Desert Tortoise

Gopherus agassizii

Geography – Range

Mojave and Sonoran deserts of southeastern California, southern Nevada, south through Arizona into Mexico.

Related Species

Tortoises are any of the land-dwelling turtles of the family Testudinidae. The Desert Tortoise is one of four species of the genus Gopherus, known collectively as gopher tortoises. Gopher tortoises are characterized by brown shells 8-15 inches long with flattened front limbs adapted for burrowing.

Berlandier's Tortoise (G. berlandieri) inhabits near-desert and wooded areas of Texas and northern Mexico. The Gopher tortoise (G.polyphemus) inhabits sandy and wooded regions of the southeastern US from Florida to Texas. The Gopherus flavomarginatus, with the common name "Bolsón Tortoise," was discovered in 1959. It lives in North Central Mexico in the states of Chihuahua, Coahuila and Durango around the Bolsón de Mapimí, a large geologic feature.

Comparisons

A tortoise is a high-domed turtle, with elephant-shaped, or "columnar," legs. It is more terrestrial than the turtle is, going to water only to drink or bathe. Tortoises do not have bodies designed for swimming. They do not have webbed feet, rather their feet are round and stumpy for walking on land, and they are not able to swim.

Description

The Desert Tortoise is an herbivore that may attain a length of 9 to 15 inches in upper shell (carapace) length. The tortoise is able to live where ground temperatures may exceed 140 degrees F, 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 while dormant, November through February or March.

The Sonoran Desert Tortoise is flat and pear-shaped, compared to the Western Mojave tortoise which is more of a butterball shape; they are usually active in spring. The Sonoran Desert tortoise is more active in summer and seeks shade under large rocks and boulders. It is possible that northern and southern Desert Tortoises may one day be designated as different species or subspecies.

The presence of soil suitable for digging burrows is a limiting factor to Desert Tortoise distribution. Some of their burrows just extend beyond the shell of the tortoise inside. Others extend for several feet. A single tortoise may have a dozen or more burrows distributed over its home range. These burrows may be used by different tortoises at different times.

Fighting may occur any time males encounter one another, and usually ends in the subordinate male running away from the other. Where there are cavities in partially consolidated gravels with room for several tortoises, males and females will share theses cover sites. The males may begin to fight upon emerging each day but the importance of adequate cover for protection against extreme heat seems to be greater than the need to maintain dominance hierarchy.

To maximize the utilization of infrequent rainfall, tortoises dig catchment basins in the soil, remember where these are, and may be found waiting by them when rain appears imminent. Water that reaches the bladder is not lost to the system but can be drawn upon as needed.

Much of the tortoise’s water intake comes from moisture in the grasses and wildflowers they consume in the spring. During very dry times they may give off waste as a white paste rather than a watery urine. Adult tortoises may survive a year or more without access to water.

Habitat

Desert tortoises inhabit semi-arid grasslands, gravelly desert washes, canyon bottoms and rocky hillsides below 3,530 ft.

Tortoises north and west of the Colorado River inhabit valleys and on alluvial fans. In the Sonoran Desert of Arizona, however, the tortoises tend to live on steep, rocky hillside slopes in Palo Verde and Saguaro Cactus communities.

Food & Hunting

Diet composition varies throughout the tortoise's range. If winter rainfall has been sufficient to result in germination of annuals, these are used heavily when the tortoises emerge from winter torpor (brumation). Other herbs, grasses, some shrubs and the new growth of cacti and their flowers comprise a major portion of the diet. If there is no summer rain, tortoises will utilize dry forage.

Breeding

Courting and copulation may occur any time that tortoises are above ground; however, there seems to be more of this behavior in late summer and early fall when the testosterone levels peak in males. Females store sperm and egg laying occurs in May, June and July.

The number of eggs varies. Female size seems to be one factor. A mature female might lay 4-8 white, hard-shelled eggs in a clutch and produce 2, sometimes 3 clutches in a season. Hatchlings from only a few eggs out of every hundred actually make it to adulthood.

Nests are often dug near the burrow opening early in the season, and farther inside late in the season. Some nests are dug away from the burrow, usually under a shrub. After laying, the female leaves the nest and the soil temperatures support growth of the embryos. Incubation periods of 90 to 120 days are typical. Data from experiments using controlled incubation temperatures show that cooler temperatures, 79-87 degrees F. produce all males; at 88-91 degrees F. all females.

Tortoises grow at varying rates depending upon forage availability. The number of growth rings in a given year may be zero to several; hence, one cannot determine a tortoise's exact age by counting those rings. Sexual maturity is a function of size rather than age, approximately 7-8 inches mid- carapace length in females. Generally, Desert Tortoises don't reach sexual maturity for 15 to 20 years.

Conservation

Ravens, Gila Monsters, Kit Foxes, Badgers, Roadrunners and Coyotes are all natural predators of the Desert Tortoise. They prey on juveniles, which are 2-3 inches long with thin, delicate shells.

In recent years Desert Tortoises of the Mojave Desert have been federally listed as a threatened species. State and federal wildlife and land management agencies and local jurisdictions are actively involved in conservation programs to help the recovery of the Desert Tortoise throughout the Mojave Desert.

Primary threats remaining to the Desert Tortoises include

Illegal collection and vandalism by humans
Urban area expansion that has destroyed habitat and increased the numbers of Ravens
Upper respiratory tract disease
The loss of forage plants due to competition with grazing livestock and replacement by invasive species.

Cautions
It is unlawful to touch, harm, harass or collect a wild Desert Tortoise. There are programs run by tortoise clubs in Arizona, California and Nevada through which legally acquired captives may be adopted.

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