Microtus oeconomus
tundra vole

Microtus oeconomus (pronounced e-KON-uh-mus), commonly known as the tundra or root vole, is one of only four Holarctic rodents and the only species of Microtus that is found on all northern continents. In the Nearctic, it is found from the extreme northern edge of British Columbia northward to the Arctic coast, and from western Northwest Territories, through Yukon Territory and nearly all of Alaska. It has the northernmost distribution of any Microtus species in North America, with occurrences to around 71°N in Alaska. In the Palearctic, tundra voles occupy a somewhat broader range, extending from Scandinavia and the Netherlands in the west, throughout northern Europe and Asia to Siberia in the east, and south as far as Mongolia. (Hoffmann and Koeppl, 1985; Musser and Carleton, 2005)

Microtus oeconomus inhabits the tundra and taiga biomes and prefers mesic sedge meadows with abundant cover. It is commonly found along the edges of lakes and streams where this and similar habitats occur. Although this habitat usually occurs in relative lowlands, tundra voles are also found in wetter areas of mountainous regions at elevations of up to 2,500 meters. In the Palearctic, M. oeconomus is found in a broader range of habitats, commonly inhabiting mixed forest, taiga, and forest-steppe biomes with similar conditions. Habitat is most strongly selected on the basis of food quality and amount of cover provided. (Bergman and Krebs, 1993; Gromov and Polyakov, 1992; Hoffmann and Koeppl, 1985; Tast, 1966)

Microtus oeconomus is a medium-sized vole and varies in color dorsally from darker brown or grayish to lighter shades of cinnamon, rusty brown, or ocher. All color variations display a mixture of black-tipped hairs in the dorsal pelage. Sides are somewhat paler and the ventral surface is much lighter, from buff or ash gray to white. The tail is slightly to strongly dichromatic, being darker above and paler below. There is a fair amount of color variation in the species, which can often be related to habitat and geography. Tundra voles are generally lighter colored in more open habitats, and have yellow or grayish tones in tundra or forested habitats, respectfully. (Gromov and Polyakov, 1992; Hall, 1981; MacDonald, 2003)

Tundra voles display sexual size dimorphism, with adult males roughly 30% larger than females. The size and weight of the species is also variable relative to latitude and geography, especially so in the Palearctic. Individuals in populations at higher latitudes are generally larger and have proportionally smaller tails as a possible adaptation to colder temperatures. In addition, insular subspecies are generally larger than continental counterparts. Weight ranges from 25 to 80 g, with an average around 50 g. Total length ranges from 118 mm in the Old World, to 226 mm in the larger subspecies found in the New World. The tail is relatively short, generally being less than 30% of the total length. The name Microtus means “small ear” and refers to the short ears hidden in pelage that are a characteristic common to members of the genus. The dental formula for M. oeconomus follows the basic pattern for all Microtus: incisors 1/1, canines 0/0, premolars 0/0, molars 3/3. (Bondrup-Nielsen and Ims, 1990; Gromov and Polyakov, 1992; Hall, 1981; Lance and Cook, 1998; MacDonald, 2003; Nagorsen, 2002; Ringens, Folk, and Berberich, 1977; Wilson and Ruff, 1999)

Microtus oeconomus maintains a relatively high basal metabolic rate (compared to most rodents) of 12.5 mL O2 per gram of body mass per hour. This is due primarily to the high thermoregulatory demands of small endothermic mammals in cold environments. (Bozinovic and Rosenmann, 1989; Ringens, Folk, and Berberich, 1977)

There is insufficient research on Microtus oeconomus for a definitive lifespan to be established. However, relatively few adults survive much longer than one year in the wild, and the maximum age for other species of Microtus is commonly around two years. (Feldhamer, Thompson, and Chapman, 2003)

Microtus oeconomus exhibits a polyphasic activity pattern, with an average of seven bouts of activity per 24 hour time period. Although this type of ultradian rhythm is most prevalent, secondary crepuscular and circadian patterns have also been observed. In general, activity is higher during the day than at night and highest during the twilight hours. Activity levels also increase with decreases in temperature and are relatively unaffected by humidity and wind due to the protectiveness of preferred habitat types. Tundra voles are active throughout the year and do not hibernate or engage in bouts of torpor. (Feldhamer, Thompson, and Chapman, 2003; Halle, 1995)

Nests are constructed out of coarse vegetation and lined with finer plant materials, such as sedges. Shallow burrows are dug by M. oeconomus in the soil and vegetation layers. These burrows are primarily used as traveling corridors along with well-developed runways created above ground and in natural features such as permafrost cracks. Tundra voles generally run in the open and walk under cover, lacking the bounding progression seen in most mammals. (Feldhamer, Thompson, and Chapman, 2003; Wilson and Ruff, 1999)

Home Range

The home range, territory size, and spacing system of Microtus oeconomus vary widely depending on habitat, population density, season, and other factors. However, several trends have been observed. When food resources are of high quality and abundant, populations become increasingly clumped around these resources and males will display mutual territorial behavior. This behavior is also seen in response to periods of synchronous female breeding, where defense of mates becomes a higher priority. (Ims, 1987; Ostfeld, 1985)

