All About the Desert Tortoise

Desert Tortoise

Gopherus agassizii

Desert tortoises live in a variety of habitats from sandy flats to rocky foothills, and are found from near sea level to around 3, 500 feet in elevation in the Mohave and Sonoran deserts. There are a number of plant communities within these areas; tree/yucca areas (Joshua tree and Mojave yucca), creosote bush and saltbush scrub habitats, and in some ocotillo-creosote habitats. Due to these distinct ecological ranges, there are variations in behavior, morphology, and DNA in populations.

The desert tortoise is well adapted both physiologically and behaviorally to live in dry desert environments. They derive almost all their water intake from the plants they eat. A large urinary bladder can store over forty percent of the tortoise's body weight in water, urea, uric acid and nitrogenous wastes. Also, they have flattened forelimbs which they use to dig burrows for escaping the heat (and predators).

Tortoise burrows vary considerably in length and type. The style of burrow appears to be dependent upon the region, geologic formations, soil type, and vegetation in which it is found. For example, tortoises in the Mohave Desert in California and the northern limits of the range in Nevada and Utah seem more inclined to construct extensive burrows, up to thirty-five feet in length. In the Sonoran Desert, tortoises frequently use rocks and boulders as sites for burrows or shelter. They may excavate a space beneath a boulder or squeeze into rock crevices. Each tortoise usually has more than one burrow. The number of burrows the tortoise uses may depend on age and sex, as well as on the season. When burrows are constructed in soil, they are the size and shape of the tortoise -- half moon for the roof and flat on the bottom. Small tortoises have small burrows and large tortoises have large burrows.

Many behavioral attributes of the desert tortoise are well documented. When confronted by a predator, tortoises typically withdraw their head, feet, and tail, folding their front knees in front of their head, thus exposing only the shell and heavy scales of the armored forelimbs. This is an effective defense against most predators except people. If attempts are made to remove a tortoise from its burrow, it will retreat to the interior or extend the legs, wedging the carapace against the roof of the burrow.

The desert tortoise, Gopherus agassizii, is characterized by a high-dome, brown shell, often with yellowish centers on each scute, and stout elephantine legs. Sexual maturity is reached at 10-20 years of age (although sexual maturity is a factor of growth and size rather than age). It reaches an average length of 6 to 14.6 inches, with males growing larger than females. A gigantic specimen, allegedly from Mexico, at the San Diego Natural History Museum, has a shell of 15.9 inches long. Other large individuals have been found in Native and introduced grasses comprise the bulk of the desert tortoise diet. Otherwise, they eat any available edible plants including spring and summer annual wildflowers, forbs and cactus fruit. Tortoises forage selectively, often sniffing or sampling various plants before consumption. Rocks and soil are also ingested, perhaps as a means of maintaining intestinal digestive bacteria and as a source of supplementary calcium or other minerals.

The desert tortoise is protected as a Threatened species under the U.S. Endangered Species Act north and west of the Colorado River in California, Nevada, and Utah where an upper respiratory tract disease has decimated many populations. Numerous other factors have contributed to this decline including residential development, road construction, agricultural and mineral development, use of off-road vehicles, overgrazing, malicious vandalism, and collection as pets.

Probably no more than one hatchling from every fifteen to twenty nests will reach sexual maturity in the wild resulting in very low recruitment to the population. The fifty to eighty year life span estimated for desert tortoises suggests population turnover is not only low but should be very episodic following fluctuating climates. The desert tortoise could reach a "point of no return" as more reproducing adults fall victim to humanity's expanding impact in fragile desert environments.

Many Arizonans are interested in having a desert tortoise for a number of reasons. This interest can be met through regulatory mechanisms and captive tortoise recycling programs. Applicants are carefully screened for intent and responsibility before being assigned a tortoise, which remains the property of the State of Arizona. All tortoises placed are urban foundlings, unwanted captives, or their progeny. The purposes of the program, sanctioned by the Arizona Game and Fish Department, are to provide appropriate care and custody for tortoises already in captivity while vigorously discouraging the taking of tortoises from the wild. Thousands of tortoises are held in captivity in Arizona. It is ironic that people's attraction to the tortoise has become a significant threat to its future. Unfortunately, release of captive tortoises is considered a high risk to existing populations because of the potential to introduce disease, disrupt population structure, and mix genetic stock from different regions. Management of the captive population separately from those in the wild may actually aid conservation of wild tortoises. Under Arizona law, one tortoise per family member may be possessed if the tortoises are obtained from a captive source and properly documented.

Sources: http://www.fws.gov/southwest/es/arizona/Sonoran_Tort.htm; http://www.desertmuseum.org; http://www.deserttortoise.org; http://www.azgfd.gov/w_c/desert_tortoise.shtml; http://www.desertusa.com/june96/du_tort.html; http://online.wr.usgs.gov/ocw/g_agassizii/; http://geochange.er.usgs.gov/sw/impacts/biology/tortoise1/index.html; http://www.tortoise-tracks.org/newsletter/ttsummer2007.pdf; http://www.werc.usgs.gov/sandiego/pdfs/RavenMgt.pdf            edited by Julie Deiter