Introduction The western harvest mouse is the smallest of the rodent species found in Channel Islands National Park. It occurs only on Santa Cruz Island within the park, but also on Santa Catalina and San Clemente islands. The species may have reached San Clemente Island via hay bales in the 1930s, whereas harvest mice may have been inadvertently introduced to Santa Cruz and Santa Catalina in native American canoes.
Quick and Cool Facts
In 1951, noted zoologist and professor at UC Berkeley, Dr. Oliver Pearson, named and described this species on the islands.
The western harvest mouse has a broad range in western North America but is only found only on three of the eight Channel Islands-Santa Cruz, Santa Catalina, and San Clemente Islands.
Females can breed at 4 months of age, and in a year will have from two to three litters.
The western harvest mouse appears to be strictly nocturnal.
The initial census of this species on the islands thought the numbers to be very small, which subsequently proved inaccurate.
Appearance 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 hind feet are ½ inch to ¾ inch in length with 5 digits.
Range The western harvest mouse, besides having a range through much of western North America, is distributed throughout California. It is common to abundant in shrub lands, grasslands, and in early several stages of most habitats statewide (except at some high elevations, although lower than 1600 feet).
Habitat 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 western harvest mouse is often considered an edge species. It is important for the western harvest mouse to have shrub or grass overstory with tall lush herbaceous cover to conceal its nests. They typically build above-ground nests in dense grass or herbaceous vegetation.
On Santa Cruz Island, their distribution was previously thought to be limited to the marsh area at Prisoner's Harbor. Recent surveys found them to be much more widely distributed on the island, but at relatively low numbers (especially when compared to the numerous island deer mice), due to their need for dense grass or herbaceous cover (which is patchily distributed on Santa Cruz. The species is more numerous on Santa Catalina Island, which may be an indication of that island's recovery from non-native grazers.
Feeding 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. Their food is mostly vegetable matter, chiefly seeds, leaves, and stems of grasses and forbs, although occasionally insects, such as grasshoppers, crickets, and beetles, may be eaten.
Reproduction 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. Each nest has a small entrance on its underside leading to a golf ball sized chamber lined with dandelion fluff or a similar material.
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. In northern parts of its range, this harvest mouse breeds only in late spring and summer (usually 2-3 litters).
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. The western harvest mouse goes through 3 pelages: juvenile, sub-adult, and adult. The juvenile pelage is rather wooly and dull grey. The adult pelage is relatively brighter. Molting starts on the ventral surface and spreads over the flanks to meet on the back. A second point of origin is on the muzzle, the new coat spreads back forming a molt line behind the ears. This specie molts once annually during the summer.
Conservation Status According to the IUCN, (International Union for Conservation of Nature and Natural Resources), this species is listed as Least Concern in view of its wide distribution, presumed large population, tolerance of a broad range of habitats, occurrence in a number of protected areas, and because it does not appear to be under threat and is unlikely to be declining at nearly the rate required to qualify for listing in a threatened category.
In 1998 the California Department of Fish and Game considered the species to be one of special concern.
The recent removal of feral pigs from Santa Cruz Island should improve habitat for harvest mice, which may become more widely and densely distributed on the island.
Jack C. von Bloeker, Jr. Land Mammals of the Southern California Islands