This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
You’ll already know what voles are. They’re blunt-nosed, comparatively short-tailed rodents with chunky bodies and rounded ears that are mostly concealed by fur. They often have open-rooted (or near open-rooted) teeth and perform well as grass-eaters (as you know, the silica crystals embedded in grass blades make them nasty, abrasive things to eat on a regular basis). There are a lot of voles: about 155 living species included within about 20 genera, the taxonomy, biogeography and phylogeny of which is hugely complex. There are several different ways of classifying voles. Some authors put them together in a group (a ‘family’), along with lemmings and muskrats, termed Arvicolidae. Others group them together with hamsters and some other muroid rodent groups within Cricetidae. [Adjacent image by Soebe.]
Here in the UK we generally have three voles, though things are made more complicated by the fact that there are several island endemics that have usually been regarded as distinct subspecies (and sometimes as distinct species). Anyway, the three are the Bank vole Myodes glareolus* (also known as the Red vole, Wood vole and Red-backed vole), Water vole Arvicola amphibius and Field vole Microtus agrestis (also known as the Short-tailed field vole, Short-tailed vole and, confusingly, as Field mouse or Meadow mouse). We also have the Orkney or Guernsey vole M. arvalis, though there are suspicions that this species was introduced to the UK from the Iberian Peninsula some time within the last 5000 years. Actually, though, the fact that the British Bank vole record only goes back to Roman times means that doubts have occasionally been cast about its presence as a native too (Flowerdew 1993). [Bank vole image below by Photopippo.]
* Like me, you might better know this vole as Clethrionomys. That generic name is still in widespread use, but it’s becoming more accepted that Clethrionomys Tilesius, 1850 should actually be included within the older Myodes Pallas, 1811 (Carleton et al. 2003).
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I want to say a few things about voles, and all stem from my familiarity with these British species. My personal encounters with voles have been few and far between, and have always involved single individuals. I assume that this is true of most non-specialists. In view of this, it’s perhaps hard to appreciate that voles are famous for occasionally erupting into vast swarms or plagues where they cover the countryside in their millions and eat everything in sight. Lemming plagues are reasonably well known (it having given rise to the infamous tales of lemming mass suicide and so on)… but vole plagues?
Plagues of Field voles occurred in the New Forest and Forest of Dean (both here in southern England) in 1813 and 1814 and caused “considerable alarm lest the whole of the young trees in those extensive woods should be destroyed by them” (according to Thomas Bell’s 1837 A History of British Quadrupeds). Another epic vole event occurred in Scotland in 1891 and 1892 and other rodent plagues that were almost certainly of Field voles occurred in Kent and Essex in 1580 and 1581 and 1648, and in Norfolk in 1754 (Freethy 1983).
The animals forming these plagues were often only vaguely described (in the ones from the 1580s, the rodents were described as “sore plagues of strange mice”), meaning that it’s difficult to be absolutely sure about their identity. Take the account from Brut y Tywysogion of AD 893 which describes the following: “vermin of a strange species were seen in Ireland, similar to moles, with two long teeth each; and they ate all the corn, all the pasture, and the roots of grasses, and the hay ground, causing a famine in the country, and it is suppose the Pagans took them there, and wished likewise to introduce them into the isle of Britain; but by prayer to God, alms to the poor, and righteous life, God sent a sharp frost during the summer weather, which destroyed those insects” (Matthews 1989).
This sure sounds like a vole plague, the problem being that voles didn’t – historically – live on Ireland. Bank voles occur there now. They were discovered in County Kerry in the south-west in 1964, and have been spreading ever since (Fairley 1969, 1971). It’s thought that these Irish voles were introduced during the 1920s. The fact that voles can be accidentally introduced and then missed for decades probably explains that 9th century plague.
Why fluctuations in vole populations occur and why vole plagues erupt has been the subject of much speculation, as has the sudden cessation of the same events. Declines in the numbers of predatory birds and mammals, rainfall patterns and other climatic events have all been suggested. Within recent decades, mustelid predators have been outed as the probable controlling factor (Hanski et al. 1993) – in other words, the vole booms have been interpreted as part of a top-down-controlled trophic cascade – but this remains contested and environmental factors have also been shown to have a likely role in some habitats (Huitu et al. 2003).
Lest we think that massive eruptions of Field voles only occurred in the past, a recent surge – occurring in 2011, again in Scotland – was termed a plague (though, I’m not wholly sure that it was a plague in the same sense as those events of centuries past). The voles were said to number in the hundreds of millions, their extraordinarily successful population increase apparently being due to snow cover resulting from an unusually cold winter. You can read more about the 2011 ‘plague’ in this BBC News article. [Adjacent Field vole image by Neil Phillips of UK Wildlife.]
