- CLASS: Reptilia (Reptiles)
- ORDER: Testudines
- FAMILY: Testudinae
- GENUS: Gopherus
- SPECIES: agassizii
A tortoise to talk about: If you’ve ever visited the Mojave or Sonoran desert in California and saw a tortoise scooting across the sand, you most likely saw a desert tortoise. This tortoise species is a slow-growing, long-lived animal that has changed little during the past 200 million years. It is the largest terrestrial turtle in the United States and is a keystone species in the Mojave Desert ecosystem, providing burrows for other animals and dispersing seeds from eating the plants and grasses there.
The tortoise’s carapace (top shell) is usually brown or gray without any pattern, but it often may be brown or tan in the center of the shell. The plastron (underside) is yellowish or brownish. A desert tortoise has good hearing, but no external ear flaps. Its domed shell provides a large space for its lungs and for efficient thermoregulation, an important adaptation for life in the desert.
HABITAT AND DIET
The Mojave Desert’s summers are harsh, making it difficult to be active, with temperatures reaching well above 105 degrees Fahrenheit (41 degrees Celsius) and with very little (if any) rain. Yet the desert tortoise is well adapted to deal with such extreme weather by going into estivation (decreased physiological activity) in a burrow during the extreme heat of the summer.
The animal’s front limbs are built like shovels, with long, sturdy nails that are good for digging. The large, cone-shaped scales on the limbs provide protection from the scratchy vegetation the tortoise may encounter and help the animal retain water. Cleverly, the tortoise digs basins in the soil to catch the infrequent rain that falls. The animal remembers where these “watering holes” are and walks directly to them after a bit of rain. Another water-saving tactic is storing up to 40 percent of its body weight in water inside the bladder, to be absorbed as necessary.
Desert tortoises are also called gopher tortoises because they live in underground burrows just as gophers do. By spending so much time underground, they are able to survive on very little food. These cold-blooded critters are the same temperature as their surroundings, so their burrow allows them to keep cool in the hot temperatures. During the winter, when food is scarce, tortoises brumate, a form of reptilian hibernation, in their burrow with a wall of dirt at the entrance to keep out the rain and cold for the entire winter season. Come spring, tortoises emerge and bask in the warm sun to jump-start their metabolism. They then devour vegetation with great relish!
Burrows also protect the tortoises from predators. Coyotes and kit foxes prey on adult tortoises. Badgers, skunks, ground squirrels, ravens, Gila monsters, and roadrunners can prey on juvenile tortoises and tortoise eggs. Interestingly, other animals such as pack rats, burrowing owls, kangaroo rats, desert jackrabbits, gopher snakes, banded geckos, and cactus wrens also use tortoise burrows. They are dug at ground level and extend down about 3 to 4 feet (1 to 1.2 meters), typically at a 45-degree angle. Normally one burrow houses a single individual, or one male and one female.
Desert tortoises may also create a den or cave, dug horizontally into the banks of dry washes and extending 8 to 30 feet (2.4 to 9 meters). Several tortoises can occupy one den at the same time, especially during brumation. One record showed 17 tortoises using the same winter den!
Rainfall and temperature control the tortoises’ movements; desert tortoises are most active in spring, early summer, and fall before the colder weather sets in. During the active season, they move across their home range to forage, using multiple burrows as needed. After foraging, tortoises may plop down in a limp, spread-eagle posture with limbs and neck extended, possibly to increase body temperature and help digestion.
Desert tortoises are herbivores, dining on grasses, flowers, fruit, and cactus. These foods contain a lot of moisture, and desert tortoises can go for up to one year without access to fresh water! Tortoises do not have teeth; instead, they have a beak and grind their food. Examples of preferred tortoise forage are prickly pear cactus, primrose, beavertail cactus, white clover, hibiscus, globemallow, desert dandelion, and desert marigold. Desert tortoises need about 20 to 30 days to digest their food, spreading the seeds from their meals across their habitat as they poop. This aids in the repopulation of native plants and grasses in the Mojave Desert.
During the active season, males spar for the privilege of breeding, using their gular horn (part of the plastron lying beneath the extended head) to hook other males and overturn them during aggressive interactions. Female desert tortoises do not mate until they are 15 to 20 years old. The females can store sperm until conditions are right, eventually laying 2 to 14 eggs the size of Ping-Pong balls in a shallow nest dug near her burrow. Eggs are laid from May to July, and hatchlings emerge from eggs from mid-August to October.
The mother does not defend the nest or raise the young; instead, after 90 to 120 days of incubation, the young hatch and fend for themselves. Hatchlings may lunge forward and hiss if disturbed. Few young tortoises make it to adulthood. Their shells remain soft for the first several years of life, and coyotes, roadrunners, gila monsters, and ravens prey upon them. Even domestic dogs attack these reptiles, often injuring their limbs before the tortoise has a chance to retreat into its shell.
Chin glands on a desert tortoise are not active until sexual maturity. They serve as chemical and visual signals to other tortoises. Dominant males have chin glands that are larger than other males’ and contain more testosterone. Desert tortoise vocalizations include hisses, grunts, and moans, and the males vocalize during mating. Head bobs are another form of communication, particularly to get a female’s attention!
AT THE ZOO
From 2009 to 2014, San Diego Zoo Global partnered with the U.S. Fish and Wildlife Service, the Bureau of Land Management, and the Nevada Department of Wildlife to operate the Desert Tortoise Conservation Center (DTCC) in Las Vegas, Nevada, to aid in the recovery of wild desert tortoise populations, as well as the Mojave Desert ecosystem. During its operation, the DTCC helped nearly 1,000 desert tortoises each year, including many acquired from people in Nevada’s Clark County who turned in pet desert tortoises they didn’t want anymore or tortoises they found wandering in developed areas. Here are some stories about some of our favorite desert tortoises!
