The desert tortoise (Gopherus agassizii) has a high domed shell, brown in color, yellow underneath without a hinge, and a pattern and prominent growth lines on the lower and upper shell. They are also characterized by stocky limbs, forelimbs covered with large scales, and a short tail. The eyes are greenish-yellow or yellow with brown near the outer edge, sometimes brown or mottled. Skin is gray, blackish-gray to black, or reddish-tan. The shell length of adults ranges from 7.8 to14.2 inches (20 to 36 cm). The desert tortoise reaches its maximum size at 5-10 years of age. The desert tortoise maintains its body temperature in the range of 25 to 35°C. This species demonstrates a delayed maturity and long life. Home range size for desert tortoises ranges from 12.4 to 124 acres (5 to 50 ha), but individuals may move several miles over weeks or years.Activity period for the desert tortoise varies by region, sex, and age class. The Mojave population of the desert tortoise is active from approximately March through October. They hibernate in burrows the remainder of the year where they conserve water and energy. Some individuals may go dormant during dry periods in the summer. Tortoises are inactive 98% of their life, in which they are often subterranean.
The Mojave population of the desert tortoise was listed as threatened on April 2, 1990. Critical habitat was designated on February 8, 1994, in portions of the Mojave and Colorado Deserts totaling 6.4 million acres. A recovery plan for the desert tortoise (Mojave population) was published in 1994 by the United States Fish and Wildlife Service. The desert tortoise is a species of special concern in the State of Arizona, threatened in the State of California, and a species of conservation priority in the State of Nevada.
The Mojave population of desert tortoises are distinguished from the Sonoran population by their more oval-shaped figure and a higher domed shell. The Sonoran population has larger scales on the top of the head and more sharply wedge-shaped snouts than the Mojave population. Hatchlings from the Mojave Desert are lighter in color and have more serrate surfaces to their marginal scutes than the hatchlings from the Sonoran Desert. The desert tortoise is the only naturally occurring tortoise in the Mojave region; however, escaped or released captive tortoises of other species are occasionally detected there.
Mating in the Mojave population of the desert tortoise begins in March and April and can extend through October. The first year of reproductive activity for a female occurs when they reach a carapace length of 7.2 inches (185 mm) at age 12 to 25.
The desert tortoise lays its eggs from April through mid-July. The desert tortoise can lay up to three clutches per year, with each clutch containing 5 to 9 eggs. Number of clutches per year is dependent on rainfall. Incubation period ranges from 85 to 125 days. Temperatures at the nest site during this period need to be above 26°C and below 35°C for eggs to survive. Hatching usually requires 48-72 hours. Hatchlings usually emerge in late summer, but some may overwinter in the nest. Desert tortoise eggs are pale, elliptical to spherical, brittle shelled, and average 1.2 to 1.6 inches (30 to 40 mm) and 0.7 to1.4 oz (20 to 40 g). Eggs are laid in depressions 3 to 4 inches deep. Sex determination is dependent on soil temperature during incubation; males are produced when temperatures are below 89.2°F (31.8°C) and females are produced when temperatures are above 89.2°F (31.8°C). Hatchlings are approximately 1.8 to 2 inches (4.5 to 5.0 cm) long and weigh approximately 0.7 to 1 oz (20.0 to 27.0 g). They are round in shape and are mustard yellow to brown in color.Juvenile tortoises (20-25 years old) are aged by counting concentric annual rings radiating outward from the center of each shell. The age of adult desert tortoises (>25 years) is indeterminable due to shell wear and shedding of juvenile rings.
The desert tortoise feeds on a variety of herbaceous vegetation including annual and perennial grasses, flowers and fruits of annual plants, cacti, and perennial shrubs. The desert tortoise will occasionally eat insects, which are a good source of lipid and protein. Forage species selected by tortoises in the west Mojave Desert include: Dwarf White Milkvetch, Widow’s Milkvetch, Booth’s Suncup, Rattlesnake Weed, Foot-hill Trefoil, and Desert Four-o’clock. In the east Mojave Desert, tortoises showed a preference for Booth’s Suncup, Creosote-bush Cat’s eye, Smooth malacothrix, Beavertail Prickly-pear, Plume-seeded Chicory, Common Mediterranean Grass, and Small Skeletonplant. Diet is based on presence and abundance of forage. Diet consists primarily of annuals during the spring and dry grasses and cactuses during the summer. The desert tortoise is able take advantage of years in which resources are abundant to sustain them through years in which resources are lacking. Individuals can tolerate large imbalance in water and energy budgets; adults can survive a year without access to water. Desert tortoises can switch from water demanding urea to uric acid for waste elimination, when needed.
