Giant Kangaroo Rat

Dipodomys ingens

Description

The giant kangaroo rat (Dipodomys ingens ) is adapted for two-footed hopping. The hind limbs are large compared to the size of the forelimbs, the neck is short, and the head is large and flattened. The tail is longer than the combined head and body length and has a dorsal crest of long hairs toward its end, terminating in a large tuft. Large, fur-lined cheek pouches open on each side of the mouth. The pouches extend as deep invaginated pockets of skin folded inward along the sides of the head.

Giant kangaroo rats are distinguished from the coexisting species San Joaquin kangaroo rat and Heermann's kangaroo rat by size and number of toes on the hind foot.

The giant kangaroo rat is the largest of all kangaroo rats, weighing about 6.4 oz (about 200 g) and reaching a length of 14 in (35.6 cm). Its tail is about 8 in (20.3 cm) long. Other distinguishing features are the five toes on each hind foot (some other kangaroo rats have only four) and short ears. The general coloration is brown above and white below. The cheeks are whitish and the eyelids black; there is a lateral white stripe along the tail; the flanks appear to be darker because of a greater number of dusky hairs. Juveniles are paler and grayer than the adults.

Behavior

Giant kangaroo rats are primarily seed eaters, but they also eat green plants and insects. They cut the ripening heads of grasses and forbs (herbs) and cure them in small surface pits located on the area over their burrow system. They also gather individual seeds scattered over the ground's surface and mixed in the upper layer of soil. Surface pits are uniform in diameter and depth, about 1 in (2.5 cm), placed vertically in firm soil, and filled with seed pods. After placing seeds and seed heads in pits, the animal covers them with a layer of loose, dry dirt. Pits are filled with the contents of the cheek pouches after a single trip to harvest seeds. Before being moved underground, the seeds, including filaree and peppergrass, are sun-dried to prevent molding. Individuals in many populations of giant kangaroo rats also make large stacks of seed heads on the surfaces of their burrow systems. The material is cured, then stored underground.

Green foliage might be an important part of the diet during lactation. Other individuals, including a young female and adult males, were captured with foliage and fruits of peppergrass and foliage of filaree in their cheek pouches. In captivity, giant kangaroo rats have been maintained for periods from two weeks to more than two years on a diet of air-dried seeds, consisting primarily of millet, oat, and sunflower, occasionally supplemented with green plants. Of the green plants, captives preferred forbs to annual grasses and usually ignored the blades of perennial grasses. Live insects of the bee and wasp family were found in the cheek pouch of a giant kangaroo rat. Giant kangaroo rats in captivity will eat seeds, lettuce, and darkling beetle larvae.

Giant kangaroo rats forage on the surface from around sunset to near sunrise, though most activity takes place in the first two hours after dark. Foraging activity is greatest in the spring as seeds of annual plants ripen. Typically, plants such as pepper-grass ripen first, and early caches, mostly in pits instead of stacks, consist of pieces of the seed-bearing stalks of this and other early ripening species. The ability to transport large quantities of seeds and other food in cheek pouches and their highly developed caching behaviors, coupled with relatively high longevity of adults with established burrow systems, probably allow giant kangaroo rats to endure severe drought for one or two years without great risk of population extinction.

The species has an adaptable reproductive pattern that is affected by both population density and availability of food. During times of relatively high density, females have a short winter reproductive season, with only one litter produced. In contrast, populations at low densities continue to breed into summer during drought. In 1990, a year of severe drought and no seed production, most females appeared not to reproduce; the few that bred apparently failed to raise young. In most years females were reproductive between December and March or April; in colonies with low densities, reproduction extended into August or September. Mating strategies seem to be flexible and may be responding to the age of males, proximity of females, and changes in sex ratios.

Giant kangaroo rats can breed the year of their birth if food and space are sufficient. Some juvenile females have been documented as having their first litters at an estimated mean age of five months. Some females had two to three litters per year.

The major time for dispersal of giant kangaroo rats seems to be following maturation of young, about 11-12 weeks after birth. However, in years of high density (when most or all burrow systems are occupied), most young appear to remain in their natal burrows until the opportunity to disperse arises or they finally are driven off by the mother or one of the siblings. Under these circumstances, death or dispersal of the resident does not leave a burrow system vacant for long. Females have a much higher survival rate than males.

Giant kangaroo rats are active all year and in all types of weather. They do not migrate or become dormant or torpid. Although primarily nocturnal, giant kangaroo rats have been seen aboveground during daylight, including midday in the hottest part of the year. They typically emerge from their burrows soon after sunset and are active for about two hours. There usually is no second period of activity before dawn. Animals are aboveground only for about 15 minutes per night. Activity patterns appear to be unaffected by distance from the home burrow, snow, rain, wind, moonlight, or season.

