Class Mammalia Order Rodentia Suborder Sciurograthi Family Heteromyidae
Small to medium nocturnal, seed-eating rodents with external, fur-lined cheek pouches, bipedal locomotion in the kangaroo rats and mice and quadrupedal locomotion in pocket mice. Rudimentary social structure; sandbathing cleans the hair and deposits scent for communication; the medium and larger sized kangaroo rats communicate by footdrumming
1.7-14.6 in (42-370 mm) total length, weight: 0.2-6.9 oz (5-195 g)
Number of genera, species: 6 genera, 60 species
Desert, grasslands, and forest Conservation status
Critically Endangered: 4 species; Endangered: 1 species; Vulnerable: 1 species; Lower Risk/Conservation Dependent: 1 species; Lower Risk/Near Threatened: 8 species
Western United States, Mexico, Central America, and northern South America
Western United States, Mexico, Central America, and northern South America
Heteromyid rodents have a long evolutionary history in North America. Hafner (1993) proposes that heteromyids diverged from their nearest relative, the subterranean pocket gopher, Geomyidae, in the Oligocene over 30 million years ago. The divergence continued as the climate of North America became increasingly cool and arid into the three distinct subfamilies and six genera extant today. Five genera diverged into multiple species in North America while one genus, Het-eromys, entered South America. Corbet and Hill (1991) list seven species of spiny pocket mice, Liomys, and five species of forest spiny pocket mice, Heteromys, in the subfamily Het-eromyinae. The subfamily Perognathinae consists of 16 species of silky pocket mice, Perognathus, and nine species of coarse-haired pocket mice, Chaetodipus). The third subfamily, Dipodomyinae, includes 21 species of kangaroo rats, Dipodomys, and two species of kangaroo mice, Microdipodops.
Heteromyids are frequently associated with the specialized morphology of kangaroo rats (weight: 1.2-6.9 oz; 33-195 g), and, to a lesser extent, the smaller kangaroo mice (0.4-0.6 oz; 10-17 g). Like kangaroos, kangaroo rats and mice move in long powerful jumps on elongated hind limbs and feet. They have especially long, beautiful tails with white tips or tufts on the end that are used for balance, and sometimes as flags. A much more generalized body type occurs in the pocket mice. Heteromys and Liomys are rat-like rodents (weights: 1.29-3.0 oz or 36.6-85.4 g and 1.2-1.8 oz or 34-50 g respectively) that have a standard quadrupedal locomotion and lack the specialized features of other species. The smaller desert pocket mice (body weights range 0.2-1.1 oz [5-31 g] in Perognathus, and 0.4-1.7 oz [10-47 g] in Chaetodipus) are quite diverse. Although they have relatively long feet, they are poor jumpers and exhibit quadrupedal locomotion. All heteromyids have external fur-lined cheek pouches that open anterior to the mouth and are nocturnal with fairly large eyes. Ears are short and rounded. Kangaroo rats and mice have inflated auditory bullae and extremely keen hearing in the low to mid-frequency ranges. The pelage varies in texture from silky and soft in kangaroo rats and mice and silky pocket mice to spiny in Liomys and Chaetodipus. Pelage color varies considerably from light to dark. Maximum longevity for these small mammals is 10 years or more.
Schmidley et al. (1993) identify five general regions occupied by heteromyid rodents in North, Central and South America. Tropical species occur in the northern neotropical areas of Mexico, Central and northwestern South America. Heteromys is confined to moist areas of Mexico, and central and northern South America; Liomys inhabits parts of Mexico and Central America. The other four regions are arid and semiarid areas of North America in the Great Plains, the Great Basin desert, southwestern Mojave, Sonoran and Chihuahuan deserts, and California. Perognathus and Chaetodipus occur in all four arid regions. Perognathus ranges as far north as British Columbia and Saskatchewan throughout the central and western United States southward to Mexico. Chaetodipus occupies the Lower Sonoran area of the western United States north into the Great Basin. Microdipodops is primarily restricted to the Great Basin desert in Nevada. Dipodomys are widespread and inhabit arid areas from the Pacific coast north to southern Canada, east to the Great Plains and south through western Texas into Mexico. California has a diverse distribution of kangaroo rats and pocket mice in the southern Mojave Desert, coastal transition zones, and the Central Valley. Twelve of 21 species of kangaroo rat are found in California, and eight of these species occur either only or primarily there.