The major causes for decline of the desert tortoise are habitat destruction, degradation, fire, and fragmentation from urban and agricultural development; livestock grazing; mining; invasion of nonnative plants; and off-road vehicle (ORV) use. Freeways, highways, paved roads, dirt roads, and railroads pose a threat to this species. Direct mortality or injury of desert tortoises caused by humans and disease are other major threats to the species. ORV activity has increased in recent years in desert habitat. Increased ORV use can have negative impacts, such as tortoises being run over by vehicles, crushing of vegetation, damage to soil crusts, soil erosion, spreading of invasive plants, and increase in fires.
Agricultural developments cause widespread reduction of the water table, increase raven populations, clear native vegetation, introduce pesticides and fertilizers to habitat, and provide a seed source for nonnative plants. Grazing can result in mortality of individual tortoises or eggs, promote soil erosion, damage soil crusts, reduce native vegetation, trample burrows, and increase rate of nonnative species invasion. The reduction of native perennial grasses reduces forage availability and protein available to desert tortoises.
Mining, energy development, utility, and energy facilities in the desert tortoise’s historical range cause construction of roads and increased vehicle use, disturbance of soil surface and vegetation, toxic byproducts, refuse of stakes and wire, transfer of title from public lands to private use, fragmentation of habitat, increased habitat for predatory birds, and creation of trenches that tortoises can fall into. Military activities in the desert tortoise’s historical range include construction, operation and maintenance of bases and support facilities; development of support communities; field maneuvers (tank traffic, bombing, testing of explosives, unexploded ordinance littering, shell casings); and chemical distribution.
Diseases, such as Upper Respiratory Tract Disease (URTD) and shell disease, are a major cause of tortoise mortality in the Mojave population, particularly in the western Mojave. URTD causes lesions of the upper respiratory tract and clinical signs vary in onset, duration, and severity. Desert tortoises infected with URTD may show symptoms of clear wet discharges from eyes and nose, loss of weight, and wheezing. Symptoms of the shell disease, cutaneous dyskeratosis, include lesions along the shell. The disease may be caused by toxins or a nutritional deficiency. Herpes virus was recently identified in desert tortoises and may have population level effects, but very little is known about it.Predators of the desert tortoise include kit foxes, bobcats, coyotes, Gila monsters, golden eagles, and common ravens. Feral and domestic dogs and cats are also predators of the desert tortoise. The common raven, whose numbers have increased in the Mojave and Colorado Deserts since 1968, is a major predator of juvenile tortoises.
Updated November 26, 2012
The desert tortoise was historically distributed in the Mojave Desert and Sonoran Desert in south central California, southern Nevada, southeastern Arizona, southwestern Utah, and Sonora and northern Sinaloa, Mexico.
Click on the map below to see the distribution range of the desert tortoise using the interactive GIS map.
The desert tortoise has declined throughout its historical range and has been extirpated in parts of its range. The desert tortoise is divided into two populations, the Sonoran and Mojave populations, based on genetic and morphological characteristics. These two distinct populations are recognized under the Endangered Species Act. The Mojave population occurs north and west of the Colorado River, and the Sonoran population occurs south and east of the Colorado River.
The Mojave population of the desert tortoise inhabits parts of the Mojave Desert in Inyo, Kern, Los Angeles, San Bernardino, and Riverside counties in California; northwestern part of Mojave County in Arizona; Clark County and the southern parts of Esmeralda, Nye, and Lincoln Counties, Nevada; and part of Washington County, Utah. The population also inhabits the Colorado Desert, a division of the Sonoran Desert, in Imperial County, San Bernardino County, and Riverside County, California. The Mojave population range in Arizona extends north and west of the Colorado River; west of the Beaver Dam Mountains; north of the Virgin Mountains; and in the Pakoon Basin in extreme northwest Mojave County.The Mojave population is present in reaches 1-6 of the Lower Colorado River Multi-Species Conservation Program (LCR MSCP) planning area in an estimated 10,660 acres in desert scrub habitat. The Desert Tortoise (Mojave Population) Recovery Plan described 14 Desert Wildlife Management Areas (DWMA). Land ownership and tortoise densities were estimated for each.
