Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 6
Biology
The western harvest mouse has not been studied comprehensively and most information is derived from studies conducted in the US. Within Canada, the only studies providing insight into the biology of the western harvest mouse are those of Sullivan (e.g., Sullivan 2004, Sullivan and Sullivan 2006a) and Klenner (unpubl. data). Nagorsen (2005) summarized the species biology.
Life cycle and reproduction
The western harvest mouse is nocturnal and forages primarily on the ground for seeds and invertebrates (e.g., caterpillars and moths; Cahalane 1961, Whitaker and Mumford 1972, Meserve 1977, Johnson and Gaines 1988, Jekanoski and Kaufman 1995). The mouse frequently climbs shrubs (as high as 1 m) in search of seeds, flowers, and invertebrates, and climbs grass stalks to harvest seeds (Cahalane 1961, Meserve 1977, Jekanoski and Kaufman 1995). In coastal California, up to 50% of western harvest mouse spring food consisted of flowers and seeds (Meserve 1976). Arthropods (primarily lepidopterans) made up to 30% of the diet (Meserve 1976).
Western harvest mice can live for up to 18 months in the wild; however, few survive more than six months (Nagorsen 2005). Females are reproductive beginning at four to 12 months of age. Gestation is 21 – 24 days with an average of 4.1 embryos (range of 1 – 9) and 2.6 young (range of 1 – 7; Hayssen et al. 1993, Nowak 1999). The maximum number of litters per season is four or five (Hayssen et al. 1993). Young are 1 – 1.5 g at birth, 7 – 8 mm in length (Jackson 1961), and are weaned at approximately 20 days (Hayssen et al. 1993). Generation time is estimated to be 6 months.
Based on an intensive study in southern BC, western harvest mice bred from March to November producing a variable number of litters per year. The proportion of breeding males was 75% of the population in three old-field and sagebrush habitats and 42.9% in an organic orchard (Sullivan and Sullivan 2005 and 2006b). Early juvenile survival (an index relating recruitment of young into the trappable population to the number of lactating females) ranged from three young per pregnant female in old fields to five in the organic orchard and to six in sagebrush habitats (Sullivan and Sullivan 2005 and 2006b).
Nests are spherical or cup-shaped, approximately 7.5 – 12.5 cm in diameter (Webster and Jones 1982, Wilson and Ruff 1999), and usually occur in shrubs up to 1 m above the ground (Webster and Jones 1982) but occasionally in burrows or on the ground (Birkinholz 1967). In general, nests are placed in shrubs and are composed of plant material with an outer layer of coarsely woven grasses and fibrous plant material and an inner layer of softer plant material such as down or dandelion fluff (Wilson and Ruff 1999).
Predation
Possible predators of western harvest mice include owls (Marti 1974, Cannings 1987), hawks, jays, shrikes, prairie rattlesnakes,raccoons, foxes, weasels, skunks, badgers and coyotes (Brant 1962, Kaufman et al. 1993, Brillhart and Kaufman 1994, Forsyth 1999, Wilson and Ruff 1999). Cannings (1987) found that western harvest mice made up <5% of the diet of northern saw-whet owls (Aegolius acadicus) in southern BC.
Physiology
Western harvest mice enter a shallow state of torpor when starved and exposed to cold temperatures in a laboratory setting (Thompson 1985). The ability to enter torpor under natural conditions is likely important for the survival of individuals in Canada due to the cold temperatures experienced by these mice at the northern periphery of their distribution (Nagorsen 2005). Although an ability to hibernate has been suggested (O’Farrell 1974), this mouse has been captured throughout the year in BC (Sullivan and Sullivan 2004).
Dispersal
O’Farrell (1978) estimated a mean home range of 1.12 ha while Meserve (1977) found home ranges between 0.44 and 0.56 ha. Dispersal distances of western harvest mice are generally <300 m (Brant 1962, Clark et al. 1988, Skupski 1995); however, long-distance movements of 375-3200 m have been reported for some individuals (Clark et al. 1988). Males generally move farther than females (Clark et al. 1988, Skupski 1995). Of five animals moving >1 km, daily average movement distance to reach the new site was 135 m (direct linear distance; Clark et al. 1988).
Individuals displaced by up to 300 m returned to their home areas (Fisler 1966). However, Kozel and Fleharty (1979) found that no mice returned after having been transported to the other side of a road on the edge of their home range.
Documented range expansions of the western harvest mouse in Illinois and Indiana indicate the potential for dispersal when appropriate habitats are available, for example along road rights-of-ways (Whitaker and Mumford 1972, Ford 1977). Western harvest mice had the highest recruitment and immigration rates of any small mammal species trapped during a study near Summerland, BC (Sullivan and Sullivan 2006b). This suggests that they should be capable of colonizing new areas of suitable habitat.
Western harvest mice have been observed to use the runways constructed by sympatric rodents, including those of the genus Microtus and Sigmodon (Hall 1946). However, harvest mice do not appear to assist with runway maintenance (Pearson 1959).
Interspecific interactions
Western harvest mice may compete locally with other similar sized rodents such as deer mice, house mice, and montane voles (Microtus montanus) (Johnson and Gaines 1988, Heske et al. 1994, Fa et al. 1996, Stapp 1997). In grassland communities of California, Heske et al. (1984) found that in years when California voles (Microtus californicus) were abundant, harvest mice became locally extinct. Removal of kangaroo rats (Dipodomys spp.) in Arizona resulted in higher densities of harvest mice (Skupski 1995). In BC, montane voles may be important competitors (Sullivan and Sullivan 2004). Competition with deer mice, the dominant mouse in habitats with western harvest mice, is less likely since these species appear to exploit different microhabitats (Cahalane 1961, Kaufman et al. 1988).
Grazing by domestic livestock decreases cover and food availability and therefore has a negative impact on western harvest mouse habitat. In eastern Colorado, Moulton et al. (1981) found that western harvest mice were not present on grazed sagebrush habitats but were common in ungrazed areas. Although this mouse may prefer the cover provided within ungrazed habitats, it is found on grazed sites in BC, when abundant shrub cover is present (W. Klenner, pers. comm.).
Adaptability
The potential for western harvest mice to enter torpor when faced with a lack of food and extreme cold (Thompson 1985), its high rates of reproduction (Bancroft 1967, Hayssen et al. 1993) and recruitment (Sullivan and Sullivan 2006b), and dispersal ability (Whitaker and Mumford 1972, Ford 1977), combine to make it adaptable to stochastic events in its environment. Given adequate connectivity among suitable patches of habitat, these mice appear to be resilient to local extirpation events. For example, despite being susceptible to fire (Kaufman et al. 1988), this mouse is a common resident throughout the fire-dependent grasslands of most of North America, and occurred at relatively high densities (up to 13/ha) just 3 years after a fire in southern BC (W. Klenner, pers. comm.).
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