Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 3

Species Information

Name and classification

Western harvest mice (Reithrodontomys megalotis Baird) belong to the Order Rodentia and Family Cricetidae and there are five species in this genus which occur in the US and Canada. Of these, only the western harvest mouse occurs in Canada. Although as many as 17 subspecies of western harvest mouse have been proposed (Hall 1981), a comprehensive taxonomic review is required to substantiate this (Nagorsen 1990). Two subspecies, designated based on morphology only, R. m. megalotis and R. m. dychei, occur in Canada, reaching their northern limits in southern British Columbia (BC) and Alberta, respectively.

Morphological description

With an average mass of 11.0 g (range 8.0-15.0; Nagorsen 2005), the western harvest mouse is one of the smallest mice in North America (Banfield 1974, Forsyth 1999). Total length is 136 mm (range: 116-151), of which nearly half is a sparsely furred, bicoloured tail that is white ventrally (Cahalane 1961, Nagorsen 2005). This mouse has prominent naked ears, tawny pelage, buff sides, white to deep grey undersides, a dark mid-dorsal stripe that runs from the forehead to tail, and white hind feet (Banfield 1974, Burt and Grossenheider 1976, Nagorsen 2005).

The western harvest mouse may be confused with larger deer mice (Peromyscus maniculatus) and house mice (Mus musculus; Nagorsen 2005). A juvenile deer mouse of comparable size to a western harvest mouse can be distinguished by its dull grey pelage, and a house mouse can be distinguished by its naked tail (Nagorsen 2005).

Designatable units

Within Canada there are two recognized designatable units (subspecies) for the western harvest mouse. The subspecies R. m. megalotis and R. m. dychei are found within southern BC and Alberta, respectively.

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