Pacific Pocket Mouse

Perognathus longimembris pacificus

Description

All members of the family Heteromyidae are nocturnal granivores (animals subsisting on seeds or grain) with external, fur-lined cheek pouches. The body pelage (hairy covering) of the little pocket mouse is silky. The little pocket mouse shows wide geographic variation in pelage color. The dorsal pelage is predominately brown, pinkish buff, or ochraceous buff. The ventral pelage is whitish. There are typically two small patches of lighter hairs at the base of the ear. The tail can be either distinctly or indistinctly bicolored. The Pacific pocket mouse is among the smallest subspecies of little pocket mice, ranging up to 5.2 in (13.2 cm) in length from nose to tip of tail. Little pocket mice weigh 0.25-0.33 oz (7-9 g).

The Pacific pocket mouse (Perognathus longimembris pacificus ) is one of 19 recognized subspecies of the little pocket mouse (P. longimembris ). This subspecies is the smallest member of the family Heteromyidae. The tail, hind foot, and skull lengths and the size of skull structures are also the smallest of all little pocket mouse subspecies. Other family members include the spiny pocket mice (Heteromys and Liomys ), pocket mice (Perognathus and Chaetodipus ), kangaroo rats (Dipodomys ), and kangaroo mice (Microdipodops ).

Behavior

Little pocket mice live up to seven and one-half years in captivity and three to five years in the wild. Pregnant and lactating females have been found from April through June, and immatures have been reported from June through September. The little pocket mouse produces one or two litters ranging in size from three to seven young in a year.

Pacific pocket mice primarily eat the seeds of grasses and forbs, but occasionally eat leafy material and soil-dwelling insects. Like most other members of the Heteromyidae family, this subspecies is nocturnal.

The Pacific pocket mouse has a high metabolic rate, continually needs food supplies while active, and loses heat rapidly. It is likely adapted for burrowing or digging, but its burrows have limited capacity to store food. Little pocket mice may stay in their burrows continuously for up to five months in winter, alternating between periods of dormancy and feeding on stored seeds as well as hibernating under adverse conditions.

While active above ground, little pocket mice have ranged up to 1,000 ft (305 m) from their burrows in a 24-hour period. Little pocket mouse home ranges vary in size from 0.3-1.4 acres (0.1-0.6 hectares), and populations range in density from 0.4 to 2.2 individuals per acre.

Little pocket mice are among the smallest mammals known to hibernate. They hibernate in winter, typically from September to April. In contrast to other hibernators that accumulate fat reserves for hibernation, little pocket mice feed on seed caches stored in their burrows. Individuals become torpid when deprived of food for 24-36 hours. During hibernation, the body temperature of pocket mice is slightly higher than, and varies directly with, the ambient temperature. Periods of dormancy have neither a daily or strictly seasonal pattern. In captivity, dormant individuals may show some activity each day within their burrows. Emergence from hibernation in spring correlates with availability of forb and grass seeds.

Habitat

Pacific pocket mice occur on fine-grain, sandy or gravelly substrates in the immediate vicinity of the Pacific Ocean. Although individual Pacific pocket mice have been collected or observed in several plant communities, the subspecies has narrow habitat requirements and typically occurs on sandy soils. The Pacific pocket mouse was known to inhabit coastal strand, coastal dunes, river alluvium, and coastal sage scrub growing on marine terraces; recent survey efforts, however, have found the subspecies in sandy substrates within coastal sage scrub.

Distribution

There are three historic localities for the subspecies in Los Angeles County—Marina del Rey/El Segundo, Wilmington, and Clifton—but there are no records of the Pacific pocket mice in the county since 1938. The Pacific pocket mouse has been confirmed at two locales in Orange County: the San Joaquin Hills and Dana Point. The only known extant Pacific pocket mouse population in Orange County occurs on the Dana Point Headlands, a historic Pacific pocket mouse locality that was discovered during the 1930s. The Pacific pocket mouse was known historically at four localities in San Diego County: San Onofre, the vicinity of the Santa Margarita River Estuary, Penasquitos Lagoon, and the lower Tijuana River Valley.

In several surveys conducted in 1993, 25-36 individual Pacific pocket mice were captured at Dana Point Headlands, spread over approximately 3.5 acres (1.4 hectares) of occupied habitat. Twenty-one individual Pacific pocket mice were captured at Dana Point Headlands in 1997 on approximately 6.8 acres (2.8 hectares) of occupied habitat.

Two locations with extant Pacific pocket mouse populations were discovered in 1995 at Camp Pendleton, a U. S. Marine Corps base in the vicinity of two historic locales. One location consisted of two separate small pockets of animals detected immediately north and south of San Mateo Creek. The San Mateo North site was approximately 0.9 mi (1.4 km) from the coast. The San Mateo South site was approximately 1.3 mi (2.1 km) from the coast. In 1995, 20 individuals were captured at the San Mateo South site on approximately 32 acres (13 hectares) of occupied habitat; 37 individuals were captured at the San Mateo North site on approximately 16 acres (6.5 hectares).

