systems while raising their young. Such relatively large and complex burrows also include food caches, which can contain up to 3 kg (6½ lbs) of seeds or roots, which the solitary burrows do not. Males, on the other hand, are not territorial at all, wandering about over a larger area in the hope of encountering as many females as they can.

Like field voles, the common species are active for several periods during the day and night, although they are less nocturnal during cold winter nights. In their case, active periods last only about two hours, but neighbours tend to synchronise their periods of wakefulness, probably to benefit from 'safety in numbers' should one of them spot a predator.

The genus Microtus, to which both common and field voles belong includes at least 60 other species. Many of these are European, including various pine voles, but many are Asian, living as far east as Japan. Others live in North America, where individual species live in places as diverse as Arizona and Newfoundland, and where the meadow vole (M. pennsylvanicus) is one of the most widespread mammalian species on the continent. The tundra vole (M. oeconomus) lives in the far north of both continents, from Nunavut to Norway, by way of Alaska and Siberia. There are also a number of species unique to Mexico, one in Guatemala, and one - the only vole in the entirety of Africa - that lives only in northern Libya.

Numerous though they are, the Microtus voles are far from the only kind. Two other species can also be found in Britain and it is to one of those that I will turn next...

[Photos by David Perez and "Dieter TD", from Wikimedia Commons.]

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