Monster: One massively large tortoise came to the DTCC from an abandoned, foreclosed home. We affectionately named him Monster, and we think he might be the largest desert tortoise on record! Seasonal staff members did a great job of digging him the largest burrow the facility ever had.
Big Guy: Big Guy’s story began in 2010 when his custodian, Mona, could no longer care for him. Big Guy had been with Mona’s family for over 25 years, so she really wanted to keep him, but she knew life in an apartment was a bad idea for a tortoise. Big Guy was a prime example of great captive care: he was the right size, weight, and color, and he showed no signs of disease or lethargy, a perfect example of what happens when you give a tortoise the proper living conditions like a burrow and native foods.
Several months after Big Guy settled into his daily routine at the DTCC, the new owners of Big Guy’s former home in Las Vegas wanted to adopt him, with Mona’s blessing, of course! They went through the legal desert tortoise adoption process and learned how to care for a desert tortoise properly. Since desert tortoises can live for 80 to 100 years with the proper care, it is really important for anyone considering adoption to realize the huge commitment involved in adopting one. Desert tortoises also have very specific diet and habitat requirements, but the new owners were definitely up to the task. After renovating Big Guy’s original burrow and adding more native plants for him to munch on in the yard, these kind folks were approved for adoption.
Big Guy explored his new-and-improved surroundings and headed straight to his old burrow like he had never left. He also picked up his old habit of knocking at the back door with his beak when he wants to join his new family inside!
Twizzler: The DTCC had a number of special-needs desert tortoises that served as education animals. Twizzler arrived in 2007 with hardened gray material all over the left side of his carapace (top shell), causing a severe deformity. We later learned that he was part of a research project in the 1990s where researchers attached radio transmitters to young tortoises’ shells to track them using telemetry, and after the project was over or when they could no longer hear the signal, they abandoned the tortoises, sometimes without removing the epoxy that kept the transmitters attached! Since the epoxy covered a large portion of the shell, including the seams between the scutes (sections of the shell), the tortoises grew up with severe deformities, if they grew up at all.
Although no longer with us, the moral of Twizzler’s story is this: do not attach anything to a tortoise’s shell, especially if it touches the seams! And on that same note, never paint your tortoise. A tortoise’s shell is full of blood vessels, so toxins may be able to get into the bloodstream through small openings in the shell and along the seams where tortoises may have suffered even slight injuries in the past. Instead, if you want to easily identify your tortoise, use a nontoxic paint pen and write its name and your phone number on its shell so you will be able to get it back if it’s lost. And an even better option to prevent from losing your pet tortoise is to have your tortoise “microchipped” just like a veterinarian would put a microchip in your dog or cat!
Lucy and Ethel: These two females arrived at the DTCC with severe shell deformities, yellow skin, and eyes swollen so severely that they could barely open them. They were also so weak that they could not support their own weight enough to move about their enclosure. All of these signs indicate that Lucy and Ethel were definitely kept indoors for the majority of their lives so they didn’t get the proper heat and light and were not fed a well-balanced diet.
After weeks of providing them with the proper food, allowing them to bask outside daily in the sun’s natural rays, and soaking them every other day in a tub of water to help them establish and maintain hydration, they went from being marginally alive to interactive, beautiful eating machines. They changed from two very sick and depressed tortoises to two very energetic and healthy (though a bit deformed) big girls. Due to the severe deformities of their carapaces, they could not be released back to the wild but were used as education animals to show students and visitors the proper way to care for a tortoise…a valuable lesson for all.
Since 1989, the US government has protected desert tortoises. Once common throughout the Mojave and Sonoran desert of California, Nevada, and Arizona, wild desert tortoise populations have declined by an estimated 90 percent in the last 20 years. As the human population in these areas increases, the tortoise population has decreased. But how are humans causing this devastation? A number of culprits have been identified.
Off-road vehicles cause enormous damage to the desert plant community. When the plants are destroyed, the tortoises are without a source of food and water. It has been estimated that it can take up to 200 years for some of the destroyed habitat to recover. Off-road vehicles often run over tortoise burrows, crushing the animal inside. Large numbers of cattle graze on publicly owned deserts, depriving the tortoises of critical forage.
New housing developments and solar energy projects have sprung up around some of the older desert cities; much of this land is prime tortoise habitat. As a result of this building boom, large numbers of tortoises have been displaced or killed outright.
Increased human populations have brought an increase of predators that feed on human garbage and also forage throughout the desert. Ravens seek out newly hatched desert tortoises, while feral dogs easily kill and eat young tortoises, and kill or seriously maim adult tortoises. There are now an estimated 150,000 wild desert tortoises living in critical habitat.
How can you help desert tortoises? It is always important to reduce, reuse, and recycle, in order to generate less waste and conserve our natural resources. If you see a desert tortoise in the wild, make sure to keep your distance. Even though you mean it no harm, being handled is frightening to a wild tortoise. It is also against the law to touch them.
If you or someone you know already has a desert tortoise and has been keeping it as a pet—keep taking care of it! Pet tortoises shouldn’t be “set free,” because they may introduce disease to the wild tortoises or be unable to survive on their own. Desert tortoises have been around for millions of years. We have to protect them, so they will be here a long, long time to come.
You can help us bring desert tortoises back from the brink by supporting the San Diego Zoo Global Wildlife Conservancy. Together we can save and protect wildlife around the globe.