Point Arena Mountain Beaver

Aplodontia rufa nigra

Description

The Point Arena mountain beaver is similar in appearance to a tailless muskrat. The general body configuration is stout, compact, and cylindrical. An average adult weighs about 4.4-5.5 lb (2-2.5 kg) and measures about 12.9 in (32.8 cm) in length. The head is broad, massive, laterally compressed, and notable for its flat upper surface and lack of postorbital processes. Long, stiff vibrissae are present. The eyes and rounded ears are quite small. This species has short limbs of about equal length, and the forelimbs have functionally opposed thumbs. All digits have long curved claws. A distinctive feature of the species is the cylindrical stump of a tail.

The Point Arena mountain beaver is a strikingly marked subspecies. The black coloration of adults is present as early as July.

Behavior

This species has a low reproductive rate compared to other rodent species. It is monestrous and usually does not give birth before its second year. All females in a population ovulate at about the same time—during a period of five to seven weeks in middle or late winter—causing the breeding season to be quite limited. Gestation period is estimated at 28-30 days. Some parturition differences between subspecies have been noted. In general, the coastal subspecies are found to have earlier parturition dates (late February and March) than the inland or mountain subspecies (April and May).

The single litter usually contains two to three young. At birth the young are naked, blind, and helpless. Vibrissae are present. Lactation probably occurs for the first two months after birth.

It was once thought that the Point Arena mountain beaver had some sort of social organization due to the extensive congregations in some localities. It now appears that some home ranges overlap, but populations consist of separate individuals independently utilizing available resources. This species vigorously defends its nests and burrows and is considered nonmigratory.

The Point Arena mountain beaver utilizes most, if not all, of the understory plants in its habitat as food. It prefers succulent herbaceous vegetation and deciduous tree bark and leaves. The main species eaten by the mountain beaver include sword fern, cow-parsnip, salal, and nettle. This species appears to forage mainly during the night.

Habitat

The known range of the Point Arena mountain beaver is limited to a small area receiving heavy rainfall along the Pacific Coast, extending from British Columbia to central California. Within this area the species is limited to cool, moist environments. Inland subspecies appear dependent upon an insulating snow pack that moderates surface and burrow temperatures.

The climate in the area where the Point Arena mountain beaver is found is characterized as mild, with little daily or annual temperature variation. The average annual temperature ranges from about 52°F (11.1°C) near the coast to 60°F (15.6°C) in southern inland areas. This area has a short frost season (the average date of the first frost is December 15) and one of the longest growing seasons in California, more than 300 days annually.

All known populations of the Point Arena mountain beaver have been found in either a sheltered gulch or on a steep, north-facing slope. Burrow systems are under dense stands of perennial vegetation where soil conditions allow for easy excavation. An abundant supply of food plants and moderately deep and firm soil with good drainage are found in this species' habitat.

The Point Arena mountain beaver is found in habitats with four main types of vegetation: 1) coastal scrub such as cow-parsnip, coyote brush, California blackberry, and poison oak; 2) coniferous forest such as Douglas fir, grand fir, and bishop pine; 3) riparian such as thimbleberry, nettle, elder-berry, giant horsetail, and willows; and 4) stabilized dunes.

This species lives in an extensive system of tunnels usually constructed about a foot from the surface. In coastal areas it burrows under the shrubby vegetation.

Distribution

No information on the historic range of the Point Arena mountain beaver is available. The subspecies is currently known from approximately 24 acres (9.7 hectares) of land in Mendocino County, California.

Only 10 populations were known at the end of the twentieth century, with a total number of individuals estimated at 100, the largest population containing 20 individuals.

The Point Arena mountain beaver is somewhat limited in maintaining its water balance and in thermoregulating. Anatomical and physiological data indicate that the species is incapable of producing a concentrated urine and, therefore, requires substantial daily amounts of water. This characteristic is believed to be responsible for its localized distribution in cool, moist areas. The Point Arena mountain beaver can thermoregulate adequately only over a relatively narrow band of ambient temperatures, 42.8-60.8°F (6-16°C), the normal temperature range within the burrows. When surface temperatures are too warm, the subspecies will either seek refuge in its burrow or orient its body to maximize its ability to lose body heat passively.

The low reproductive rate is also a limiting factor for the subspecies. Decreased genetic variability due to the small number of remaining individuals is another problem.

Threats

The Point Arena mountain beaver appears to have suffered most from habitat alteration or loss. Development, cattle and sheep grazing, and farming have greatly reduced coastal scrub in the species' habitat. Habitat has also been opened somewhat by livestock trails. Private and county road construction has encroached on the Point Arena mountain beaver habitat and caused higher mortality through vehicular traffic.

A proposed microwave tower within habitat occupied by the largest known population was the greatest threat at the close of the twentieth century. Construction of the tower would destroy habitat used by 10 of the 20 animals at this site.

The species' reputation as a burrower and forager in gardens, croplands, and forests has caused decreases in its numbers through poisoning and trapping. However, none of the subspecies endemic to California are known to cause substantial damage to crops, nor are they generally found in intensively managed forestlands.

Predation by domestic and feral dogs as well as cats is a mortality factor for the species. Due to the small numbers of individuals, even one predator can seriously impact the population.

Conservation and Recovery

Potential recovery action for the Point Arena mountain beaver includes establishing a buffer around each population site and excluding further urban or other development within this zone (about 100 acres [40.5 hectares] of total habitat and adjacent potential habitat); installing protective fencing; implementing cooperative agreements to manage the species; and restricting pesticide application. Other recovery actions include minimizing human and domestic animal impact; land acquisition or conservation easement; and development of federal or state recovery programs.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

Sacramento Ecological Services Field Office
2800 Cottage Way, Room W-2610
Sacramento, California 95825-1846
Telephone: (916) 414-6446
Fax: (916) 414-6486

References

Smurthwaite, Donald. May 1986. "Mountain Beaver: The Rodent That Gets No Respect." American Forests 46-50.

Steele, Dale T. 1986. "A Review of the Population Status of the Point Arena Mountain Beaver (Aplodontia rufa nigra )." U. S. Fish and Wildlife Service, Sacramento, Calif.

U.S. Fish and Wildlife Service. 1991. "Determination of Endangered Status for the Point Arena Mountain Beaver." Federal Register 56 (239):64716-64722.

U.S. Fish and Wildlife Service. 1998. "Point Arena Mountain Beaver (Aplodontia rufa nigra (Rafinesque) Recovery Plan." Region 1, Portland, Oregon. 71 pp.