Family: HETEROMYIDAE (Kangaroo rats, pocket mice)
Merriam's kangaroo rats (MKR), like other kangaroo rats and pocket mice, are members of the family Heteromyidae. Each species within this family has fur-lined food storage pouches. The cheek pouch is utilized as a portable cache for food while foraging. Kangaroo rats are named for their extremely long, kangaroo-like hind feet and they are almost completely bipedal. They hop or jump rather than scurry or run. Because of this, most heteromyid rodents also have a relatively long tail that acts to counterbalance the hopping/jumping form of locomotion.
Fur color varies between populations within the species' range, but the back color is generally light brown or tan. MKR are smaller than most of the other kangaroo rats in the southwest. Adults average about 14 inches in length. The tail is relatively long with a large tuft of hair at the tip. MKR have four toes on each hind foot in contrast to the pacific kangaroo rat (PKR), Dipodomys simulans and the Stephens' kangaroo rat (SKR), Dipodomys stephensi, both also found in San Diego County. PKR and SKR have five toes on each hind foot.
Range and Habitat
Range: Merriam's kangaroo rats are found in the upper and lower Sonoran life zones of the southwestern United States, Baja California, and northern Mexico. In San Diego County they are found in the low desert and the washes east of Laguna, Volcan, and Palomar Mountains.
Three subspecies of MKR occur in San Diego County. Dipodomys merriami arenivagus is frequently found throughout the low desert of Anza-Borrego State Park. D. m. trinidadensis occurs in the Jacumba and Mountain Springs area (Lidicker 1960), while D. m. collinus is found in the upper elevations of the desert near San Felipe Valley, La Puerta Valley, and Oak Grove.
Habitat: MKR can be found in desert scrub, alkali scrub, sagebrush, pinyon-juiper, and Joshua tree habitat throughout the southwest United States and Mexico (Zeiner 1990).
Behavior: MKR live individually within a maze of underground burrows. Male and female MKR each establish individual territories. They defend their territories against other male and female MKR, primarily to protect often scarce food resources. It is typical that MKR locate multiple entrances to their burrow complex at the base of shrubs near the middle of their territory. This allows more opportunities for MKR to escape from predators.
Most Kangaroo rats are exclusively nocturnal. Even so, they tend to avoid being outside their burrows when the moon is full. The greater the amounts of moonlight the less time MKR spend collecting food, defending their territory, or searching for mates. When the amount of moonlight is great they retreat underground in order to avoid predation. In some areas, above ground activity is limited to two hours or less (Zeiner 1990). During the day, they remain in their cool burrows. They often seal entranceways to their burrows with soil to prevent exposure to heat. When they are active above ground they move about within their territory and attempt to fill their cheek pouches with seeds and plant material. When their cheek pouches are full they retreat to their burrows where they disgorge the seeds they have collected. Some of the food is eaten immediately while the remainder is stored (often building a seed cache of considerable size) in several chambers within the burrow system.
Reproduction: MKR produce up to three litters per year, with an average of four young in each litter. Weaning of young occurs 24-33 days after birth (Chew and Butterworth 1964).
Diet: The diet of MKR is almost exclusively plant seeds (they are "granivorous"). The bulk of their diet consists of the seeds of desert and grassland plants. MKR rarely drink water. They obtain water through metabolic processes augmented by the moisture content of their food.
Predators: Kangaroo rats are a common prey items for many other desert animals. Typical predators of the MKR include barn and great horned owls, coyotes, foxes, badgers, bobcats, and several snake species including sidewinders and glossy snakes.
In general we have not caused adverse impact to the Merriam's kangaroo rat through most of its range. However, in southern California one subspecies of MKR is at risk due primarily to urban development including construction of dams and alteration of hydrologic regimes throughout its range. Once common on alluvial plains in the washes of San Bernardino and Riverside counties, the United States Fish and Wildlife Service listed the San Bernardino kangaroo rat as endangered in 1998.
Text by Scott Tremor and Bill Haas
Photograph from Ocean Oasis
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