147.

Townsend’s Vole

Microtus townsendii

French: Campagnol de Townsend / German: Townsend-WihIimaus / Spanish: Topillo de Townsend

Other common names: Triangle Island Vole (cowani)

Taxonomy. Arvicola townsendii Bachman, 1839, “Columbia River.” Restricted by V. Bailey in 1900 to “Lower Columbia River, near mouth of Willamette, on or near Wappatoo (or Sauvie) Island,” Oregon, USA.

Six subspecies are recognized.

Subspecies and Distribution.

M.t.townsenduBachman,1839—fromSWBritishColumbia,Canada,StoHumboldtBay,NWCalifornia,USA.

M.t.cowaniGuiguet,1955—TriangleI,BritishColumbia,Canada.

M.t.cumming:Hall,1936—TexadaandBowenIs,BritishColumbia,Canada.

M.t.laingiR.M.Anderson&Rand,1943—NVancouverI,BritishColumbia,Canada.

M.t.pugetiDalquest,1940—SanJuanandCypressIs,Washington,USA.

M. t. tetramerus Rhoads, 1894 — S Vancouver I and nearby islands, British Columbia, Canada.

Descriptive notes. Head-body 121-160 mm, tail 48-75 mm, ear 15-17 mm, hindfoot 20-26 mm; weight 47-83 g. Males are significantly heavier than females. Townsend’s Vole is relatively large and dark; ears extend beyond fur. It is morphologically variable across its distribution along the west coast of North America. Dorsum is dark brown, with intermixed black-tipped guard hairs. Venter is gray to grayish brown or smoky, and tail is black or brown and slightly bicolored. Adult males have large hip glands. Head-body is 2:2-2-4 times longer than tail. Skull is long and angular and can become heavily ridged in older individuals. Incisive foramina are elongate, narrow, and posteriorly constricted. M? has four closed triangles. Diploid number is 2n = 50, and karyotype is entirely acrocentric.

Habitat. Mesic habitats with high grass cover from sea level to elevations of ¢.1830 m, such as moist fields and meadows, but occasionally salt marshes along the coast, willow thickets along streams, or wet forests, and even alpine meadows (Vancouver Island). Townsend's Voles nest on or aboveground in wetter habitats, potentially allowing them to inhabit seasonally inundated meadows; they are good swimmers. They nest underground in drier habitats.

Food and Feeding. Diet of Townsend's Vole contain green vegetation, including many species of grasses and sedges. Rushes ( Juncus sp., Juncaceae), horsetail ( Equisetum sp., Equisetaceae), alfalfa ( Medicago sativa), and clover ( Trifolium sp.), both Fabaceae, are commonly eaten.

Breeding. Breeding season of Townsend's Vole varies with density and stage of multiannual population cycles. Gestation lasts 21-24 days;litters average 4 young (range 1-9). Young are weaned at 15-17 days old.

Activity patterns. Townsend’s Voles are terrestrial and active night and day and throughout the year.

Movements, Home range and Social organization. Townsend's Vole have been the subject of extensive research on population dynamics, including a series of manipulative experiments to understand population eruptions. Survival, dispersal, and other aspects of demography have been studied. Larger males had higher survival rates, and survival was inversely correlated with density and growth rates of populations. In contrast, smaller females had higher survival, and their survival was less related to density and population increases. Following population highs (densities up to 800 ind/ha have been recorded), adults and subadults disperse during spring declines, followed by subadult and juvenile dispersal that can last into autumn. Home range averaged 900 m® for males and 500 m?for females.

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

Bibliography. Bailey (1900), Boonstra & Krebs (1979), Boonstra et al. (1987), Bradley, Ammerman et al. (2014), Cornely & Verts (1988), Krebs et al. (1976), Hall (1981), Maser et al. (1981), Musser & Carleton (2005), Verts & Carraway (1988).