North American voles
Voles are small mouse-like mammals in the family Muridae, order Rodentia. Other members of this family include the gerbils, hamsters, lemmings, rats, and mice. Voles occur in a wide range of open, often grassy habitats, such as alpine and arctic tundra, prairies, savannas, and pastures and other types of agricultural fields.
Voles have a body length of about 3-5 in (8-12 cm), and typically weigh 1.1-1.8 oz (30-50 g), with mature males being slightly larger than females. Voles have a stout, plump body, a small head, blunt nose, small eyes, short ears, a short neck, and a short, stubby tail. Their pelage is dense, and is typically colored brownish or grayish.
Voles are herbivores, eating a wide range of plant tissues, especially the shoots and rhizomes of grasses and sedges. Voles typically live in tunnels and runways that they dig in the ground and surface litter, equipped with numerous resting dens and food-storage areas. These tunnels are often lived in by communal groups of voles. The runways are kept free of obstructions, and the voles are intimately familiar with the twists and turns of these paths. Consequently, voles can move along their runways much faster than they can run along the ground surface. This is an important advantage when these small animals are attempting to flee from a predator.
Voles remain active throughout the year. During the winter, voles develop tunnels beneath the snow at the snow-ground interface, and they feed on rhizomes and food that they have stored from the previous autumn.
Populations of voles often irrupt in a cyclic fashion, with regularly occurring years of great abundance punctuated by longer periods of time during which these small animals may be rather scarce. Some species of voles have enormous potential for population growth because of their intrinsic fertility. For example, females of the common vole (Microtus arvalis) of Europe can become sexually mature before they are two weeks old, and while they are still suckling on their mother's milk! Potentially, a female vole can give birth to her first litter of four to seven young when she is only five weeks old. One captive common vole bore 33 litters, and had a total of 127 young. Because these animals breed more or less continuously during the growing season, and sometimes even in the winter, their population growth rates are potentially enormous.
What prevents an unmitigated growth of voles is mortality associated with predation, coupled with unfavorable environmental conditions, such as the availability of adequate food. In most cases, it appears that the most important ecological factors that allow rapid population growth in voles is the availability of an abundance of food, associated perhaps with relatively favorable growing conditions for one or several years.
Voles are always an important prey item for many species of predators. However, when voles are abundant, they are avidly hunted by virtually all small predators, including weasels, foxes, owls, and hawks, and even some larger animals such as wolves and bears.
Voles are sometimes considered to be pests, because they can cause serious damages to crops growing in fields, and to stored grains and other foods. These damages are caused by the actual eating of foods, as well as by the contamination of stored grains with vole feces, which can render the crops unsalable.
There are about 50 species of voles. Most voles are included in the genus Microtus. The meadow vole (M. pennsylvanicus) is the most common and widespread species of vole in North America, occurring through most of Canada south of the high arctic tundra, and in most of the northern United States. The meadow vole is a familiar species of fields, wet meadows, and disturbed forests. Because of its wide distribution and periodic irruptions of abundance, the meadow vole is both ecologically important as a component of ecological food webs, and economically important as an occasional pest.
The tundra vole (Microtus oeconomus) is another widespread species, occurring in arctic tundra and open boreal forests of Alaska, Yukon, and Eurasia. The woodland vole (M. pinetorum) is widespread in forests of southeastern North America. The montane vole (M. montanus) occurs in alpine grasslands and tundras of the mountains of the western United States. The prairie vole (M. ochrogaster) occurs in the grasslands of the interior of the continent, while the rock vole (M. chrotorrhinus) occurs in boulder slopes and other rocky places in northeastern North America. The chestnut-cheeked vole (M. xanthognathus) occurs in local populations in the boreal forest of northwestern North America. The long-tailed vole (M. longicaudus), singing vole (M. miurus), Townsend's vole (M. townsendii), Richardson's vole (M. richardsoni), and California vole (M. californicus) are all species of coniferous forests of western North America.
The heather vole (Phenacomys intermedius) occurs widely in boreal and northern temperate forests of Canada and the northwestern United States. This species is very similar in appearance to the meadow vole, and was overlooked as a distinct species by most field biologists until the 1950s, when reliable, diagnostic characters were discovered (these involve the shape of the cheek teeth).
The northern red-backed vole (Clethrionomys rutilus) occurs in tundra and open boreal forests of northwestern Canada and Alaska, and also in Siberia and eastern Scandinavia. There is also a disjunct population in coniferous rain-forests of Oregon and Washington. Gapper's red-backed vole (C. gapperi) occurs more widely throughout temperate North America, reaching as far south as the mountains of Arizona.
The sagebrush vole (Lagurus curtatus) occurs in high altitude sagebrush steppes and semi-deserts of southwestern North America. Richardson's water vole (Arvicola richarsoni) is a species of alpine meadows and streams of the Rocky Mountains.
See also Rodents.
MacDonald, David, and Sasha Norris, eds. Encyclopedia of Mammals. New York: Facts on File, 2001.
Wilson, D. E., and D. Reeder. Mammal Species of the World. 2nd ed. Washington, DC: Smithsonian Institution Press, 1993.