The Biogeography of  the Giant Kangaroo Rat (Dipodomys ingens)

 by Seth Hiatt, student in Geography 316,  Spring 2005

 

	Kingdom:  Animalia Phylum:  Chordata Class:  Mammalia Order:  Rodentia Family:  Heteromyidae Genus:  Dipodomys Species: Dipodomys ingens
Photo Credit: David J. Germano

Description of the Species: As the name implies, the giant kangaroo rat is considerably larger than other species of kangaroo rat.  It weighs approximately 131–180 grams, which is nearly three times the weight of the Tipton kangaroo rat (Dipodomys nitratoides nitratoides) (Jameson 1988, Steinhart 1990).  Its body length is about five inches, with an elongated tail extending approximately 130 percent of its body length (Jameson 1988).  It has short brownish fur, with darker spots behind the eyes and ears, and a white stripe that runs along its thigh.  The thin tail has tuft of fur at the tip, and has a brown dorsal stripe with lighter color ventral stripes (Jameson 1988).   Much like a kangaroo, the giant kangaroo rat rests on its enlarged hind limbs and has significantly reduced forelimbs.  The head appears oversized for its body because of its enlarged auditory bullae. This gives the head a triangular shape that appears somewhat heart-shaped from above (Jameson 1988).

Natural History: 

The unique physical features of the giant kangaroo rat are not without purpose.  The oversized ear bullae allow the kangaroo rat to hear low frequencies such as those made by a striking snake or a swooping owl, and its oversized feet give it the ability to leap quickly to avoid the strike.  Not only can the kangaroo rat jump quickly, it can further evade predators by using its long tail to change direction mid-air.  It also uses its elongated feet for various types of communication with other giant kangaroo rats by rapidly drumming the ground.  The purpose of drumming is usually to declare territory, but in the event that diplomacy fails, the rapid-fire feet are also used to kick at each other (Steinhart 1990).   Compared to the other species of kangaroo rat that footdrum, the giant kangaroo rat has the longest drum roll, which is applied with greater tolerance than other kangaroo rat species before physical conflict ensues (Randall 1997).  The kangaroo rat is also equipped with external fur-lined cheek pouches that allow it to transport seeds to either its underground nest or to hidden ‘pit caches’ above ground (Jamison 1988).

The giant kangaroo rat has evolved to live comfortably in an arid and sparsely vegetated environment.  In order to retain water, its urine is highly concentrated and it is able to secrete salt so efficiently that it could feasibly drink seawater and still maintain a healthy hydration level.  Giant kangaroo rat feces is very dry, and its primary water source is its food that consists of seeds and sometimes leaves of annual grasses and forbs (Jameson 1990).     

The giant kangaroo rat is nocturnal and lives in colonies that range from only a few to several thousand members, but individuals occupy solitary territories called precincts. Precincts have a radius of about three to four meters, and each contains a shallow burrow that is deep enough for the kangaroo rat to escape the afternoon heat and avoid predators.  Burrows have two to four entrances that often surround a bush or shrub and are identifiable by both the mound of soil at the base of the plant, and numerous tracks in the immediate surrounding area (Jameson 1988)  The above ground portion of the precinct is used both for foraging as well as sand bathing. (Randall et al. 2001).   

The breeding period is generally between February and June, but sometimes lasts later into the year.  There is some correlation between reproduction periods and rainfall along, with an increase in consumption of fresh leaves during breeding (Jameson 1988). Litter size is typically from four to six, which considered large for kangaroo rats. Despite this fact, the giant kangaroo rat is an endangered species.  Its decline is due largely to the conversion of grasslands and shrubland to agriculture (Steinhart 1990). 

Distribution and Habitat:

The giant kangaroo rat is endemic to the San Joaquin and Tulare Basins in the southern end of the Great Central Valley
 in California.  Its distribution is discontinuous, and comprised of six major populations collectively extending from San Benito County in the north, to the northeastern border of Santa Barbara County at the southern end of the range.  Individual populations include the northern Fresno/San Benito, Kettleman Hills, San Juan Creek, Western Kern County, Carrizo Natural Area, and Cuyama Valley (See fig. 1).  Each of the six major populations are further fragmented by steep topography and unsuitable vegetation, as well as human induced barriers such as cropland, roads, and urban development (Williams et al. 1998).

