Packrat (Neotoma spp.) middens are valuable in paleoecological research because of the preserved plant and animal material they contain, which is readily identifiable and ideal for macrofossil analysis. In addition to animal and plant macrofossils, middens also may contain pollen, epidermal fragments, and microfossils. The initial discovery of these deposits was made in 1960 at Indian Springs Ranch, around 65 kilometers northwest of Las Vegas, Nevada. Midden deposits were radiocarbon-dated and developed as a paleoecological method after it was discovered that these middens contained evidence of coniferous woodland at what are now desert elevations in the arid Mojave Desert region of southern Nevada (Wells, 1976). Plant remains in ancient packrat middens, which are preserved in dry rockshelters in many North American deserts, have allowed reconstructions of local vegetation and regional climate for the last 10,000 to 45,000 years of the Wisconsinan/Holocene glacial-interglacial cycle (Van Devender et al., 1994).
Packrat Behavior and Ecology
Packrats are members of the rodent family Muridae and belong to the New World subfamily Sigmodontinae. They are compact, long-tailed rodents that can attain adult weights from 100 to 400 grams. They have large eyes and ears, and strongly built feet for grasping branches or rocks when climbing. Colorwise, they range from pale buff in some desert dwellers to nearly black for packrats that inhabit areas around lava beds. Packrats are known to inhabit alpine tundra, forests, grasslands, desert scrub, chaparral, and tropical thorn scrub. They can be found from below sea level, such as at the Salton Sea of California, to high altitudes, such as the top of Pikes Peak in Colorado (Vaughan, 1990).
Fossilized packrat middens are important in paleoecological reconstruction of past climate because of the plant and animal material they contain. This material is collected by the packrat within a radius up to 100 meters from their dens (Allen et. al., http://biology.usgs.gov/luhna/chap9.html). They accumulate this material, mostly in the form of sticks, twigs, and other plant material because of their basic need for succulent plant food, which is by definition 50% water by weight. They release copious amounts of urine and depend on moist forage to maintain their water balance (Vaughan, 1990). Packrats are also known to collect leaves, seeds, flowers, bones, shells, exoskeletons, etc. from the local flora and fauna. Usually during the night, the packrat makes many forays from its den to collect food and building materials, which are stored in the den. The deposits are preserved by the packrat impregnating the material with its viscous urine, which dries and crystallizes quickly to form a glossy, dark brown veneer, which is known as "amberat." This amberat commonly stains rock walls in vertical streaks or curtains below ledges or crevices, and commonly permeates and coats the middens. It works so well as a fossilizing agent that intact, delicate plant structures have been preserved in middens for time periods exceeding 40,000 radiocarbon years. Packrat deposits have a better chance for preservation if they are deposited within dry rockshelters or caves. (Wells, 1976). Because deposition of midden material may contain temporal gaps, caused by absence of packrats, midden specimens must be dated in order to obtain accuracy (Grayson, 1993).
Click Here to view a massive complex Pleistocene packrat midden, which is located in the alcove in the center of the image. Note dark streaks of amberat which coat rock below the midden (Courtesy, USGS).
Click here for an image of a 13,000 year old midden in the Grand Canyon (Courtesy of K. Cole, USGS).
Click here to view a 28,000+ year old midden under an overhang at Capital Reef National Park. Notebook in photo is 4" by 7" (Courtesy of K. Cole, USGS).
Six species of Neotoma are important to researchers in the Southwest. Neotoma albigula, commonly known as the white-throated packrat, is primarily a desert species that occurs in the Sonoran and Chihuahan deserts, and can be found in Arizona, New Mexico, Texas, and northern Mexico. It commonly builds dens in caves, grottos, and rock fissures. The bushy-tailed packrat (Neotoma cinerea) inhabits the boreal areas from northern Canada to central Arizona, and lives among spruce-fir forest, pinyon-juniper woodland, Great Basin sagebrush, desert scrub, alpine talus, and shortgrass prarie. The bushy-tailed packrat also has a tendency to collect bones of large and small invertebrates, and because of its large size, it has been known to collect completely articulated skeletons. The desert packrat (Neotoma lepida) ranges from Washington to Utah, and prefers rock outcrops, cactus patches, or at the base of desert trees such as catclaw (Acacia greggii) or joshua trees (Yucca brevifolia). The Mexican packrat (Neotoma mexicana) inhabits a wide variety of plant communities in northern Colorado, Utah, Arizona, New Mexico, and Central America. In the American Southwest, it occupies montane areas mostly dominated by ponderosa pine (Pinus ponderosa), pinyon-juniper, and Gambel's oak (Quercus gambelii). It tends to live around cliffs, boulders and rock outcrops. Stephens' packrat (Neotoma stephensi) is geographically restricted to parts of northern Arizona, extreme south-central Utah, and western New Mexico. It typically builds dens at the base of junipers or in rock outcrops. The Western Arizona packrat (Neotoma devia) is found in desert mountains and low desert areas in western Arizona, and usually makes skimpy dens which rarely contains fossilized midden deposits (Vaughan, 1990).