Order: Testudines - turtles and tortoises
Family: Testudinidae - tortoises
Scientific Name: Gopherus agassizii
Description: The desert tortoise is a medium-sized tortoise with a high-domed shell. The shell can be light or very dark brown, and the center of each scute is usually yellowish. The eyes are greenish-yellow. The skin is dry and scaly, with enlarged, armor-like scales on exposed parts of the legs. Their legs are short and elephantine, and the front legs are flattened as an adaptation for digging. They have short, strong claws. Both sexes have a gular horn – an extension of the plastron below the neck. Males have a longer horn than females, and also have a longer, thicker tail. The male’s plastron is slightly concave. Adult desert tortoises grow to 11-16 inches in carapace length, and can weigh 10 pounds or more. Males are larger than females.
Home Range: The Mojave and Sonoran Deserts of southwestern United States and northern Mexico
Habitat Type: Semi-arid grasslands, sandy or gravelly desert areas, canyon bottoms, and rocky hillsides
Reproduction: Desert tortoises can breed throughout the year, but most breeding takes place when males’ testosterone levels peak in late summer or early fall. Sperm is probably stored by the females, and the eggs are not laid until the following spring. Females dig nests in a burrow or under shrubs, and lay their eggs inside. They lay 4-8 hard-shelled eggs per clutch, and can produce up to 3 clutches per year. The soil temperature within the nest determines the gender of the hatchlings – temperatures below 89.3o F produce a clutch of males, and higher temperatures produce females. The eggs hatch 90-120 days after laying. The hatchlings are less than 2 inches long. Like most other reptiles, desert tortoises do not care for their eggs or young. The survival rate for hatchlings is very low – out of 100 eggs laid by desert tortoises, only a few will live to adulthood. Sexual maturity depends on size instead of age, and females begin laying eggs when they are 7-8 inches long. It takes 12-20 years to reach this length.
Diet in the Wild: Grasses, some shrubs, and the new growth of cacti and their flowers; also annual flowers, when available.
Diet in the Zoo: Rabbit pellets and fresh fruits and vegetables such as apples, carrots, and lettuce.
General Information: Desert tortoises can be found in places where the ground surface temperature sometimes exceeds 140o F. They dig burrows into sandy or gravelly soil, creating places where they can avoid the temperature extremes of the desert. Their burrows are sometimes just long enough to admit the tortoise, and rarely reach 30 feet in length. A single tortoise may dig more than a dozen burrows in its home range, and these are used by many other animals such as gopher frogs, hares, burrowing owls, snakes, and many insects. Desert tortoises spend the winters in their burrows, usually staying in torpor from November through February or March. In very dry areas, they may also enter torpor in the summer, when food is not available. During seasons when they are active, they feed in the early morning and late afternoon, spending cool nights and the hottest part of the days in their burrows.
Adult tortoises get most of their water from moisture in the plants that they eat. They can survive without access to water for a year or longer. When they have free access to water, they can drink more than 40% of their body weight in approximately an hour.
Desert tortoises are usually silent, but they occasionally make hissing and popping sounds. These are thought to be fear or distress calls. During active periods, males use their gular horns to fight for dominance. By inserting the horn under the carapace of their opponent and twisting to one side, they can flip the opponent onto his back. In most fights, the subordinate male runs away before either tortoise is flipped over. During torpor or overnight, several males and females may gather together in a single burrow. Males do not appear to fight inside the burrows, although they may begin fighting as soon as they emerge each morning.
Conservation Status: Desert tortoises are listed as Threatened by USFWS; IUCN Red List – Vulnerable; CITES, appendix ll. Once widespread across the southwestern US, they are now only found in small, isolated populations. A key factor in their decline is habitat destruction, as desert areas are developed for agriculture or human habitation, or used to graze sheep and cattle. Predation is a major problem for this species (see below). Desert tortoises are also captured for the pet trade, and many succumb to respiratory disease, which was probably introduced into the wild population by captive turtles that were released into the wild. Because of their low reproductive rate, their populations are very slow to recover from disturbances. Desert tortoises are protected by law – it is illegal to touch, harm, harass, or collect a wild tortoise, or release a captive one back into the wild. Conservation efforts focus on protecting desert habitats and educating the public.
Predators: Ravens are one of their primary predators, causing more than 50% of juvenile deaths in some parts of their range. Other predators include gila monsters, kit foxes, skunks, badgers, roadrunners, and coyotes.
Atlas of Reptiles and Amphibians for the Terrarium. 1988. F.J. Obst, K. Richter, and U. Jacob. T.F.H. Publications, New Jersey. pp. 399-400.
The Conservation Biology of Tortoises. 1989. I.R. Swingland and M.W. Klemens. IUCN/SSC Tortoise and Freshwater Turtle Specialist Group. pp. 5-7.
Tortoises and Freshwater Turtles: An Action Plan for their Conservation. 1989. IUCN/SSC Tortoise and Freshwater Turtle Specialist Group.
Turtles and Tortoises of the World. 1988. D. Alderton. Facts on File Publications, New York. p. 39