Figure 1 (Illustration by Seth Hiatt. Source: Williams et al. 1995)	Figure 2 (Illustration by Seth Hiatt. Source: Williams et al. 1995)

The preferred habitat of the giant kangaroo rat consists of open, gently sloping annual grasslands with friable soils, only sparse shrubs and arid climate conditions. Some smaller populations live in shrub-dominated areas with a variety of soil types (Williams et al. 1998).  Ideally slope is less than 11 percent with an optimum elevation of approximately 400 meters (1,300 ft), and total annual precipitation less than 16 centimeters (6.3 in).  Although total annual precipitation may be less than 16 cm per year, giant kangaroo rat populations are highly susceptible to fluctuations in precipitation due to impacts upon the surrounding vegetation (Good et al. 1997).  Annual grasses and forbs that serve as food sources include annual fescue (Vulpia microstachys and Vulpia myuros), red brome (Bromus rubens), peppergrass (Lepidium nitidum), and common shrubs in the area are California ephedra (Ephedra californica) and saltbush (Atriplex polycarpa) (Williams 1998, McNeely 2004).  The giant kangaroo rat shares this habitat with sympatric species including the heermann’s kangaroo rat (dipodomys heermanni) and the antelope squirrel (Ammospermophilus nelsoni), as well as predators such as the coyote (Canis latrans), kit fox (Vulpes macrotis mutica), barn owl (Tyto alba), and rattlesnake (Crotalus viridis) (Randall 1993, California Dept. of Fish and Game 1981).

Easily friable soil types such as Kettleman loam and Panoche sandy loam generally support larger populations.  Well-drained loamy soils are preferred in-part for easy digging of burrows and seed caches, but also in order to discriminate soil particles from seeds, as the giant kangaroo rat uses texture to differentiate between the two.  As soil particles become increasingly coarse, the ability for the giant kangaroo rat to efficiently identify and collect food decreases dramatically (Randall 1993).  The ability to harvest seeds quickly and dig caches (as much as 10 cm deep) for food storage are essential for survival in the event of drought conditions.  Soil characteristics help to explain the preference of open spaces with sparse shrubs, as well as the avoidance of steeper slopes.  While higher shrub density results in more concentrated seed distributions, the soils beneath shrubs are typically coarser than those in open areas (Randall 1993).  Soil texture also becomes increasingly coarse as slope increases.  Unfortunately, over the past few decades agricultural encroachment has forced some giant kangaroo rat colonies into marginal shrub dominated areas and hillsides with slopes up to 22%. (Williams et al. 1998).

Predatory conditions also possibly help to determine giant kangaroo rat habitat. While foraging for seeds during moonlit nights, shrubs provide shelter from avian predators such as owls.  Although foraging in shrubs places the giant kangaroo rat in closer proximity with snakes, moonlight increases the ability to avoid snakes visually. Darker nights, on the other hand, permit foraging under the cover of darkness, allowing it utilize other adaptations to avoid predation such as bipedality and auditory detection of low frequencies (Randall 1993).  

Evolution:

Dipodomys ingens is one of about 2,050 modern species of Rodentia (Huchon et al. 2002).  The adaptive radiation in the order Rodentia during the Eocene epoch has been described as ‘explosive’, and during this time period (about 50 million years ago) two families of rodent, Geomyidae and Heteromyidae, diverged from a common superfamily, Geomyoidea (see figure 2) (Eisenberg 1981).  The single distinctive trait shared by these two families is their external fur lined cheek pouches.   During the Oligocene three subfamilies diverged from the Heteromyidae family; Heteromynae, Perognathinae, and Dipodomyinae, and these further divided into the six genera (see figure 3).  Because of the similarities in appearance, it should be noted that kangaroo mice (Microdipodops) are fundamentally different ecologically and physiologically from kangaroo rats, and have even been periodically placed within the Perognathinae subfamily (Hafner 1993)

Figure 3. Source: Eisenberg 1981

Figure 4. Source: Hafner 1993

Although Heteromyid species inhabit environments with a variety of climatic conditions, most are well suited to arid regions in North America.  In fact, so long as the basic requirements of friable soils and plant seeds are present, heteromyid species can be found in every xeric environment on the continent.  Unlike other heteromyids, however, Dipodomys and Microdipodops have developed a series of evolutionary novelties allowing them to be particularly well adapted to arid environments.  These morphological features and their adaptive function are listed below in figure five. 

Figure 5. Source: Hafner 1993

The degree to which Dipodomys is ideally suited to aridity is further emphasized by the fact that through the process of convergent evolution, several other rodent species have independently adopted similar features and behaviors (Mares 1993).  The jerboa (Jaculus jaculus) for instance, inhabits parts of northern Africa and the Middle East, and although it is only remotely related (See Dipodidae family in fig. 3), it shares remarkably similar morphological features to the giant kangaroo rat.  The most obvious similarities are bipedalism and shortened forelimbs.  In arid environments with minimal vegetation as protection from predators, the agility associated with bipedalism is an important mechanism in predator evasion.  If extinctions are taken into consideration, every arid habitat throughout the globe has supported a bipedal small mammal species (Mares 1993). 

