AppearanceThese tortoises may attain a length of 10 to 14 in , with males being slightly larger than females. A male tortoise has a longer gular horn than a female, his plastron is concave compared to a female tortoise. Males have larger tails than females do. Their shells are high-domed, and greenish-tan to dark brown in color. Desert tortoises can grow to 4–6 in in height. They can range in weight from .02 to 5 kg . The front limbs have sharp, claw-like scales and are flattened for digging. Back legs are skinnier and very long.
NamingTestudo agassizii — Cope, 1875
Xerobates agassizi [sic]Garman, 1884
Gopherus agassizii — Stejneger, 1893
Testudo aggassizi [sic]Ditmars, 1907
Testudo agassizi — Ditmars, 1907
Gopherus agassizi — V. Tanner, 1927
Testudo agasizzi [sic]Kallert, 1927
Gopherus polyphemus agassizii — Mertens & Wermuth, 1955
Gopherus agassiz [sic]Malkin, 1962
Gopherus polyphemus agassizi — Frair, 1964
Geochelone agassizii— Honegger, 1980
Scaptochelys agassizii— Bramble, 1982
Scaptochelys agassizi— Morafka, Aguirre & Murphy, 1994
StatusRavens, Gila monsters, kit foxes, badgers, roadrunners, coyotes, and fire ants are all natural predators of the desert tortoise. They prey on eggs, juveniles, which are 2–3 inches long with a thin, delicate shell, or, in some cases, adults. Ravens are thought to cause significant levels of juvenile tortoise predation in some areas of the Mojave Desert – frequently near urbanized areas. The most significant threats to tortoises include urbanization, disease, habitat destruction and fragmentation, illegal collection and vandalism by humans, and habitat conversion from invasive plant species .
Desert tortoise populations in some areas have declined by as much as 90% since the 1980s, and the Mojave population is listed as threatened. It is unlawful to touch, harm, harass, or collect wild desert tortoises. It is, however, possible to adopt captive tortoises through the Tortoise Adoption Program in Arizona, Utah Division of Wildlife Resources Desert Tortoise Adoption Program in Utah, Joshua Tree Tortoise Rescue Project in California, or through Bureau of Land Management in Nevada. When adopted in Nevada, they will have a computer chip embedded on their backs for reference. According to Arizona Game and Fish Commission Rule R12-4-407 A.1, they may be possessed if the tortoises are obtained from a captive source which is properly documented. Commission Order 43: Reptile Notes 3: one tortoise per family member.
The Fort Irwin National Training Center of the US Army expanded into an area that was habitat for about 2,000 desert tortoises, and contained critical desert tortoise habitat . In March 2008, about 650 tortoises were moved by helicopter and vehicle, up to 35 km away.
Another potential threat to the desert tortoise's habitat is a series of proposed wind and solar farms. As a result of legislation, solar energy companies have been making plans for huge projects in the desert regions of Arizona, California, Colorado, New Mexico, Nevada, and Utah. The requests submitted to the Bureau of Land Management total nearly 1,800,000 acres .The Desert Tortoise Preserve Committee protects roughly 5,000 acres of desert tortoise habitat from human activity. This area includes 4,340 acres in Kern County, 710 acres in San Bernardino County, and 80 acres in Riverside County.
HabitatDesert tortoises can live in areas with ground temperatures exceeding 140 °F because of their ability to dig underground burrows and escape the heat. At least 95% of their lives are spent in burrows. There, they are also protected from freezing winter weather while dormant, from November through February or March. Within their burrows, these tortoises create a subterranean environment that can be beneficial to other reptiles, mammals, birds, and invertebrates.
Scientists have divided the desert tortoise into two types: Agassiz's and Morafka's desert tortoises, with a possible third type in northern Sinaloan and southern Sonora, Mexico. An isolated population of Agassiz's desert tortoise occurs in the Black Mountains of northwestern Arizona. They live in a different type of habitat, from sandy flats to rocky foothills. They have a strong proclivity in the Mojave Desert for alluvial fans, washes, and canyons where more suitable soils for den construction might be found. They range from near sea level to around 3,500 feet in elevation. Tortoises show very strong site fidelity, and have well-established home ranges where they know where their food, water, and mineral resources are.
