Desert Tortoise

The desert tortoise (Gopherus agassizii) is a species of tortoise native to the Mojave desert and Sonoran desert of the southwestern United States and northern Mexico. The epithet agassizii is in honor of Swiss-American zoologist Jean Louis Rodolphe Agassiz.

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Description

The carapace of these tortoises may attain a length of 6" to 15" (15 to 38 cm), with males being slightly larger than females. Their shells are high-domed, and greenish-tan to dark brown in color. Desert tortoises can grow from 4–6" in height and weigh 8–15 lb (4–7 kg) when fully grown. The front limbs have heavy, claw-like scales and are flattened for digging. Back legs are more stumpy and elephantine.

Habitat

The tortoise is able to live where ground temperature may exceed 140 degrees Fahrenheit (60 degrees Celsius) because of its ability to dig underground burrows and escape the heat. At least 95% of its life is spent in burrows. There, it is also protected from freezing winter weather while dormant, from November through February or March. With its burrow, this tortoise creates a subterranean environment that can be beneficial to other reptiles, mammals, birds and invertebrates.

Scientists have divided the desert tortoise into two types: the Mojave and Sonoran Desert tortoises, with a possible third type 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. It is believed that, in their entire lives, these tortoises rarely move more than two miles from their natal nest. They also live to be 80-100 years old.  

Diet

The desert tortoise is an herbivore. Grasses form the bulk of its diet, but it also eats herbs, annual wildflowers, some shrubs, 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 and/or 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 forty percent 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. Adult tortoises can survive a year or more without access to water.

One defense mechanism the tortoise has when it is handled or molested is to empty its bladder. This can leave the tortoise in a very vulnerable condition in dry areas, and they should never be alarmed, handled or picked up in the wild.

Tortoises may also be vulnerable to diseases and viruses. Coming into contact may cause them to catch unfamiliar strains.

Reproduction

The mating season for the desert tortoise is lengthy. It occurs from spring to fall, with a peak in late summer/early fall (September). They typically lay 4-8 eggs per clutch, with 1-2 clutches per year. The eggs are hard, chalky and elliptical or spherical and buried in a funnel-shaped nest. They are incubated for 90-120 days. Hatchlings from only a few eggs out of every hundred actually survive the 7-15 years it takes to reach full adulthood.

Predators and conservation status

  Ravens, 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 hypothesized 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, habitat destruction and fragmentation, illegal collection and vandalism by humans, and competition with cattle for forage plants. The eggs they lay can get so shiny that they can look like they've been hard boiled.

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 (TAP) in Arizona, or through the Bureau of Land Management in Nevada. When adopted in Nevada, they will have a computer chip embedded on their back for reference. Under Arizona law, one tortoise per family member may be possessed if the tortoises are obtained from a captive source which is properly documented. Captive sources include urban foundlings, unwanted captives, and their progeny.

References

• Tortoise & Freshwater Turtle Specialist Group (1996). Gopherus agassizii. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Listed as Vulnerable (VU A1acde+2cde, E v2.3)

• Gopherus agassizii (TSN 173856). Integrated Taxonomic Information System. Accessed on 6 February 2006.
• The Desert Tortoise
• The Biogeography of The Desert Tortoise, by Kerrie Bathel
• The Arizona-Sonora Desert Museum