The Mojave population of the desert tortoise is found in Mojave Desert scrub dominated by creosote bush, creosote bursage, shadscale, other sclerophyll shrubs, and small cacti. They also occur in Joshua tree forests and occasionally blackbrush habitat. Native desert grasses, particularly galleta grass and Indian rice grass, are associated with high desert tortoise densities. The most preferable desert tortoise habitat is where there is a high density of shrubs that provide cover and high densities of perennial and annual forbs and grasses. Desert tortoises prefer sandy loam to rocky soils in valleys, bajadas, and hills. Their elevation range is from sea level to 4921 feet (1500 m). The desert tortoise basic habitat requirements are sufficient, suitable plants for forage and cover, and suitable substrates for burrow and nest sites. Burrows can be up to 10 meters deep and are usually directly below vegetation or in caves in washes. Desert tortoises prefer areas that receive from 100 mm to 300 mm of rainfall annually.
Burrows used in the spring and summer, when tortoises are active, have the following characteristics: 1) usually larger and longer than the tortoise, often extending 1 to 8 feet in length; 2) a mean floor declination of 15 degrees; 3) opening faces north, northwest, or northeast; 4) often under a shrub; and 5) has a single opening.Burrows used in the winter, when desert tortoises hibernate, have the following characteristics: 1) extends up to 30 feet in length; 2) often used by more than one tortoise; 3) opening faces south; 4) often enhanced by chambers and interconnections between dens; and 5) hold air masses with stable, high relative humidity reaching 40%.
LCR MSCP Conservation Measures
The Habitat Conservation Plan provides conservation measures specific to each species. Listed below are the species specific conservation measures for the desert tortoise. Click on the arrows to expand the table.
|DETO1—Acquire and protect 230 acres of existing unprotected occupied habitat (completed)|
Consistent with the mitigation measures identified in the document “Compensation for Desert Tortoise” (Desert Tortoise Conservation Team 1991), the LCR MSCP will acquire and protect 230 acres of unprotected occupied desert tortoise habitat. The acquired habitat will be transferred to an appropriate management agency for permanent protection of species’ habitat. Although creation of replacement habitat is not considered feasible, protecting existing occupied habitat will ensure that implementation of covered activities and LCR MSCP conservation measures do not adversely affect the existing distribution, abundance, or population viability of the desert tortoise within the LCR MSCP planning area.
|DETO2—Avoid impacts on individuals and their burrows|
To avoid and minimize impacts on desert tortoise, the following measures, which are derived from USFWS's Field Survey Protocol for Any Federal Action That May Occur within the Range of the Desert Tortoise (U.S. Fish and Wildlife Service 1992) and the Desert Tortoise Council's Guidelines for Handling Desert Tortoises during Construction Projects (Desert Tortoise Council 1994), will be implemented.
|AMM5—Avoid impacts of operation, maintenance, and replacement of hydroelectric generation and transmission facilities on covered species in the LCR MSCP planning area|
To the extent practicable, before implementing activities associated with OM&R of hydroelectric generation and transmission facilities, measures will be identified and implemented that are necessary to avoid take of covered species where such activities could otherwise result in take. These measures could include conducting surveys to determine if covered species are present and, if so, deferring the implementation of activities to avoid disturbance during the breeding season; redesigning the activities to avoid the need to disturb covered species habitat use areas; staging of equipment outside of covered species habitats; delineating the limits of vegetation control activities to ensure that only the vegetation that needs to be removed to maintain infrastructure is removed; stockpiling and disposing of removed vegetation in a manner that minimizes the risk of fire; and implementing BMPs to control erosion when implementing ground disturbing activities.
Research and Monitoring Activities
The LCR MSCP conducts a variety of research and monitoring activities along the LCR encompassing both MSCP and non-MSCP species. For a complete list of all activities, please see the Research and Monitoring Activities web page.
You can find research and monitoring activities for the desert tortoise here.
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