Desert Tortoise


ORDER: Chelonia

FAMILY: Testudinidae

GENUS: Gopherus

SPECIES: agassizi

DESCRIPTION:
8-15 inches. Has a high-domed shell, usually with prominent growth lines. The carapace is brown or horn-colored and the plastron is yellowish and has no hinge. The hind margin of the shell is somewhat serrated. Limbs are stocky and covered with large conical scales. The tail is short. Males have a noticeably larger chin gland than females.

GEOGRAPHICAL RANGE AND HABITAT:
Southeastern California, western Arizona, southern Nevada, southeastern Utah and in coastal Sonora in Mexico. Found in desert oases, riverbanks, washes, dunes and rocky slopes. Soils must be friable enough to allow for burrows.

DIET:
Herbs, grasses, cacti. Spring grasses and wildflowers are their primary nutritional source of both food and water. Dry grass stems and cactus pads provide food in dryer times.

LIFE CYCLE/SOCIAL STRUCTURE:
Terrestrial. Mating may occur at anytime the tortoises are above ground but most occurs in late summer and early fall. Females store sperm and egg-laying occurs in May, June and July. Clutches of 3-14 hard-shelled round eggs may be laid as many as three times in a season. Soil temperatures support the growth of embryos during an incubation period of 3 to 4 months. Controlled experiments show that cooler temperatures of 79-87 degrees F. produce all males while at 88-91 degrees F. only females are produced. Hatchlings are about two inches in length. Slow growth and thin shells make juveniles vulnerable to predators and they have a very high mortality rate. Maturity is reached at 14 to 20 years. Lifespan is 80-100 years.

SPECIAL ADAPTATIONS:
Large limbs and well developed claws enable them to burrow into desert soil to escape the heat. Short tunnels afford temporary shelter; longer ones (15 feet) are used for estivation and hibernation. Foraging takes place in the morning or late afternoon from March to October. By November, most tortoises ( except those in the extreme south of their range) have begun hibernation. May go for years without drinking, ingesting most of their water from plants and then storing it in their bladders. Sometimes dig shallow basins in impermeable soil to catch rainwater. An anterior extension of the plastron is longer in males and is used in fighting other males. By inserting this horn under the anterior edge of the carapace and twisting, the other male may be flipped on its back. The opponent attempts to stand high to prevent this.

INTERPRETIVE INFORMATION:
The four species of this genus are known as gopher tortoises because of their flattened front limbs adapted for burrowing. 95% of their life is spent in underground burrows. Desert Tortoises are able to live where ground temperatures may exceed 140 degrees F. These tortoises make hisses, pops and "poink" sounds, perhaps as distress calls. Males grunt when mating. Growth rings in a given year may be zero to several; therefore one cannot determine exact age by counting these rings. The Desert Tortoise is the California State Reptile.

OUR ANIMALS:
One male in the Education Department. He was donated in 2008 after being kept as a pet for 48 years.

STATUS IN WILD:
Listed as a threatened species in 1990 by the US Fish and Wildlife Service and vulnerable on the CITES Redlist. Populations have declined more than 90 percent since the 1980s. This is due to: 1) direct loss of individual (poaching, collection for pets, military activities, vehicular impact, livestock trampling, disease and raven encroachment); and 2) habitat degradation (urban sprawl and livestock grazing practices). California state law makes it illegal to "sell, purchase, needlessly harm, take, or shoot any projectile at a desert tortoise".

BIBLIOGRAPHY:

  1. Internet: usgs.gov/sw/impacts/biology/tortoise1 2004. ; desertusa.com
  2. Pritchard, Peter. Encyclopedia of Turtles. 1979. tfh publications, Inc. Neptune, NJ. pp. 418-420..
  3. Stebbins, Robert C. Western Reptiles and Amphibians. 1985. Houghton Mifflin Co. Boston, MA, p.104. CL:2008

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