Desert pocket mice and kangaroo rats inhabit a variety of arid habitats consisting of wind-blown sand, sagebrush, desert scrub, grassland, creosote bush flats, rocky hillsides, and chaparral. In contrast, kangaroo mice are mainly restricted to sandy habitats. A characteristic of heteromyid communities is their complexity in which multiple species co-exist. Brown and Harney (1993) describe a community in the Great Basin desert in Nevada composed of three species of kangaroo rat (D. deserti, D. merriami, D. ordii), a kangaroo mouse (M. pal-lidus), and a pocket mouse (P. longimembris). Heteromys occupies tropical wet forests, and Liomys is found in tropical dry forests and thorn scrub. In contrast with the high diversity of heteromyids in desert habitats, Sanchez-Cordero and Fleming (1993) note the low diversity of species in tropical habitats, probably a result of deserts being more productive in seeds, the main food source for all heteromyid rodents.
Heteromyid rodents have not evolved complex social systems. Most heteromyids live alone in individual burrows, except for females with young, and show the common mammalian pattern of overlapping home ranges. Male home ranges overlap those of other males and females, and females tend to occupy areas exclusive from other females. An exception is seen in the larger-sized kangaroo rats: D. spectabilis, D. ingens, and D. deserti. Both males and females maintain exclusive territories where they defend large seed caches, except during the breeding season when males travel to female territories for mating. Heteromyid home ranges average 6,458-26,910 ft2 (600-2,500 m2).
Although heteromyids are solitary species, they are not "asocial" and exhibit a rudimentary social structure. Randall (1993, 2002) views the social system of kangaroo rats as an orderly one promoted by establishment of familiarity with
close neighbors, philopatric dispersal patterns so that relatives may live nearby, and well developed means of communication. Kangaroo rats recognize neighbors from strangers by footdrumming, a characteristic thumping or drumming sound made with the hind feet. They are much less aggressive toward familiar neighbors than unfamiliar ones. Neighbor recognition seems independent of the type of spacing, but species do differ in the degree of tolerance they show toward conspecifics. Little detailed information about the social behavior of other heteromyids is available. Heteromys, however, may be somewhat social. Eisenberg (1963) noticed that adults and mature young seem to share the same burrow, and behavioral tests in the laboratory demonstrated tolerance of con-specifics. Randall (1994) has found that the lack of sociality in heteromyids is not unusual for desert rodents.
A well developed system of communication helps to promote social order in heteromyid rodents. Sandbathing, which consists of rubbing the sides and ventrum in loose soils, is performed frequently by pocket mice and kangaroo rats. The behavior functions both to remove oils from the pelage and to deposit scent for communication. The rodents deposit scent from oils associated with the hair and from a specialized scent gland on the dorsum at sandbathing sites to communicate species, sex, individual identity, familiarity, and reproductive condition, depending on the species.
Kangaroo rats drum their large hind feet on the ground to generate both airborne and seismic signals. Randall (2001) compared footdrumming in four species of territorial kangaroo rats (D. spectabilis, D. ingens, D. deserti, and D. heermanni) and found that the complexity of the signal and its messages are species specific and vary with context. Footdrumming patterns range from individually distinct footdrumming signals to single foot thumps to communicate territorial ownership, competitive superiority, submission, and readiness to mate to conspecifics.
As in many small prey, heteromyids rely on cryptic coloration and avoidance for predator defense. If a predator is encountered, the erratic path of bipedal locomotion often provides an effective escape for kangaroo rats and mice. Desert heteromyids also have sensitive low-frequency hearing, which enables them to detect approaching predators. At least three species of kangaroo rat (D. spectabilis, D. ingens, and D. deserti) have evolved a unique defense against snakes. Instead of avoiding a snake, they approach it to within striking distance, jump back and footdrum. Although mothers may drum to warn pups in the burrow, the drumming appears di-
rected at the predator to communicate that the kangaroo rat is alert, cannot be ambushed, and to tell the snake to go away. D. deserti also kicks sand at the snake during defensive interactions
Heteromyid rodents are primarily granivores. Seeds comprise their main food source in all habitats, supplemented by green vegetation and insects in some species. Kenagy (1973) found that the chisel-toothed kangaroo rat, D. microps, climbs up in saltbrush, Atriplex, to consume the leaves by stripping off the epidermis with specialized chisel-shaped teeth. Desert species are physiologically adapted to survive lack of water during long droughts. Each night, the rodents emerge from their burrows to dig through the soil with their forelimbs to gather seeds. They scoop up the seeds, rapidly stuff them into their fur-lined cheek pouches until they look like they have the mumps, and transport them to caches. Factors such as seed size, density and nutritional value, predation risk, and type of soil influence foraging choices. Most heteromyids scatter-hoard and bury seeds in widely spaced surface caches
throughout their home ranges. The Merriam's kangaroo rat can even remember the location of seed caches. Territorial kangaroo rats store seeds in a central larder in the territory and actively defend the caches from other rodents by chasing away intruders and footdrumming.