Males generally occupy and utilize much larger territories than females, and this territory size is positively correlated with body size. Territories can range up to 3900 m^2 for males, but average 804 m^2 and have little overlap occurring between them. Females occupy much smaller territories, averaging 377 m^2, but are more philopatric than males and may share a territory with several other related females. The majority of male territories contain several female territories, and the resident male will often defend the breeding rights to all of the females contained within. Males travel approximately twice as far as females and may utilize an area as large as 12,000 m^2 in a single day. (Gliwicz, 1997; Lambin, Krebs, and Scott, 1992; Litvin and Karaseva, 1968; Tast, 1966)

Olfaction is presumably the most important and well-developed sense in Microtus oeconomus. Scents are commonly used by the species to identify familiar or unfamiliar individuals and to determine their age, sex, reproductive condition, diet, or social status. Conspicuous hip and preputial glands used for scent production are located on both sexes of adult tundra voles. The glands are largest in males and older individuals, and are only present in females at higher latitudes. Scent marking behaviors such as scratching or rubbing are also displayed by M. oeconomus. Hearing is well-developed in tundra voles and they may occasionally use vocalizations for communication, although little is known about this behavior. (Feldhamer, Thompson, and Chapman, 2003; Hall, 1981)

Tundra voles are strictly vegetarian and preferentially feed on sedges (Carex sp. and Eupharium sp.) that grow in their favored habitat of wet, marshy tundra. Sedges make up around 70 to 80% of their diet, with the remainder comprising herbs, mosses, lichen, and small woody shrubs. These percentages vary seasonally, and a 30% decrease in sedge consumption is common in winter, with mosses and lichens becoming a relatively major component of the diet. Other plant foods that are preferred when available include Equisetum sp., Dryas integrifolia, Salix sp. and various grasses. (Batzli and Lesieutre, 1991; Tast, 1966)

In the fall, Microtus oeconomus creates large caches of stored seeds and rhizomes to supplement its winter diet. Native peoples of North America occasionally sought out these caches in order to obtain large quantities of desired foods, such as licorice root. (Wilson and Ruff, 1999)

Microtus oeconomus is an important prey source for many carnivores. During cycles of peak density, they have been known to comprise the majority of the diet for species such as the arctic fox (Vulpes lagopus) in Alaska. Other known terrestrial predators of M. oeconomus include weasels, martens, red foxes, and wolverines. In addition, numerous avian species prey on tundra voles, including owls, falcons, hawks, jaegers, gulls, and shrikes. (Anthony, Barten, and Seiser, 2000; Sundell et al., 2004; Wilson and Ruff, 1999)

Populations of Microtus oeconomus experience cyclical fluctuations, reaching peak densities of 70 to 80 voles per hectare. As explained above, during these peak density years, tundra voles provide a large food base for many predator species. When present in such large numbers, they can also significantly alter the biological production of the tundra ecosystem through their foraging activities. Tundra voles share their habitat preferences with several similar species (e.g., M. longicaudus, M. pennsylvanicus, M. agrestis and M. xanthognathus) but generally either avoid competition through niche specialization, or out-compete the other species, as in the case of M. agrestis. (Tast, 1968; Whitney, 1976)

Tundra voles are host to a variety of internal and external parasites. Endoparasites include cestodes (Echinococcus sp., Paranoplocephala sp., and Taenia sp.), nematodes (Heligmosomoides sp.), and trematodes (Quinqueserialis nassalli). Ectoparasites include fleas (Siphonaptera sp.), lice (Polyplax sp.), and ticks (Ixodes angustus). (Timm, 1985)

Tundra voles are known carriers of several diseases. Of most importance to humans is the role of Microtus oeconomus as a carrier of the bacterium (Francisella tularensis) that causes tularemia in humans. This disease is transmitted by direct contact and is fatal in about 7% of untreated human cases. Additionally, at peak densities they can compete with livestock for forage and cause damage to trees by gnawing on the roots. (Feldhamer, Thompson, and Chapman, 2003)

Microtus oeconomus is commonly considered an ideal species for scientific research due to its adaptability to laboratory settings, quick development, and rapid population growth. Many important hypotheses have been tested using tundra voles as model species. As discussed above, tundra voles also provide an important food base for many carnivores (such as mink and foxes) that are highly valued for their fur. (Feldhamer, Thompson, and Chapman, 2003; Gromov and Polyakov, 1992)

Overall, Microtus oeconomus is a relatively stable species and not currently listed as threatened or endangered on any listing. It is also categorized by the IUCN as a species of “least concern”. However, insufficient knowledge of several isolated subspecies has resulted in their categorization of “data deficient” by the IUCN. These subspecies include M. o. amakensis, M. o. elymocetes, M. o. innuitus, M. o. popofensis, M. o. punukensis, and M. o. sitkensis. Furthermore, two subspecies, M. o. mehelyi and M. o. arenicola, are categorized by the IUCN as “vulnerable” and “critically endangered”, respectively. Population declines in some subspecies have been attributed to habitat deterioration. In the case of M. o. arenicola, competition with M. arvalis has additionally contributed to population declines on some islands. (Gippoliti, 2006)

Microtus oeconomus is most commonly described as comprising 10 subspecies in the Nearctic and 15 or more subspecies in the Palearctic. However, these numbers are subject to frequent debate and revision. New evidence suggests that Microtus oeconomus is a relatively recent colonizer of the Nearctic, and most likely crossed the Beringian land bridge during the Wisconsian (ca. 80,000-100,000 years ago) or Illinoian (ca. 130,000-300,000 years ago) glacial period. Recent research based on molecular evidence suggests that the species can be divided into four major clades. (Brunhoff et al., 2003; Galbreath and Cook, 2004; Hall, 1981; Lance and Cook, 1998)