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giant kangaroo rat
Dipodomys ingens

Status

U.S.A. and California Endangered

Life History

Giant kangaroo rats are curious and bold inhabitants of the most arid, southwestern edge of central California's San Joaquin Valley and adjacent valleys and plateaus of the Inner Coastal ranges. They are found from elevations of about 90 to 885 meters, though today, most extant habitat lies at elevations above 200 meters. Few colonies are located above 760 meters. Giant kangaroo rats mainly inhabit sandy-loam soils located on level and gently sloping ground vegetated with annual grasses and forbs and widely-scattered desert shrubs. Extant habitat has been fragmented, mostly by irrigated croplands, into six major geographic units, which in turn have been broken into dozens of smaller colonies by agricultural and petroleum developments. Fossil giant kangaroo rats are known from late Pleistocene (38,000 years before present) brea deposits in an area still inhabited.

Though they often emerge from their burrows in the twilight hours around sunset, and less frequently in the daytime, giant kangaroo rats are mainly active at night. Except when harvesting seeds at the end of the growing season of herbaceous plants, they are out of their burrows for only about 15 minutes per night during a 1.8-hour period starting near sunset.

Giant kangaroo rats are inveterate diggers, frequently remodeling their burrows, closing old entrances and creating new ones. They usually live solitary lives within their shallow burrows, which average less than 30 centimeters in depth and have branching side tunnels that typically do not reconnect. Besides an enlarged chamber for nesting, which may or may not contain nesting material such as plant fibers and animal hairs, there are several enlarged chambers where seeds are stored. There may be up to 24 of these larders in an individual's burrow, some empty or containing seed hulls and others each containing 1-9 liters of seeds, mostly peppergrass (Lepidium spp.), filaree (Erodium cicutarium), Arabian grass (Schismus arabicus), and brome grasses (Bromus spp.). As herbaceous plants ripen, giant kangaroo rats cut the seed head and place them either in shallow, thimble-shaped pits arranged in a honey-comb pattern and covered with dirt after filling, or in densely compacted piles on the surface. Surface piles are sometimes enormous for such a small animal. In one study, a stack was found that measured 1.2 by 1.8 meters and was 10 centimeters deep; in another, stacks averaged 29 liters, and ranged from about 2.5 to 53.2 liters. Seedheads in stacks cure in the sun for about 4-6 weeks before being moved to larders in the burrows. Besides seeds, giant kangaroo rats eat small amounts of green foliage such as leaves of clover (Trifolium depauperatum) and filaree, and insects.

Long-term occupancy of a site by giant kangaroo rats results in a mima-mound topography, with burrow systems located on mounds a few to several centimeters above the intervening ground. The mound is the center of the individual's territory, which is tiny in comparison to most kangaroo rats and other small mammals, averaging about 0.04 ha. These precincts of individual kangaroo rats support lusher, greener vegetation richer in nitrogen than herbaceous plants on surrounding ground. Productivity of plants on precincts averages about 3-5 times greater, and plant species composition on precincts differs from vegetation on surrounding ground, consisting mostly of plants with larger seeds favored by the kangaroo rats. Following harvest of the seed heads and dying of the annual vegetation, the kangaroo rat clears the plant litter from its precinct, creating a barren contrast with surrounding, vegetated ground.

The owner of a precinct with its granaries vigorously defends it against all intruders. A giant kangaroo rat produces long, rapid drumming sounds by alternately striking its huge hind feet on the ground while standing only on its hind feet. Presumably, this is one aspect of their territoriality. Drumming in the burrows and on the surface is greatest in late spring and summer during and after seed harvest. During the day, interloping birds and antelope squirrels (Ammospermophilus nelsoni), bent on stealing from the surface caches, are aggressively chased from the precinct. The giant kangaroo rat emerges from its burrow through a vertical shaft, and, with its boldly black-and-white-striped tail held stiffly curved upward, runs at top speed at the intruder. When it reaches the boundary of its territory, it reverses, and at full speed dives down the shaft, its long, upward-projecting tail being the last to disappear.

The better habitats for giant kangaroo rats are shared with few or no other small, nocturnal mammals. San Joaquin antelope squirrels are the only other common mammal. During the day they enter the openings of giant kangaroo rat burrows, usually reappearing shortly. Presumably they are using only shallow portions of tunnels not plugged at the surface to shelter from heat and predators. Occasionally, the occupant of a burrow emerges and chases off the antelope squirrel.

Giant kangaroo rats have major impacts on their communities, increasing and enriching plant productivity, being the base of the food chain for most predatory vertebrates, providing sheltering burrows for the endangered blunt-nosed leopard lizard (Gambelia sila), threatened antelope squirrel, and other animals, and providing favorable microhabitats for the endangered plants, San Joaquin woolly-threads (Lembertia congdoni and California jewelflower(Caulanthus californicus). Where shrubs such as saltbushes (Atriplex spp.) and snakeweed (Gutierrezia spp.) are recruited during rare, heavy, late-spring rains, giant kangaroo rats gnaw through the stems, eventually eradicating most or all shrubs on their precincts and surrounding ground.

Most females enter estrus in the cool, wet winter in central California, in mid or late December or January. When population density is high and most precincts are occupied, adult females may have only a single litter of from 1-4 young after a gestation period of about 32 days. Under these circumstances, young-of-the-year do not breed. During years of drought and low or no seed production, females are monestrous or anestrous. During years with a prolonged wet season or where population density is low and there are many vacant precincts, adult females may have 2-3 litters and young-of-the-year females may begin breeding when about 12-13 weeks old. Young giant kangaroo rats appear on the surface when they weigh about 50-70 or more grams and are presumably about 6.5 to 8.5 weeks old. Severe drought results in population decline, mainly by reduction or cessation of reproduction; whereas torrential rainfall over several days results in great, rapid population decline, presumably by drowning, death from wetting and hypothermia, and other factors related to too much moisture.

Classification

Order RODENTIA, Suborder SCIUROGNATHI, Family HETEROMYIDAE

Subspecies

None.

Recent Synonyms

None.

Other Common Names

None.

References

Braun, Suzanne E. 1985. Home range and activity patterns of the giant kangaroo rat. J. Mammal., 66:1-12; Grinnell, Joseph. 1932. Habitat relations of the giant kangaroo rat. J. Mammal., 13:305-320; Williams, Daniel F. 1992. Geographic distribution and population status of the giant kangaroo rat, Dipodomys ingens (Rodentia, Heteromyidae). Pp. 301-328, in Endangered and sensitive species of the San Joaquin Valley, California: their biology, management, and conservation (D.F. Williams, S. Byrne, and T.A. Rado, eds.). The California Energy Comm., Sacramento, 388 pp.; Williams, D.F., and K.S. Kilburn. 1991. Dipodomys ingens. Mammalian Species, 377:1-7.

Size

Total length :
312-347 (333.2) mm (males)
323-348 (328.9) mm (females)
Length of Tail:
157-197 (185.5) mm (males)
179-194 (182.9) mm (females)
Weight:
93-180 (138) g (males)
101-195 (132) g (females)

Identification

Of the species of kangaroo rats sharing its geographic range, D. ingens is distinguished from Dipodomys heermanni by longer hind foot, wider head, and greater mass. D. nitratoides is much smaller in all characters and has four instead of five toes on the hind foot.

Authors of Profile

D.F. Williams

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