The Pacific little pocket mouse (Perognathus longimembris) fills its cheek pockets. (Photo by Mike Couffer. Bruce Coleman, Inc. Reproduced by permission.)
The extreme environmental conditions associated with arid habitats constrain reproduction in heteromyid rodents to times of the year when enough moisture is available in the diet to support lactation. D. merriami reproduces any time of the year in response to rainfall and the subsequent growth of green vegetation. D. spectabilis and D. ingens have a winter breeding season but will continue to breed into the summer under the right conditions. D. microps may breed only once during a few weeks in the spring when saltbrush produce new leaves. Chaetodipus and Perognathus escape winter extremes by being dormant and then usually breed in the spring before summer droughts. Those in more mesic conditions may have an extended breeding season. Tropical het-eromyids reproduce during both the dry and rainy seasons and generally have longer breeding seasons than desert species.
Females are polyestrus and can produce several litters under favorable environmental conditions. Litter sizes range from one to nine and average between three and four for most species. In field studies, Randall (1991) and Randall et al. (2002) found that D. ingens and D. spectabilis females produced an average of only two pups during a breeding season.
Details of mating behavior in heteromyids are generated from field studies by Randall (1991, 1993) and Randall et al. (2002) of three species of kangaroo rats: D. spectabilis, D. merriami, and D. ingens. Mating is opportunistic and ranges from exclusive matings between one male and female to polygynous matings where males compete for access to an estrous female. Males always go to a female's home area or territory to mate, and one male, usually a close neighbor, attempts to chase away all other males. Larger and medium sized kangaroo rats footdrum during these competitions. Females seem to prefer to mate with familiar males, but they will mate with more than one male if possible. They also prefer the scent of familiar males and allow more amicable contact with familiar than with unfamiliar males in both natural and experimental contexts. Males, however, do not appear to discriminate and approach all females alike. Mating interactions of small to medium sized species can last for up to 30 minutes and include multiple mounts alternating with mutual circling. Larger species may mount only one to three times.
Excessive habitat destruction and fragmentation and the loss of plant communities increasingly threaten populations of heteromyids. Currently, all heteromyid species listed as endangered by the United States Fish and Wildlife Service occur in California as a result of habitat loss from urban and industrial development, cultivation, grazing, oil and mining exploration and extraction, highway construction, and flooding. Listed species include D. ingens, D. stephensi, D. nitratoides exilis, D. n. tipton, D. heermanni morroensis, D. merriami parvus, and P. longimembris pacificus. Recovery plans for these species have been implemented. Fifteen species of heteromyids are on the Red List of Threatened Species of the IUCN. They include eight species of kangaroo rat, three Heteromys, two Li-omys, and two Perognathus.
Kangaroo rats are often considered keystone species because their burrows provide habitat for a variety of plants and animals, thus their presence is important to maintain biodiversity in desert environments. Many heteromyids are agents of seed dispersal. These attractive rodents can be observed in the desert at night by humans who wish to enjoy nature.
1. Hispid pocket mouse (Chaetodipus hispidus); 2. Pale kangaroo mouse (Microdipodops pallidus); 3. San Joaquin pocket mouse (Perognathus inornatus); 4. Heermann's kangaroo rat (Dipodomys heermanni); 5. Giant kangaroo rat (Dipodomys ingens); 6. Desmarest's spiny pocket mouse (Heteromys desmarestianus); 7. Salvin's spiny pocket mouse (Liomys salvini); 8. Banner-tailed kangaroo rat (Dipodomys spectabilis). (Illustration by Michelle Meneghini)
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