The second, separate Camp Pendleton locale with an extant population occurs on a marine terrace north of the Santa Margarita River in an area that is designated by Camp Pendleton as training area Oscar-1. The extent of occupied habitat in the Oscar-1 training area ranged from approximately 1.6-2.5 mi (2.6-4 km) from the coast in 1996. Fifty-four individual Pacific pocket mice were captured in 1995 at the Oscar-1 site, estimated to possibly be as large as 865 acres (350 hectares) of occupied habitat. Although supporting data are presently lacking, both Camp Pendleton populations could be part of a metapopulation operating in the long term with populations occupying a dynamic mosaic of habitats. Even though the Pacific pocket mouse appears to be extirpated from suitable patches today, it does not necessarily mean that these populations do not function as a metapopulation.

Less than 150 individual mice were live-captured at these sites from 1993 to 1997. Numerous recent surveys within the historic range of the subspecies have failed to detect additional extant populations. Given the small numbers of known populations, the small area known to be currently occupied, and total individual animals captured to date, the Pacific pocket mouse remains highly endangered and vulnerable to extinction.

Threats

The quantity of potential Pacific pocket mouse river alluvium substrate has significantly declined since the subspecies was last recorded in numbers in the 1930s. With few exceptions, essentially all of the rivers and creeks within the historic range of the Pacific pocket mouse are now partially or completely channelized. In many cases, stream and sediment flows are regulated or inhibited by dams, reservoirs, or other water conservation or impoundment facilities.

In Los Angeles County, two of the three historic locales for the Pacific pocket mouse have been developed, and the third, Marina del Rey/El Segundo, has been substantially altered since the species was last detected there. The Hyperion area, which formerly contained relatively large expanses of coastal strand and wetland habitats, has been extensively developed.

In Orange County, the development of the Spyglass Hill area began in 1972. This development resulted in the destruction of the formerly occupied habitat at that site.

Although portions of the San Onofre area and the Santa Margarita River mouth in San Diego County remain relatively undisturbed, recent survey and small mammal trapping efforts at these locations failed to detect the presence of the Pacific pocket mouse. During the 1930s, the Camp Pendleton base did not yet exist and the city of Oceanside was immediately adjacent to the Santa Margarita River estuary. Much of the southern half of this estuary was destroyed in the early 1940s during the establishment of Camp Pendleton and the related construction of a boat basin and harbor facilities. In addition, the Oceanside area has been extensively developed since the Pacific pocket mouse was last recorded there in 1931, and little, if any, suitable habitat remains at that location.

Within the remaining undeveloped range of the Pacific pocket mouse, areas that contain suitable habitat for the species represent less than 10% of the remaining habitat.

The proliferation of non-native populations of the red fox in coastal southern California is well documented. It has been speculated that the red fox "may have hastened the demise of the Pacific pocket mouse in the El Segundo area," where the species apparently was well-represented historically.

Feral and/or domestic cats are threatening the population of the Pacific pocket mouse at Dana Point Head. A resident living immediately adjacent to the known population reported that domestic cats had, during the late 1990s, repeatedly brought home a number of "tiny gray mice." Of all rodent captures at Dana Point Headlands in 1993, 81% were Pacific pocket mice.

The vegetation of the marine terraces from Dana Point to southern Camp Pendleton today is heavily influenced by annual grasses and other non-native species, partly in response to exotic plant introductions and agricultural use of these lands during the late nineteenth and early twentieth centuries. Such dense exotic vegetation, in concert with relatively hard soils, probably precludes occupation by the Pacific pocket mouse, whereas relatively low densities of animals may have historically occurred in the same areas in association with the original, more open, native vegetation.

Artificial night-time lighting may cause problems for nocturnal rodents such as the Pacific pocket mouse through potential modification of predation rates, obscuring of lunar cycles, and/or causing direct habitat avoidance. Artificial lighting has the potential to increase the efficiency of predators. Illumination of foraging habitat by artificial light during the Pacific pocket mouse's periods of surface activity likely makes detection by predators easier, potentially increasing the predation rate by owls, coyotes, fox, house cats, etc. Artificially lit habitat areas may also be directly avoided by Pacific pocket mice for unknown behavioral reasons. In a survey performed on Fallbrook Naval Weapons Station, artificially lit but otherwise apparently suitable habitat was avoided by Heteromyid rodents, while adjacent unlit habitat areas were occupied.

In another potential threat, exotic Argentine ants are invading coastal sage scrub areas near Pacific pocket mouse habitats. These ants could have adverse direct or indirect effects on Pacific pocket mouse populations. Upon invasion into coastal southern California habitats, Argentine ants are known to drive out most native ant species. Since ants are a major ecosystem component of most terrestrial ecosystems, Argentine ants could adversely affect Pacific pocket mouse individuals and burrow sites directly. They could also affect seed producing plants, or could disrupt key ecosystem functions, including those typically carried out by native ants. The invasion of these ants may be expedited by development and associated irrigation.