Desert tortoises inhabit elevations from below mean sea level in Death Valley to 5,300 feet in Arizona, though they are most common from around 1,000 to 3,500 feet . Estimates of densities vary from less than eight individuals/km2 on sites in southern California to over 500 individuals/km2 in the western Mojave Desert, although most estimates are less than 150 individuals/km2. The home range generally consists of 10 to 100 acres . In general, males have larger home ranges than females, and home range size increases with increasing resources and rainfall.
Desert tortoises are sensitive to the soil type, owing to their reliance on burrows for shelter, reduction of water loss, and regulation of body temperature. The soil should crumble easily during digging and be firm enough to resist collapse. Desert tortoises prefer sandy loam soils with varying amounts of gravel and clay, and tend to avoid sands or soils with low water-holding capacity, excess salts, or low resistance to flooding. They may consume soil to maintain adequate calcium levels, so may prefer sites with higher calcium content.
ReproductionTortoises mate in the spring and autumn. Male desert tortoises grow two large white glands around the chin area, called chin glands, that signify mating season. A male circles around female, biting her shell in the process. He then climbs upon the female and insert his penis into the vagina of a female, which is located around the tail. The male may make grunting noises once atop a female, and may move his front legs up and down in a constant motion, as if playing a drum.
Months later, the female lays a clutch of four to eight hard-shelled eggs, which have the size and shape of ping-pong balls, usually in June or July. The eggs hatch in August or September. Wild female tortoises produce up to three clutches a year depending on the climate. Their eggs incubate from 90 to 135 days; some eggs may overwinter and hatch the following spring. In a laboratory experiment, temperature influenced hatching rates and hatchling gender. Incubation temperatures from 81 to 88 °F resulted in hatching rates exceeding 83%, while incubation at 77 °F resulted in a 53% hatching rate. Incubation temperatures less than 88 °F resulted in all-male clutches. Average incubation time decreased from 124.7 days at 77 °F to 78.2 days at 88 °F .
FoodThe desert tortoise is an herbivore. Grasses form the bulk of its diet, but it also eats herbs, annual wildflowers, and new growth of cacti, as well as their fruit and flowers. Rocks and soil are also ingested, perhaps as a means of maintaining intestinal digestive bacteria as a source of supplementary calcium or other minerals. As with birds, stones may also function as gastroliths, enabling more efficient digestion of plant material in the stomach.
Much of the tortoise’s water intake comes from moisture in the grasses and wildflowers they consume in the spring. A large urinary bladder can store over 40% of the tortoise's body weight in water, urea, uric acid, and nitrogenous wastes. During very dry times, they may give off waste as a white paste rather than a watery urine. During periods of adequate rainfall, they drink copiously from any pools they find, and eliminate solid urates. The tortoises can increase their body weight by up to 40% after copious drinking. Adult tortoises can survive a year or more without access to water. During the summer and dry seasons, they rely on the water contained within cactus fruits and mesquite grass. To maintain sufficient water, they reabsorb water in their bladders, and move to humid underground burrows in the morning to prevent water loss by evaporation.
Emptying the bladder is one of the defense mechanisms of this tortoise. This can leave the tortoise in a very vulnerable condition in dry areas, and it should not be alarmed, handled, or picked up in the wild unless in imminent danger. If it must be handled, and its bladder is emptied, then water should be provided to restore the fluid in its body.
UsesEdwards ''et al.'' reported that 35% of desert tortoises in the Phoenix area are hybrids between either ''Gopherus agassizii'' and ''G. morafkai'', or ''G. morafkai'' and the Texas tortoise, ''G. berlandieri''. The intentional or accidental release of these tortoises could have dire consequences for wild tortoises.
Before obtaining a desert tortoise as a pet, it is best to check the laws and regulations of the local area and/or state. Desert tortoises may not be moved across state borders or captured from the wild. They may, however, be given as a gift from one private owner to another. Desert tortoises need to be kept outdoors in a large area of dry soil and with access to vegetation and water. An underground den and a balanced diet are crucial to the health of captive tortoises.
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