Desert Tortoise, Gopherus agassizii
Desert tortoises were listed as a threatened species in April 1990. Their numbers have declined due to habitat loss and fragmentation. U.S. Fish and Wildlife Service Status: Endangered. International Union for the Conservation of Nature Status: Vulnerable.
Desert tortoises may grow to be 10 to 15 inches long (males are larger than females). The males have a longer horn-like forward projection on their plastron (lower shell) called the gular shield and their plastron is concave compared to female tortoises, so they fit together when mating. Their shells are high-domed and greenish-tan to dark brown in color with prominent growth lines. Desert tortoises can reach 4–6 inches in height and can weigh approximately 8–15 pounds when fully grown. The front limbs have heavy, claw-like scales that are flattened for digging. Back legs are more stumpy and elephantine.
Habitat and Range
The desert tortoise is found in the Mojave and Sonoran deserts in southwestern Utah, southern Nevada (near Las Vegas), southeastern California, and western Arizona south into Mexico. They range from near sea level to around 3,500 feet in elevation. They have the ability to live where the ground temperature may exceed 140 degrees Fahrenheit. They need soils that are soft enough to dig burrows in.
Being herbivores, their diet consists of herbs, grasses, cacti, and wild flowers. Desert tortoises derive almost all their water intake from the plants they eat, making it possible in some circumstances to live a year without drinking.
Mating season is August to October. Desert tortoises reach sexual maturity between ages 12-20. A typical clutch size is 1-15 eggs. In years with low rainfall, females may lay few to no eggs. Females can store sperm for five years or longer, meaning they can reproduce for several years after mating. Females dig holes with back legs and lay eggs in May to July. After laying the eggs, the mother leaves the nest. Once the hatchlings appear, they must survive on their own.
They are elusive animals, spending at least 95% of their life in burrows. The burrow protects the animal from extreme heat and cold ground temperatures. They will lie in a dormant state during winter months (November to February or March). Their shell also provides protection. Tortoises can completely withdraw their head and hind limbs within their shells, leaving only horny scaled front limbs visible to predators.
- Life span: 50-100 years old.
- They have a very large urinary bladder that can store over 40% of the tortoise’s body weight in water. As a defense they will empty their bladder, leaving them at a disadvantage during dry periods (a good reason not to handle them if encountered in the wild).
- Tortoises at the extreme northern reaches of their range (southern Utah) have been shown to have a “summer range” on the desert flats, resting in shallow burrows under bushes, and a “winter range” in the foothills, residing in deep, communal burrows.