Conservation:

The giant kangaroo rat was listed as an endangered species by the U.S. government in 1987.  Due to rapid agricultural development since the 1960s, its habitat has been reduced to approximately 3% of its historic geographic range (Good et al. 1997, Williams et al. 1998).   While habitat conservation and recovery is slow, some progress is being made.  Over the past 100 years, crop farming and livestock grazing in San Luis Obispo County have reduced habitat there by 20-40%, however, recent habitat protections have permitted secondary succession and population increases in fallow cropland (Good et al 1997).  Protecting the giant kangaroo rat from extinction remains a challenge as fire, drought, and incompatible land use continue to represent serious threats.  And as of 1998, public or conservation lands were designated in only three of the six population areas indicated (fig. 1) (Williams et al. 1998)

A further dilemma associated with giant kangaroo rat habitat conservation is that the species has become dependent invasive plants for its food supply.  Originally brought as ornamentals from the Mediterranean, introduced grass species such as red brome (Bromus rubens) help support population stability of the giant kangaroo rat because their seeds are considerably larger than those of native plants.  The relationship is mutualistic because as the giant kangaroo rat disrupts the soil, it assists the germination of the invasive. Protecting native plants unable to compete with the invasives is then difficult because it compromises protection of the giant kangaroo rat as an endangered species (McNeely 2004).

Bibliography

Ben-Moshe A., T. Dayan, D. Simberloff. 2001. Convergece in Mophological Patterns and Community Organization between Old and New World Rodent Guilds. The American Naturalist, Vol. 158, 5 484-494. (Ben-Moshe et al. 2001)

California Department of Fish and Game. 1981. Wildlife and Habitat Data Analysis Branch M106. Website accessed on 4/26/2005 http://www.dfg.ca.gov/whdab/html/M106.html (California Dept. of Fish and Game 1981)

Eisenberg, J.F. 1981. The mammalian radiations : an analysis of trends in evolution, adaptation, and behavior. Chicago : University of Chicago Press. (Eisenberg 1981)

Good, Sara V., Daniel F. Williams, Katherine Ralls, and Robert C. Fletscher. 1997.  Population Structure of Dipodomys Ingens (Heteromyidae): The Role of Spatial Heterogeneity in Maintaining Genetic Diversity. Evolution.  Vol. 53 1296 –1310 (Good et al. 1997)

Hafner, J.C. 1993. “Heteromyid Rodents: Heterochrony and Adaptation in Phylogeny” pp. 290-302. in Genoways, H.H., J.H. Brown, eds. 1993. Biology of the Heteromyidae. Special Publication No. 10. [Place of publication unknown]: American Society of Mammalogists. (Hafner 1993)

Huchon D., O. Madsen, M. Sibbald, K. Ament, M.J. Stanhope, F. Catzeflis, W.W. de Jong, E. Douzery. 2002. Rodent Phylogeny and a Timescale for the Evolution of Glires: Evidence from an Extensive Taxon Sampling Using Three Nuclear Genes. Molecular Biology and Evolution, Vol. 19 1054 – 1065. (Huchon et al. 2002)

Jameson Jr., E.W. and H.J. Peeters. 1988. California Mammals: California Natural History Guides: 52 Berkeley: University of California Press. (Jameson and Peeters 1988)

Mares, M.A. 1993. “Heteromyids and Their Ecological Counterparts: A Pandesertic View of Rodent Ecology and Evolution” pp. 262-690. in Genoways, H.H., J.H. Brown, eds. 1993. Biology of the Heteromyidae. Stillwater, OK: American Society of Mammalogists. (Mares 1993)

McNeely, Jeffrey A. 2004. Strangers in Our Midst. Environment, Vol. 46, 6 17-31. (McNeely 2004)

Randall, J.A. 1993. Behavioral Adaptations of Desert Rodents (Heteromyidae). Animal Behaviour.  Vol. 45 263-287. (Randall 1993)

Randall, J.A., E.R. Hekkala, L.D. Cooper, J. Barfield. 2001. Familiarity and Flexible Mating Strategies of a Solitary Rodent, Dipodomys ingens. Animal Behaviour, Vol. 64 11-21. (Randall et al. 2001)

Randall, J.A. 1997. Species-specific Footdrumming in Kangaroo Rats: Dipodomys ingens, D. deserti, D. spectabilis.  Animal Behaviour, Vol. 54 1167-1175. (Randall 1997)

Steinhart, P. 1990. California’s Wild Heritage: Threatened and Endangered Animals in the Golden State. California Department of Fish and Game. (Steinhart 1990)

Williams, D.F., E.A. Cypher, P.A. Kelly, K.J. Miller, N. Norvell, S.F. Phillips, C.D. Johnson, and G.W. Colliver. 1998. Recovery plan for upland species of the San Joaquin Valley, California. U.S. Fish and Wildlife Service. Region 1, Portland, OR. pp 85-96. (Williams et al. 1998)

 Williams, D.F., M.K. Davis, L.P. Hamilton. 1995 Distribution, Population Size, and Habitat Features of Giant Kangaroo Rats in the Northern Segment of their Geographic Range. California Department of Fish and Game, Bird and Mammal Conservation Program Rep. 95-01, 38 pp.  (Williams et al. 1995)

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