The southwestern United States is lucky to have a wide variety of animals throughout its desert region. One special and ecologically important animal that is found here is the desert tortoise. This land-loving prehistoric critter has worked its way into the hearts of the public and has gained a lot of attention due to its declining populations resulting from encroachment on its habitat, droughts, and exposure to bacteria that cause upper respiratory tract disease.
The desert tortoise is one of four North American tortoise species that are uniquely adapted to the different habitats in which they thrive. These four tortoise species are all members of the genus Gopherus, and it can be tricky to tell them apart. We have seen all four species here at the Desert Tortoise Conservation Center (DTCC)! Following are some characteristics and life history traits of each species.
Gopher tortoise Gopherus polyphemus:
The gopher tortoise is a master burrower found in the southeastern U.S. from southern Louisiana all the way up to the southern tip of South Carolina, as well as throughout Florida (except in the range of the Everglades). The gopher tortoise is found in shrubby, prairie-like areas with sandy soil, as well as in longleaf pine/scrub oak habitat. Gopher tortoises are known for their long deep burrows, with some burrows reaching up to 40 feet (12 meters) long. These amazing burrows serve to protect gopher tortoises and over 300 other animals such as snakes, skunks, lizards, and armadillos from predators and harsh weather conditions.
Texas tortoise Gopherus berlandieri:
The smallest of the four Gopherus species, the Texas tortoise is found in southern Texas and northeast Mexico. It can be found in a variety of habitats but is mostly found in dry scrub and grassland areas. Unlike its cousin the gopher tortoise, the Texas tortoise doesn’t dig its own long burrows but instead uses a burrow dug by another animal or digs a shallow burrow under small shrubby plants. The Texas tortoise population has declined because of the pet trade in recent years, but new laws in Texas will be giving the tortoise a chance to recover.
Bolson tortoise Gopherus flavomarginatus:
The Bolson tortoise, also called the Mexican tortoise, is the largest of the four Gopherus species and is believed to be the rarest as well. It is found in a small region in north-central Mexico and only at an altitude of 3,300 to 4,300 feet in arid, sandy desert habitat characterized by the unique flora and fauna of that region. The significant declines in the populations of this species are attributed to cattle ranching, road construction, the pet trade, and even collection by locals as food. Recently, a biosphere reserve was established for the conservation of the Bolson tortoise and its habitat.
Desert tortoise Gopherus agassizii:
The desert tortoise that we know and love right here in the Mojave Desert is naturally found in northwest Mexico, southern California, southern Nevada, southern Utah, and throughout much of Arizona. The desert tortoise’s range spans two different deserts, the Sonoran and the Mojave. Scientists have determined that there are genetic differences between the tortoises found in those two locations. Like the gopher tortoise, the desert tortoise is an excellent burrow digger, and it uses burrows to escape the extreme temperatures of the desert, which range from above 120 degrees Fahrenheit in the summer to below freezing in winter. Burrows also help to protect the desert tortoise from predation by coyotes, ravens, and even mountain lions. Desert tortoises have been known to live up to 100 years in managed care. They can lay up to two clutches of eggs in a single year if resources like food and water are abundant. Unfortunately, desert tortoises are a threatened species, and populations are declining due to upper respiratory tract disease and human encroachment on their habitat.
So how do we tell the difference between these four animals if they are all lined up in front of us?! Well, Texas tortoises and desert tortoises show one small characteristic on their carapace (top shell) that is different from the others and from each other. Texas tortoises are small and have a slightly different head structure from the others, while Bolson tortoises are big and do not show sexual dimorphism (you can’t tell males from females externally). Finally, gopher tortoises sometimes have a darker and slightly different-shaped carapace than the others. Not very definitive, is it?! That’s why we rely on genetic testing to be certain which species we have here at the DTCC!
Daniel Essary is a research associate at the San Diego Zoo’ Desert Tortoise Conservation Center. Read his previous post, Desert Tortoises: Unexpected Nests.