Harvest mice are small rodents found in both the Old World and the New World, with distinct species inhabiting different regions and exhibiting unique adaptations to their environments. This article explores the diet and habitat preferences of both the Old World harvest mouse (Micromys minutus) and the New World harvest mice (Reithrodontomys sp.), highlighting their ecological roles and conservation status.
Old World Harvest Mouse (Micromys minutus)
The harvest mouse (Micromys minutus) is a small rodent that can be found across Europe and Asia.
Habitat and Distribution
The Old World harvest mouse has an extensive natural range covering Europe and Asia. Across this vast expanse, the species has evolved to thrive in a variety of differing habitats, including some created- and managed- by man, such as grasslands, meadows, cereal crops, hedgerows, and reed beds. They never enter into houses; are carried into ricks and barns with the sheaves; abound in harvest; and build their nests amidst the straws of the corn above the ground, and sometimes in thistles. The harvest mouse is common in all east coast counties of England, reaching the North York Moors. Harvest mice reside in a large variety of habitats, from hedgerows to railway banks. Harvest mice in Japan like making wintering nests near the ground from grasses that are dried, which indicates that they require vegetative cover in the winter, as well as in the warmer seasons. Grasslands with a mix of perennials and annual grasses are required to balance the increases in nesting periods and the mice's need to secure nutrients.
Physical Characteristics
The harvest mouse ranges from 55 to 75 mm (2.2 to 3.0 in) long, and its tail from 50 to 75 mm (2.0 to 3.0 in) long; it weighs from 4 to 11 g (0.14 to 0.39 oz), or about half the weight of the house mouse (Mus musculus). Its eyes and ears are relatively large. The upper part of the body is brown, sometimes with a yellow or red tinge; the under-parts range from white to cream coloured. It has a prehensile tail which is usually bicoloured and furless at the tip. The mouse's rather broad feet are adapted specifically for climbing, with a somewhat opposable, large outermost toe, allowing it to grip stems with each hindfoot and its tail, thus freeing the mouse's forepaws for food collection.
Diet
Wild Harvest Mice are considered omnivorous, yet their diet consists mostly of seeds and grains. This is what we use in captivity to form the basis of their diet. I would recommend a reasonably low-fat seed mix- for example canary seed- which is readily available in most pet shops, and even online. Millet sprays are commonly used when feeding Harvest Mice. In the wild, the species will also eat berries and invertebrates, which can be offered in small amounts in captivity. A great way to provide berries is through the browse you add to the setup. Throughout the year many UK native plant species, for example hawthorn, grow berries. Live invertebrates can also be placed in bowls or scattered around the enclosure for the mice to scavenge. These mice have been known to eat grasshoppers and various other species- great alternatives for your captive individuals are small locusts and mealworms. If you would rather not use live feeds, dried mealworms made for birds can also be used, but these tend to have little nutritional value. Some research suggests that Harvest Mouse diets vary seasonally, with the species consuming more invertebrate prey in the colder months.
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Nesting Behavior
These mice spend much of their time off the ground, navigating branches, or the stems of grasses and reeds. Grass is the main life support system for Harvest Mice; they use it for food and shelter, eating its seed heads and cereals, and creating spherical nests out of shredded living stems.
They breed as many as eight at a litter, in a little round nest composed of the blades of grass or wheat. One of these nests I procured this autumn, most artificially platted, and composed of the blades of wheat; perfectly round, and about the size of a cricket-ball. During breeding season this mouse constructs globular nests of grass suspended between vertical stems up to 13 cm above ground; during the rest of the year, nests are located in holes in the ground, beneath haystacks, or in buildings.
Conservation Status
Harvest mice are listed as a BAP (Biodiversity Action Plan) species because they are thought to have become much scarcer in recent years, and they require conservation plans to reverse the decline. Harvest mice are thought to be ‘Critical’ in Scotland and ‘Vulnerable’ in Wales due to changes in habitat management and agricultural methods, such as through the use of stubble burning, combine harvesting, and pesticides. These are thought to be the main cause for the loss of populations from certain areas, although there have been no reliable studies to quantify this change.
New World Harvest Mice (Reithrodontomys sp.)
The 20 species of American harvest mice are widespread, being found from southern Canada to northern South America at elevations ranging from below sea level to above the timberline in the northern Andes Mountains.