Crashes in rabbit numbers have sometimes been linked to vole population surges, and weasels are known to increase in numbers when Field voles have good years. Owls move in and have an easy life when vole numbers surge (especially Short-eared owls Asio flammeus), and domestic cats and dogs are said to become so satiated with vole during plague events that they simply lose interest in eating them. Voles themselves become cannibalistic when their numbers are unusually high: they quarrel a lot, kill each other in fights, and seem not to like wasting the opportunity to eat a deceased neighbour.
Intuitively, it seems most likely that a combination of factors come together to give the population an ideal season. The Scottish events of the 1870s and 90s were severe enough that sheep had to be given supplementary feed since the voles had eaten most available grass (Matthews 1989); their burrows had also made the ground treacherous underfoot in places. Even in places where Field voles have not erupted in plagues but are simply highly numerous, considerable damage can be incurred – the British forestry industry is thought to be responsible for massively increasing the vole population, and it is in these forests that the voles bite the bark away from young trees, thereby deforming or killing them (Matthews 1989).
Field voles are predominantly grassland voles, though they also occur in moorland, forests and dunes where sufficient grass cover is available. The weaving runway/burrow systems made by these voles – concealed by tall grass stems and going beneath tussocks, leaf litter and metal sheets – are often marked with small piles of cut, shredded grass fragments and droppings. Indeed, the numbers and whereabouts of voles can be easily tracked in some areas by counting their droppings (it’s much easier than trapping live animals in marked-off areas, or studying burrows or feeding piles). Field voles produce an obscene number of dropping – more than 1050 per day according to some counts. Bank voles are mostly woodland animals, sometimes associated with hedgerows. Field voles have an especially distinctive cheek dentition: the edges of the crowns form sharp, zig-zagging margins and the teeth are rootless. Bank vole cheek teeth have rounded edges and two roots are present on each tooth.
Like many rodents, voles possess an impressive number of scent glands. Their bodies are covered in them, meaning that they have the chance to deposit scent whenever they walk over an object or brush against one. Bank and Field voles both have anal glands and preputial glands (located on either side of the penis or clitoris). In addition, Bank voles have glands on the lips and corners of the mouth, on the pads on the soles of their feet, and on their hips. The hip glands become much enlarged during the breeding season. Field voles have flank glands as well (these are more anteriorly positioned than hip glands) – you might be familiar with those if you keep (or ever kept) pet hamsters.
I was hoping to say more about voles, but it’ll have to wait to another time. I’ve covered some very familiar, extensively studied species here, but there’s a huge number of obscure ones. The UK’s island-endemic voles are also interesting and worthy of discussion, as is the evolution, diversity and systematics of the Water vole(s). Well, more on voles some other time. Tet Zoo needs more on rodents.
For previous Tet Zoo rodent articles, see…
It’s not a rhinogradentian: it’s the most fantastic jerboa, Euchoreutes
The mysterious tree-creature revealed (squirrels)
New, obscure, and nearly extinct rodents of South America, and… when fossils come alive
Refs - -
Carleton, M. D., Musser, G. G. & Pavlinov, LYa. 2003. Myodes Pallas, 1811, is the valid name for the genus of red-backed voles. In Averianov, A. & Abramson, N. (eds) Systematics, Phylogeny and Paleontology of Small Mammals. Russian Academy of Sciences (Saint Petersburg), pp. 96-98.
Fairley, J. S. 1969. Bank voles Clethrionomys glareolus Schr. in Co. Cork. The Irish Naturalists’ Journal 16, 209.
- . 1971. Bank voles Clethrionomys glareolus Schreber in Cos. Clare and Tipperary. The Irish Naturalists’ Journal 17, 23-24.
Flowerdew, J. 1993. Mice & Voles. Whittet Books, London.
Freethy, R. 1983. Man & Beast: The Natural and Unnatural History of British Mammals. Blandford Press, Poole.
Hanski, I., Turchin, P., Korpimäki, E. & Henttonen, H. 1993. Population oscillations of boreal rodents: regulation by mustelid predators leads to chaos. Nature 364, 232-235.
Huitu, O., Norrdahl, K. & Korpimäki, E. 2003. Landscape effects on temporal and spatial properties of vole population fluctuations. Oecologia 135, 209-220.
Lawrence, M. J. & Brown, R. W. 1963. Mammals of Britain: Their Tracks, Trails and Signs. Blandford Press, Poole.
Matthews, L. H. 1989. British Mammals. Bloomsbury Books, London.