21.

Common Muskrat

Ondatra zibethicus

French: Rat-musqué commun / German: Bisamratte / Spanish: Rata almizclera

Other common names: Muskbeaver, Muskrat

Taxonomy. Castor zbethicus Linnaeus, 1766, eastern Canada.

Sixteen subspecies are recognized.

Subspecies and Distribution.

O.z.zibethicusLinnaeus,1766—fromextremeSEManitoba,COntario,andCQuebec,Canada,SthroughEUSAtoextremeNETexas,extremeNLouisiana,Mississippi,Alabama,NGeorgia,andNWSouthCarolina.

O.z.albusSabine,1823—Saskatchewan,Manitoba,andNOntario,Canada.

O.z.aquiloniusBangs,1899—NQuebecandLabrador,Canada.

O.z.bernard:Goldman,1932—SNevada,SECalifornia,andWArizona,USA,andNBajaCaliforniaandextremeNWSonora,Mexico.0.z.aannamominusHollister,1910—fromSEAlberta,SSaskatchewan,andSManitoba,Canada,SthroughCUSAtoNTexas.

O.z.goldmaniHuey,1938—SWUtah,SENevada,andNWArizona,USA.

O.z.macrodonMerriam,1897—fromSEPennsylvaniaSalongthecoasttoNorthCarolina,USA.

O.z.mergensHollister,1910—SEOregon,NC&NECalifornia,NNevada,andextremeNWUtah,USA.

O.z.obscurusBangs,1894—Newfoundland,Canada.

O.z.ocapitalisElliot,1903—WWashingtonandWOregon,USA.

O.z.osoyoosensisLord,1863—fromSBritishColumbiaandSWAlberta,Canada,SalongtheRockyMtstoNNewMexico,USA.

O.z.pallidusMearns,1890—fromSCArizonaEtoWNewMexico,USA.

O.z.nipensisBailey,1902—fromCNewMexico,USAStoNChihuahua,Mexico.

O.z.nwaliciusBangs,1895—fromSEcoastalTexasEtoextremeNWFlorida,USA.

O.z.spatulatusOsgood,1900—fromPacificCoastofCAlaska,USA,SEtoSBritishColumbia,AlbertaandWSaskatchewan,Canada.

O. z. zalophus Hollister, 1910 — SW & SC Alaska, USA. Introduced to most of Europe, Siberia, C Asia, Russian Far East, China, NE Korea, Japan (Honshu), S Chile, and S Argentina.

Descriptive notes. Head-body 256-299 mm, tail 200-254 mm, ear 20-21 mm, hindfoot 65-78 mm; weight 700-1800 g. The Common Muskratis the largest arvicoline; the largest individuals reach up to 700 mm in total length. Fur is dark black or brownish to white and silver, with venter lighter than dorsum. Underfuris waterproof. Skull is massive and has interorbital ridge. Incisors are slightly procumbent, unrooted, and without grooves. Lips can be closed behind incisors to allow gnawing underwater. Molars are rooted, and first molar has six triangles. Incisive foramina are long and narrow, and there is a spine at posterior edge of palate. Tail is long, laterally flattened, scaled, and nearly naked, except sparse fringe of hair along ventral keel. Large hindfeet are partially webbed, and toes are lined by stiff guard hairs providing greater surface area for swimming. Front feet are relatively small and used primarily for digging and manipulating food. Karyotype is 2n = 54, and sex chromosomes are acrocentric.

Habitat. Mesic habitats, such as streams, rivers, lakes, ponds, and marshes.

Food and Feeding. Common Muskrats eat aquatic vegetation such as cattails (Typha latifolia, Typhaceae), foraging especially on roots and basal parts of plants. They also eat animals including crayfish (Cambarus spp.), numerous fish species, turtles, and freshwater mussels (Unio sp., Anodonta sp.).

Breeding. In the southern USA, Common Muskrats breed year-round; those in northern latitudes reproduce primarily in spring and summer, with earliest litters born in late April or May. Average litter size is 4-8 young, which are born nearly hairless, blind, and with relative short round tails. At c¢.2 weeks old, young are haired and capable of swimming, and they are weaned by c.4 weeks. At c¢.2 months old, tail has become laterally compressed. At higher latitudes, 2-3 litters/year are common.

Activity patterns. Common Muskrats are highly aquatic and can be active day or night throughout the year.

Movements, Home range and Social organization. Up to 40% of Common Muskrats disperse in spring (March-April) in an attempt to establish new territories in vacant areas. Dispersal is male-biased and averages c.3 km at high latitudes. Weather conditions, densities, age, and hormonal changes also might impact dispersal. Common Muskrats construct burrows or conical mud houses above water but with multiple underwater access burrows. Nest chambers within these are lined with fresh leaves and twigs. Two distinct kinds of houses are built, either main dwelling or smaller feeding houses. Home ranges are relative small with most activity occurring within 15 m of burrows or conical mud homes, although much larger territories have been noted. Females defend territories more aggressive than males. The common name, muskrat, refers to odor secreted by perineal musk glands located near base oftail. Males and females secrete musk from these glands, and odoris thought to communicate important reproductive and territorial cues.

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

Bibliography. Cassola (2016n), Errington (1963), Hafner et al. (1998), Hall (1981), Musser & Carleton (2005), Skyriené & Paulauskas (2012), Willner et al. (1980), Wilson & Ruff (1999).