Habitat

Historically, giant kangaroo rats were believed to inhabit 1) annual grassland communities with few or no shrubs and, 2) well-drained, sandy-loam soils located on gentle slopes in areas with about 6.3 in (16 cm) or less of annual precipitation and no risk of flooding in winter. However, more recent studies found that giant kangaroo rats inhabit both grassland and shrub communities on a variety of soil types and on steeper slopes up to 2,850 ft (868.7 m) above sea level. This broader concept of habitat requirements probably reflects the fact that most remaining populations are on poorer and marginal habitats compared to those of the large, historical populations in areas now cultivated. Still, these studies demonstrated that the preferred habitat of giant kangaroo rats is the annual grassland community on gentle slopes of sandy-loam soil. Few plots in flat areas were inhabited, probably because of periodic flooding during heavy rainfall.

Distribution

Substantial kangaroo rat populations survive only in a few areas at the southern edge of the historic range in six counties of California (Merced, San Benito, Fresno, Kings, San Luis Obispo, and Santa Barbara). In 1980 kangaroo rat colonies were widely scattered within a total area of less than 76,800 acres (31,000 hectares). Since then, the range has been reduced by at least 50%. The giant kangaroo rat probably has been completely exterminated in Merced County, and only a few small isolated colonies survive in San Benito, Fresno, and Kings counties. The last remaining large tracts of suitable habitat are in the upper Buena Vista Valley of western Kern County, the Elkhorn and Carrizo plains of eastern San Luis Obispo County, and the Cuyama Valley of northern Santa Barbara County.

The species population is currently fragmented into six major geographic units that are further fragmented into more than 100 smaller populations. Many of these smaller populations are isolated by several miles of barriers (such as steep terrain with plant communities unsuitable as habitat) or by agricultural, industrial, or urban land without habitat for this species.

Within the area of currently occupied habitat, populations of giant kangaroo rats have expanded and declined with changing weather patterns since 1979. At their peak in 1992-1993, there were probably about six to 10 times more individuals than at their low point in the spring of 1991.

Threats

The principal cause of the giant kangaroo rat's decline has been the conversion of native grassland to agricultural production. Because of habitat fragmentation, remaining populations are likely to become genetically isolated. Kangaroo rats may also be jeopardized by human recreation in their habitat, by predation, and by poisons used to control the California ground squirrel. The evidence linking these rodenticides to kangaroo rat decline is circumstantial but strong.

Researchers believe that oil exploration may cause loss of food and cover through removal of vegetation and destruction of burrow systems. Vehicles may crush mammals, and 14 giant kangaroo rats were found dead in a drainage contaminated by oil in the Buena Vista Valley. Although the extent of effects of oil and gas development on the species is not known, intensive development, which requires recontouring of soil surface profiles, could adversely affect this species.

Kangaroo rat populations are known to fluctuate naturally because of climate, disease, or predation by owls, coyotes, kit foxes, badgers, and snakes. Nonetheless, many populations of the giant kangaroo rat in Fresno, Kern, and San Luis Obispo counties have experienced recent precipitous declines. Although the cause of these declines is not clearly understood in many instances, the overall trend is dramatic and negative.

The species cannot survive in areas where field cultivation destroys its burrows and food caches. As recently as the late 1950s kangaroo rat population densities remained high over a substantial portion of its range, but major water diversion projects in the late 1960s and 1970s stimulated new agricultural development. This trend continues near remaining giant kangaroo rat populations in western Kern and southeastern San Luis Obispo counties.

Several human-induced factors other than agricultural production have affected the giant kangaroo rat and its habitat. Among these are disturbances from mining activity, construction of a rifle range, trampling of a population by campers, partial destruction of a large colony from road widening, construction of several structures along the edge of a colony, and direct impacts to colonies from off-road vehicle use.

Conservation and Recovery

Though substantial habitat for giant kangaroo rats is now in public ownership, recovering giant kangaroo rats requires additional habitat protection. Land acquisition, purchase of conservation easements, or other incentive mechanisms that will ensure that suitable habitat will be maintained in perpetuity are also needed to protect key local populations. Some existing public lands could be inhabited or support larger populations if suitably restored.

Of highest priority for habitat protection is proper land use and management on publicly owned and conservation lands in the Carrizo Plain Natural Area, Naval Petroleum Reserves in California, Lokern Natural Area, and Ciervo-Panoche Natural Area. Of equal priority is supporting research on habitat management and restoration, focusing on effects of livestock grazing on habitat quality, and habitat restoration on retired farmland, especially abandoned dryland farms. Other conservation measures should include restricting the purchase of anticoagulant-type rodenticides by the public and prohibiting off-road vehicle travel and recreation in the kangaroo rat's habitat.

Contact

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/

Reference

Braun, S. E. 1985. "Home Range and Activity Patterns of the Giant Kangaroo Rat, Dipodomys ingens. " Journal of Mammalogy 66:1-12.