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WESTERN HARVEST MOUSE
Reithrodontomys megalotis
Family Muridae - Rats and Mice
Order Rodentia - Rodents
Risk Status
Official status
Because of its limited range and apparent rarity, the Western Harvest
Mouse was placed on the provincial Blue
List by the Ministry of Environment (CDC = G5
S2S3). COSEWIC
designated the British Columbia population of Western Harvest Mouse
as Vulnerable in 1994.
Image Credits: Top sketch by Michael
Hames for the Royal British Columbia Museum. Bottom sketch in Mammals
of British Columbia, Cowan and Guiguet, 1978.
Historical facts
The Western Harvest Mouse was first discovered in Okanagan valley
of British Columbia in 1942.
Distinguishing features
The Western Harvest Mouse is the smallest mouse in British Columbia
with an average total length of 136 mm and weight of 11 g; it rivals
the Olive Backed Pocket Mouse for the title of smallest rodent in
Canada. Both are about the size of large shrews. It is often confused
with the Deer Mouse and House Mouse, two species that coexist with
the harvest mouse in southern British Columbia. It can be distinguished
from these two species by: more buffy dorsal colour, a smaller size,
and the outer surface of each of the upper incisors have distinct
shallow grooves.
This species is a slim, long-tailed mouse that has prominent,
naked ears, a slender sparsely haired tail, grey above and whitish
below. The harvest mouse has three pelages:
juvenile, sub-adult, and adult. The juvenile pelage is rather woolly
and dull grey. The adult pelage is the brightest. Moulting starts
on the ventral surface
and spreads over the flanks to meet on the back; then the new fur
spreads fore and aft. A second point of origin is on the muzzle,
the new coat spreading back to form a moult-line behind the ears.
There is one annual moult each summer. The Western Harvest Mouse
has four toes on each front paw and five on each hind foot.
The Western Harvest Mouse is a naturally rare species associated
with the grassland rodent communities of western North America.
Although it has been extensively studied in the United States, little
is known about its biology in Canada, where it reaches the northern
limits of its range.
Distribution
Map
Red dots indicate specimen records or confirmed breeding sites.
British Columbia
In British Columbia, the Western Harvest Mouse is known only from
the lower elevation grasslands in the southern Okanagan and Similkameen
valleys at low elevation. Records exist for Oyama, Osoyoos Lake,
Chopaka, Penticton, Summerland and a disjunct population around
Vernon. Chopaka and Keremeos Creek records demonstrate presence
in the Similkameen valley however, the population may extend as
far north as the Ashnola River of Hedley. However, intensive sampling
revealed that that species is absent from adjacent arid valleys,
such as the Thompson River and Kettle River valley.
North America
Two geographically distinct subspecies of Western Harvest Mouse
are found in Canada which presumably reflects the different biogeographic
histories and ecology of where they are found. Reithrodontomys
megalotis dychei is found in extreme southern Alberta while
Reithrondontonys megalotis megalotis occurs only in the interior
of southern British Columbia. The Western Harvest Mouse has a wide
distribution across the central and western United States and throughout
the central plateau of Mexico.
Habitat
Across North America, the Western Harvest Mouse inhabits sagebrush
steppe and agricultural
areas in areas below elevations of 500 m. It forages
in grasslands bordering riparian
areas such as irrigation right-of-ways, coastal salt marshes, streams
or lakes, and in deciduous ravines of willow, rose and trembling
aspen. In British Columbia, Western Harvest Mice are usually associated
with edge habitats
bordering agricultural areas and rangeland. Shrub or grass overstory
with tall lush herbaceous
plants and natural debris are important for cover and concealing
nests.
All known Western Harvest Mouse populations in British Columbia
are associated with intermontane
grasslands of the Okanagan Basin Ecosection in the Southern Interior
Ecoprovince. The
elevational range extends from 300 m to 780 m, with most occurrences
below 600 m. Populations found at higher elevations (>600 m)
were associated with south-facing slopes. Most occurrences are in
the very dry hot subzone of the Bunchgrass biogeoclimatic zone.
This zone, which is confined to the hot valley bottoms, is dominated
by bunchgrass and big sage.
Typical shrub species co-existing with the Western Harvest Mouse
in British Columbia include: wild rose, mock orange, Saskatoon berry,
chokecherry, currants, Douglas maple, snowberry, and black hawthorn.
Giant wildrye was present at many capture sites, and undisturbed
grassy edges habitats and roadside habitats with high grass and
weeds, such as thistle, support Harvest Mice. In the Similkameen
Valley, a population of mice was found in disturbed edge-habitat
of smooth brome, thistle, giant wildrye, and goldenrod which bordered
hay and alfalfa fields.
Greasewood replaces big sage in the sandy
soils of the south Okanagan. A few Harvest Mice in the south Okanagan
were in the Ponderosa Pine biogeoclimatic zone, and, in the north
Okanagan. The Western Harvest Mouse also is associated with the
Interior Douglas-fir biogeoclimatic zone, and strangely. this species
is absent from the Bunchgrass zone in the Thompson Basic Ecosection.
The Western Harvest Mouse may maintain more than one nest within
its home range to use as rest sites. The nest is about the size
of and shape of a baseball, consisting of grass lined with fine
plant materials and can be found on the ground in clumps of grass,
shrubs or logs or hanging from vegetation. There is a tiny entrance
underneath, which leads into a golfball-sized chamber, lined with
the finest of plant material such as down or dandelion fluff. This
species does not construct burrows, although it will use other small
mammal burrows for shelter.
Why is it at risk?
Grassland habitats in southern British Columbia have been affected
by cattle grazing, agriculture, and urbanization.
