145.

North American Water Vole

Microtus richardsoni

French: Campagnol de Richardson / German: Richardson-Wiihimaus / Spanish: Topillo acuético norteamericano

Other common names: Richardson's Meadow Vole, Richardson's Vole, Richardson's Water Vole

Taxonomy. Arvicola richardsoni DeKay, 1842, “near the foot of the Rocky Mountains.” Restricted by V. Bailey in 1900 to “vicinity ofJasper House, Alberta,” Canada. Four subspecies are recognized.

Subspecies and Distribution.

M.r.richardsoniDeKay,1842—SEBritishColumbiaandSWAlberta,Canada.

M.r.arvicoloidesRhoads,1894—SWBritishColumbia,Canada,SalongtheCascadesRangetoSOregon.

M.r.macropusMerriam,1891—EWashington,EOregon,Idaho,WMontana,andWWyoming,USA.

M. r. myllodontus Rasmussen & Chamberlain, 1959 — from extreme SE Idaho S to C Utah, USA.

Descriptive notes. Head-body 125-178 mm, tail 66-98 mm, ear 15-20 mm, hindfoot 25-34 mm; weight 72-150 g. The North American Water Vole is one of the largest species of Microtus . Pelage is long, with dark-tipped hair thatis gray to reddish brown on back and dull gray on venter. Five plantar tubercles are on feet. Males and females have distinctive side glands and a musk-bearing anal gland. Tail is relatively long. Skull is large and angular and has wide, well-developed zygomatic arches; bullae are small. Incisors extend beyond premaxillae, and molars are constricted with tightly closed sections. M?* has four closed triangles, M® has 2-3 closed triangles, M, hasfive closed triangles, and M; has three transverse loops.

Habitat. Nearrivers and clear spring-fed or glacial-fed streams in mountain meadows at elevations of ¢.900-3000 m.

Food and Feeding. North American Water Voles eat various green herbaceous plants including leaves, stems, roots, and flowers. They also eat grasses, sedges, willows, seeds, and insects. They do not cache food for winter.

Breeding. North American Water Voles are polygynous. Estrus in femalesis induced by reproductively active males; annual onsetis correlated with new spring vegetation and extends for 3—4 months. Pregnant females are found from May until early September, and gestation is 22 days. Litters have 2-10 young (generally 4-7), increasing with age of the mother.

Activity patterns. North American Water Volesare terrestrial and primarily nocturnal.

Movements, Home range and Social organization. In winter, North American Water Voles build tunnels through snow. In summer, they use aboveground runways through vegetation and also underground tunnels. Burrow entrances are often located in riverbanks. Females give birth in underground nests of leaves and grasses. Females maintain non-overlapping territories; males have larger home ranges that allow them to breed with multiple females. Densities vary yearly or seasonally and have been estimated at 8-1-32-5 ind/ha.

Status and Conservation. Classified as Least Concern on The IUCN Red List.

Bibliography. Bailey (1900), Hall (1981), Klaus (2003), Ludwig (1984, 1988), Ludwig & Anderson (2009), Musser & Carleton (2005).