Other non-native species, including the red fox, are also potential predators of the Pacific pocket mouse. The explosive proliferation of non-native populations of red foxes in coastal southern California is well-documented. Given the relative abundance of the red fox in coastal southern California and the fact that the diet of red foxes invariably include mice, red foxes could substantially have an impact on populations of Pacific pocket mice where their ranges overlap.

Conservation and Recovery

The immediate recovery goal is to avert the extinction of the Pacific pocket mouse by focusing on short-term strategies to improve the subspecies' prospects for survival. Foremost among these are the immediate protection and restoration of existing populations and the habitat of the subspecies. Considering the extremely small population size of the Pacific pocket mouse (less than 150) and the fragmentation and depletion of the coastal strand, river alluvium, and coastal sage scrub habitats upon which the subspecies depends, further losses of occupied or potential habitat would seriously reduce the probability that the mouse will survive. Given the small sizes of the populations at the three known extant locales, the apparently sedentary nature of the subspecies, and the severe fragmentation and diminution of the subspecies' habitat, the Pacific pocket mouse is highly susceptible to extinction as a result of environmental or demographic factors alone.

The recovery strategy for the Pacific pocket mouse consists of two components. The first is to stabilize the existing populations by protecting currently occupied habitat and searching for additional populations. The second component involves establishing additional populations through natural colonization into nearby and adjacent habitats, coupled with habitat management in these areas, and translocation of captive-bred individuals.

Camp Pendleton has authorized, coordinated, and funded multiple surveys for the Pacific pocket mouse in recent years. For purposes of continued military training and land management activities, the level of survey effort is considered sufficient to confirm the absence of the Pacific pocket mouse on those training areas where no Pacific pocket mice were found by this or previous studies. Nevertheless, construction projects in areas typical of Pacific pocket mouse habitat may still require specific trapping surveys.

Survey efforts authorized by Camp Pendleton in 1995 led to the discovery of two of the three locales with known extant populations of the Pacific pocket mouse. The base also developed a habitat evaluation model. Camp Pendleton is funding some additional surveys on the base, with trapping in 1998 on Edson Range Impact Area and in the Smart Mesa area. The base has noted that they would authorize the translocation of animals from the Base to off-base locales for purposes of establishing new populations. Camp Pendleton personnel are additionally preparing a management plan for upland habitats on the base and have begun the process of identifying potential habitats by developing a model that relies on data pertaining to soils, vegetation, elevation, and distance from the coast.

The Federal Highway Administration has required and authorized focused surveys for Pacific pocket mice in conjunction with the proposed Foothill Transportation Corridor project. The project applicant, the Transportation Corridor Agencies, apparently is redesigning the proposed project to avoid direct impacts to the local Pacific pocket mouse occupied habitat as it was defined in the last survey effort. As of the late 1990s, the applicant's proposed preferred alternative paralleled San Mateo Creek. One alternative alignment for the proposed project avoided San Mateo Creek drainage altogether by connecting with Interstate-5 farther to the north.

In an attempt to conserve the coastal California gnatcatcher's coastal sage scrub habitat and other sensitive plant and animal species, 30 cities, two counties, 35 landowners, and three land management agencies have formally committed to develop or to assist in the development of conservation plans.

The city of Dana Point has expressed an interest in the conservation of the Pacific pocket mouse population and other sensitive animals and plants within the city's jurisdiction. In this regard, the city suggested the potential need for conservation measures above and beyond those in the Orange County Central/Coastal NCCP (Natural Community Conservation Planning) plan. On April 28, 1998, the Dana Point City Council unanimously supported a residential development proposal that required 70 acres (28.5 hectares) of open space within a 122-acre (49.5-hectare) Dana Point Headlands site. Apparently most of the open space, which includes the Dana Point Headland Pacific pocket mouse population, would be privately owned.

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
Telephone: (503) 231-6121
http://pacific.fws.gov/

U. S. Fish and Wildlife Service
Carlsbad Fish and Wildlife Office
2730 Loker Avenue West
Carlsbad, California 92008-6603
Telephone: (760) 431-9440
Fax: (760) 431-9624

References

Erickson, R. A. 1993. Pacific Pocket Mouse (Perognathus longimembris pacificus ). Draft manuscript to be included in Endangered Rodents of the World, to be published by the Species Survival Commission of the International Union for the Conservation of Nature and Natural Resources (IUCN).

U. S. Fish and Wildlife Service. February 3, 1994. "Emergency Rule to List the Pacific Pocket Mouse as Endangered." Federal Register 59 (23): 5306-5311.

U. S. Fish and Wildlife Service. 1998. Pacific Pocket Mouse (Perognathus longimembris pacificus) Recovery Plan. Portland, Oregon. 112pp.