Western Harvest Mouse megalotis subspecies
Scientific Name: Reithrodontomys megalotis megalotis
Other/Previous Names: Western Harvest Mouse (British Columbia population)
Taxonomy Group: Mammals
COSEWIC Range: British Columbia
COSEWIC Assessment Date and Status Change: November 2019
COSEWIC Status: Endangered
COSEWIC Status Criteria: B1b(iii)c(iv)+2b(iii)c(iv)
COSEWIC Reason for Designation: This tiny mouse occurs at the northern edge of its distribution within the Okanagan and Similkameen valleys of British Columbia, and is one of two designatable units of the species in Canada. It is among Canada’s shortest-lived mammals. This species demonstrates extreme fluctuations in the number of mature individuals over time, increasing vulnerability to disturbances when populations are at a low in spring and early summer. The species’ limited distribution, extreme fluctuations, and habitat loss are the reasons for designation. Change in status from Special Concern to Endangered is the result of inclusion of extreme fluctuations in the latest assessment. Continued urban and agricultural expansion threaten the persistence of this mouse.
COSEWIC History of Status Designation: Designated Special Concern in April 1994 and in April 2007. Status re-examined and designated Endangered in November 2019.
SARA Status: Schedule 1, Special Concern
Date of Listing (yyyy-mm-dd): 2009-03-05
Please note that this information is provided for general information purposes only. For the most up to date and accurate list of species listed under the Species at Risk Act, please see the Justice Laws Website.
Image of Western Harvest Mouse megalotis subspecies
The Western Harvest Mouse is one of the smallest mice in North America; it averages 13.6 cm in length, half of which is its tail. This brownish mouse has a dark dorsal stripe that runs from forehead to tail. The hindfeet are white, its flanks and cheeks are a buff-coloured and the fur on its belly can be white to deep grey. Its long, sparsely-haired tail is grey above, whitish below. The prominent ears are hairless. This rodent is easily confused with the Deer Mouse and the House Mouse, two larger and more common species. However, the juvenile Deer Mouse is most often grey in colour and the House Mouse's tail is completely bare.
Distribution and Population
The Western Harvest Mouse is widespread throughout western North America, from southern Canada down to central Mexico. In Canada, it is restricted to the grasslands of south-central British Columbia and southern Alberta. The Alberta and British Columbia populations, separated by the Rocky Mountains, are of the dychei and megalotis subspecies, respectively. In British Columbia, the Western Harvest Mouse megalotis subspecies occurs throughout the Okanagan Valley, as far north as Vernon, and in the Similkameen Valley, as far north as Keremeos. The mouse has not been captured in central Okanagan near Kelowna, suggesting that the Vernon population is isolated from populations to the south. The species is also absent from adjacent valleys, including the Thompson River and Kettle River valleys. However, low trapping success makes the absence of records an unreliable means of assessing distribution. In contrast to the US, where the Western Harvest Mouse is a dominant member of grassland small mammal communities, this species is rather rare in Canada, typically comprising less than 10% of such communities. However, population densities as high as 80 individuals/hectare have been reported in British Columbia. Western Harvest Mouse populations appear to peak in late fall or early winter and decline to low levels in midsummer. There are no data available on population size or trends at either a provincial or national level.
Little is known about the detailed habitat requirements of the Western Harvest Mouse, but the species is reported to prefer habitats characterized by thick herbaceous cover of tall grasses or shrubs. The British Columbia population appears restricted to very hot, dry valley bottoms where bitterbrush and big sagebrush grow. It is found in various habitats, including dry gullies with dense shrub cover that border grasslands, old fields, apple orchards, Ponderosa pine forests and grassy areas bordering cultivated fields. Although preferring herbaceous cover in habitats without heavy grazing, the Western Harvest Mouse megalotis species is found in grazed habitats in British Columbia, as long as there is enough cover provided by shrubs such as bitterbrush or sagebrush. Since the 1930s, the extent of available habitat for the Western Harvest Mouse in the Okanagan Valley has been declining because of the combined effects of cattle grazing, agriculture and urbanization. Its principal natural habitat, as well as old fields, are declining. Furthermore, old apple orchards where the mouse has been caught are being converted into vineyards. The disappearance rate of its habitat is accelerating.