Habitat and Distribution
American harvest mice live in prairies, grassy fields with shrubs or trees, meadows, temperate and tropical forests, and cultivated fields. One, the salt-marsh harvest mouse (R. raviventris), lives only in the tidal salt marshes surrounding San Francisco Bay in California and is listed as an endangered species under federal and state laws.
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The western harvest mouse has a range through much of western North America, is distributed throughout California. Across its range the western harvest mouse inhabits sagebrush, steppe, and agricultural areas. The western harvest mouse dwells in areas below 1600 feet. It forages in grasslands bordering riparian areas such as irrigation right of ways, coastal marshes, streams, or lakes. The eastern harvest mouse is found across the southeastern United States from Maryland to Florida to eastern Oklahoma and Texas. The eastern harvest mouse is found mainly in areas dominated by grasses and other herbaceous plants including abandoned fields, roadside ditches, wet bottomlands, marshes, wet meadows, and briar thickets. These mice require dense vegetation (>80% herbaceous cover).
Physical Characteristics
Weight varies among species from 6 to 20 grams (0.2 to 0.7 ounce) and body length from 5 to 15 cm (1.9 to 5.9 inches); the slender, scantily haired tail may be either shorter or longer than the body.
The western harvest mouse is a slim, medium sized mouse with adult lengths ranging from 4 ½ to 6 ½ inches in length and weight from 3 to 8 ounces. It is distinguishable from the island deer mouse by its smaller head, body size and it's relatively much longer tail, including the grooves on the anterior face of the upper incisors. It has a brownish back, buff-colored sides, and a white underside. There is an indistinct dark broad stripe along its spine. The tail is from about 2 to 4 inches in length, sparsely haired, dorsally grey while white underneath. The ears are naked giving them a prominent, flesh-color or a buffy-cinnamon color. The western harvest mouse only has 4 digits on the forefeet. The eastern harvest mouse is a small mouse, 5 in/12.6 cm with a tail as long as the head and body. Its back is deep brown or gray with some black, usually with a dark stripe running down the back. The sides of the eastern harvest mouse are lighter in color. The grayish belly is distinguished by a strong line on the sides. Ears are blackish all over. The tail is bicolored matching the body, darker above and lighter below.
Diet
Their diet includes seeds, flowers, cactus fruit, succulent green sprouts, and invertebrates. Harvest mice eat mostly seeds of grasses and forbs, but like many granivorous animals they also eat some insects as available.
Nesting Behavior
Although terrestrial, they are excellent climbers and build globular nests of vegetation either on the ground or above it in grass, sedge, shrubs, or trees. The western harvest mouse may have more than one nest within its home range to use as rest sites. The nests are about the size and shape of a baseball, consisting of grass lined with fine plant material. These nests are located on the ground in clumps of grass, shrubs, or logs, or hanging from vegetation. Eastern harvest mouse habitat can be grazed by livestock. They construct baseball-sized nests of shredded grass and other plant fibers on the ground in tangled vegetation, under debris, or above ground in a clump of grass.
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Behavior
American harvest mice are nocturnal and are active all year. The western harvest mouse appears to be strictly nocturnal. It is most active before midnight, on moonless or overcast nights. Minimum activity occurs between 6:00 am and noon. This mouse is active year round utilizing trails built by other small mammals. The eastern harvest mouse is nocturnal but may be active during daylight hours during cold weather.
Reproduction
The western harvest mouse has a high potential reproductive rate. Females can breed at 4 months of age. It births litters averaging 4 young, but ranging anywhere from 1-9 young. The female is able to become pregnant more than once a year, with a gestation period of 23 or 24 days. It breeds throughout the year with the exception of late winter in the southern part of its range. The young weigh ½ ounce or less at birth and are naked, pink, and blind. The mouse's eyes open 10-12 days after birth and they are weaned 19 days after birth. Eastern harvest mice breed nearly year-round with peak in spring.
Conservation Status
Harvest mice are classified as ‘Critical’ in Scotland, ‘Vulnerable’ in Wales, and ‘Near Threatened’ in Great Britain overall. In 2021, the Mammal Society launched the National Harvest Mouse Survey - a volunteer-led annual nest survey that aims to improve understanding of the status of harvest mice across the UK. One, the salt-marsh harvest mouse (R. raviventris), lives only in the tidal salt marshes surrounding San Francisco Bay in California and is listed as an endangered species under federal and state laws.