The most intensive grazing in the Okanagan Valley occurred in the
late 1880s. Over-grazing by cattle has altered the plant species
composition and cover in British Columbia grasslands and, presumably,
this has affected habitat quality and small mammal populations.
Heavy cattle grazing reduced cover in ravines and open grasslands,
which impacted Western Harvest Mouse habitat. Grazing also reduced
the availability of food since the Western Harvest Mouse's diet
consists primarily of seeds from grasses.
In British Columbia, ongoing habitat loss through urbanization
has had the greatest impact on this species. Historically, grasslands
were distributed continuously throughout low elevations of the Okanagan
and Similkameen valleys. The conversion of grasslands to irrigated
orchards, vineyards, and cultivated fields, and recent urban development
has eliminated much of the original grassland-steppe in these valleys
and contributed to habitat fragmentation.
The rapid growth of urban centres, such as Penticton, Kelowna, and
Vernon, in the past decade also has resulted in significant habitat
loss and contributed to fragmentation of the range of the Western
Harvest Mouse.
The amount of protected habitat in parks and ecological reserves
in the Okanagan region is small. The size of these protected areas
may be insufficient to maintain viable populations of the Western
Harvest Mouse if they become isolated and surrounded by unsuitable
habitat. A fire in July 1993 burned the entire Hayne's Lease Ecological
Reserve at Osoyoos Lake, demonstrating the vulnerability of small
reserves. It is also noteworthy that these protected areas are widely
separated, probably beyond the dispersal distance of the Western
Harvest Mouse. Large "non-protected" areas with significant
habitat are the Inkaneep Aboriginal Reserve on Osoyoos Lake, the
Penticton and Okanagan reserves near Okanagan Lake, and the Upper
and Lower Similkameen reserves. These reserves support large tracts
of low elevation grassland that are relatively undisturbed.
Because of its rarity in Canada, the Western Harvest Mouse is
of little economic concern in agricultural areas. Rodentcides
are routinely used by orchardists
in British Columbia to control vole and pocket gopher populations,
but the harvest mouse is uncommon in cultivated habitats and mortality
from poisoning is probably low. Potential limiting factors other
than those described above are: competition with other small mammals,
especially those species driven out by agricultural and urban development,
and habitat disturbance from hay mowing, cultivation and fire.
Biology
Breeding
The potential reproductive rate of the Western Harvest Mouse is
high. The female are polyoestrous
, breeding throughout the year in southern parts of the range
with the exception of late winter. Towards the northern limits of
its range , where
the seasons are more pronounced, Western Harvest Mice breed only
in late spring and summer. Females are capable of breeding at about
four months old. Pregnant or lactating
females were captured between June and September in British Columbia.
Although females may produce as many as 14 litters per year in captivity,
wild females in Canada probably produce two or three litters. The
gestation period is 23-24 days, with a litter size at birth averaging
2.6, but the number of young varies from one to nine.
The young harvest mice are approximately 1.0 to 1.5 g at birth.
They are pink, naked, and blind. Their lower
incisors appear at four days, the pelage is visible at five
days, the eyes open between the tenth and the twelfth day, and they
begin to walk at about the same time. Their young are weaned at
19 days and continue to develop rapidly
Behaviour
There are no comprehensive behavioural studies on this species.
The available data consist of descriptive observations from field
studies or from captive individuals. Wild and captive Western Harvest
Mice are nocturnal,
and most active before midnight, and on moonless or rainy nights.
It is active year-round, utilizing trails built by other small mammals.
Western Harvest Mice have been described as 'ferocious,' 'cannibalistic,'
'not gentle,' and 'nervous,' and some claim that it dislikes being
handled. In colonies, harvest mice appear rather sedentary and spend
much of their time clustered together. However, they are remarkably
compatible in mixed colonies of house mice and deer mice, and often
cluster together with these species, even forming interspecific
social hierarchies.
Captive Western Harvest Mice can be induced to enter shallow torpor
by exposure to temperatures below 10°C. The ability to enter
shallow torpor presumably helps conserve energy during periods of
stress (food deprivation,
water shortage, or cool ambient temperatures). Torpor may be critical
to survival of Canadian populations at the extreme northern limits
of the species’ range, where they are exposed to seasonal
cool temperatures. Western Harvest Mice are not known to hibernate,
and they may be captured year-round in the Okanagan Valley.
Diet or Growing requirements
Dietary data, based on studies done in California and the Great
Plains of the United States. The summer diet of the Western Harvest
Mouse consists primarily of seeds and invertebrates including: blue
grass, fescue, bromegrass, oats, vetch, fruit and insects. Flowers,
herbaceous material,
and Endogone fungi are also consumed. Small caches of sectioned
grass blades and stems may be found in runways of harvest mice.
In late summer and fall, the diet consists mainly of seed from grasses
and forbs. Some food
is cached prior to the arrival of inclement weather. The average
food consumption is estimated at 1.63 g of oats per day. The arboreal
activity of Western Harvest Mice probably is related to
foraging for seeds, flowers, and insects.
Predators
Rodents as diminutive as the Western Harvest Mouse must be on guard
against a host of possible predators
including: snakes, owls, and shrikes, and carnivores, such as weasels,
skunks, and coyotes.
Sources of more information
Related On-line Sites to Visit
Publications
Mammals of BC , Nagorsen 1990
Status Report, Wildlife Working Report No. WR-71, March 95, Nagorsen
Status Report, COSEWIC, Nagorsen, 1994
Habitat Conservation Fund, August 1992
Banfield, A.W.F. 1974. Mammals of Canada. University of Toronto
Press.
Kritzman, E.B. 1977. Little Mammals of the Pacific Northwest.
North Vancouver: J.J. Douglas Ltd.
Museum Specimens
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