In British Columbia, the breeding season extends from March through November. Females can breed at four months of age and have up to five litters per season with an average litter size of three. The Western Harvest Mouse builds a small grass nest on the ground or in shrubs up to 1 m above ground. It does not build burrows, but sometimes uses burrows of other small mammals. The Western Harvest Mouse appears to be able to enter torpor to cope with cold temperatures. This dormant state, where vital functions slow down, may be essential to the survival of Canadian populations. Some researchers speculate that they hibernate, although this does not appear to occur in southern British Columbia as this species was observed throughout the year. This nocturnal mouse is omnivorous; it mainly feeds on seeds and insects, such as caterpillars and moths, but also on new plant growth and flowers. In Canada, birds of the Strigidae family, such as the Northern Saw-whet Owl, are the only officially confirmed predators of the Western Harvest Mouse, but other likely predators include prairie rattlesnakes, hawks, jays, shrikes, raccoons, foxes, weasels, skunks, badgers and coyotes. Although the Western Harvest Mouse can live naturally for 18 months, most do not live past six months. The Western Harvest Mouse is sometimes in competition with rodents of similar size. In British Columbia, the Montane Vole may be an important competitor.
Habitat fragmentation and loss caused by grazing, cultivation or other agricultural activities, along with urban development, are likely the most significant threats to populations in Canada. The Western Harvest Mouse's habitat is mostly threatened by cattle and horse grazing, agriculture and urbanization. In British Columbia, conversion of grasslands to orchards, cultivated fields, and more recently, urban development and vineyards, has eliminated large areas of shrub-steppe habitats important to this species. The use of roadside habitats and habitats along cultivated fields may be particularly important for the dispersal of this species among suitable habitat fragments. The Western Harvest Mouse is susceptible to habitat change, such as the disappearance of herbaceous cover and food, resulting from fire, but populations can recover quickly, provided there is suitable unburnt habitat nearby. The creation of new roads may represent a significant barrier to dispersal and movement of the Western Harvest Mouse. Finally, the use of pesticides to control vole and pocket gopher populations in old field and orchard habitats in British Columbia may have significant impacts on local populations of the Western Harvest Mouse. However, orchards are not the species' preferred habitat; thus, mortality from poisoning is assumed to be low.
The Western Harvest Mouse megalotis subspecies is protected under the federal Species at Risk Act (SARA). More information about SARA, including how it protects individual species, is available in the Species at Risk Act: A Guide.
Provincial and Territorial Protection
PLEASE NOTE: Not all COSEWIC reports are currently available on the SARA Public Registry. Most of the reports not yet available are status reports for species assessed by COSEWIC prior to May 2002. Other COSEWIC reports not yet available may include those species assessed as Extinct, Data Deficient or Not at Risk. In the meantime, they are available on request from the COSEWIC Secretariat.
12 record(s) found.
- COSEWIC Status Reports (2 record(s) found.)
- Response Statements (2 record(s) found.)
- Management Plans (1 record(s) found.)
- Orders (2 record(s) found.)
- COSEWIC Annual Reports (2 record(s) found.)
- Consultation Documents (2 record(s) found.)
- Recovery Document Posting Plans (1 record(s) found.)
COSEWIC Status Reports
COSEWIC Annual Reports
COSEWIC Annual Report - 2007 (2007-08-30)2007 Annual Report to the The Minister of the Environment and the Canadian Endangered Species Conservation Council (CESCC) from the Committee on the Status of Endangered Wildlife in Canada.
COSEWIC Annual Report 2019 to 2020 (2020-09-02)Over the past year COSEWIC assessed a total of 21 wildlife species, none of which were assigned a status of Not at Risk. Of these 21, COSEWIC re-examined the status of nine wildlife species; of these, 44% were reassessed at the same or lower level of risk. To date and with the submission of this report, COSEWIC’s assessments now include 810 wildlife species in various risk categories including 363 Endangered, 190 Threatened, 235 Special Concern, and 22 Extirpated (i.e. no longer found in the wild in Canada). In addition, 19 wildlife species have been assessed as Extinct, 59 wildlife species have been designated as Data Deficient, and 198 have been assessed as Not at Risk.