Sonoran Desert Tortoises

Some readers have asked me about Tonka, my desert tortoise, so I thought I'd write a little bit about the Sonoran Desert tortoise.

There are thirty-nine species of tortoises worldwide and they differ from turtles in that they are entirely land-based, rather than living in and near the water. In fact, they cannot swim and will drown in water over their head. Four species of tortoises have been living in North America for thirty million years, essentially unchanged.

Tonka is a Sonoran Desert tortoise (Gopherus agassizii). These tortoises live in the Sonoran Desert, in Arizona and Sonora, Mexico, where they are called la tortuga de tierra (land turtle). They prefer rocky slopes, washes and bajadas (alluvial fans at the base of mountains). The other Southwestern desert tortoise is called the Mohave Desert tortoise and occurs in the flat desert valleys and basins of southeastern California, southwestern Utah and southern Nevada.

The desert tortoise has a high domed shell (carapace) composed of individual scutes, which are usually light brown in adults and dark tan in younger individuals. The tortoise's hind limbs differ markedly from the forelimbs. Whereas the hind limbs are elephantine, the forelimbs are flattened with well-developed muscles and are covered in thick protective scales. The forelimbs are used for digging burrows. The females use their hind limbs to dig their nests.

Both sexes have a gular horn under their chin—the horn is an extension from the plastron (lower shell). The horn is longer in males, and is used in fighting in an attempt to flip the other male over onto their back. The tortoise also has a short stubby tail, longer in males than females.

Tortoises can live up to 100 years. Unlike some of the giant tortoises, such as the Galapagos tortoise, the desert tortoise will only reach a length of 8 to 15 inches and weigh 8 to 15 pounds. They are herbivorous and their primary food is native and introduced grasses. They also eat the leaves, stems and flowers of many desert plants, including prickly pear cactus pads and fruit.

They are well adapted to living in the desert environment, surviving temperatures up to 140° F. by using burrows, rock outcrops and the shade of bushes to help regulate their internal temperature. They spend up to 95% of their life in these small dens. They hibernate in burrows during the colder winter months, and are active during the day and evening from spring through autumn. During the hottest part of the summer day, though, they will remain in their burrows to conserve moisture and energy.

Tortoises obtain most of their water from the plants they eat. Adults can survive a year or more without drinking any water. During periods of drought, they retain water in their bladders, able to reabsorb it if needed. As a defense mechanism if molested, tortoises will empty their bladder if they feel threatened, so please do not pick up tortoises in the wild. You may very well be the cause of its subsequent death if it cannot quickly replace the lost moisture.

Tortoises are primarily solitary creatures, though they may share burrows. Males seek out females during the mating season, but will fight other males they come across. After breeding, females lay 4 to 8 eggs in a shallow nest dug into the soil usually near the burrow. Gestation is 90-120 days. Data from experiments using controlled incubation temperatures show that cooler temperatures, 79-87 degrees F. produce all males; at 88-91 degrees F. all females.

Hatchling tortoises are small (about 1½ to 2 inches in length), and have thin, delicate shells. They are prey to many animals, including coyotes, foxes, bobcats, ravens, roadrunners and Gila monsters. Less than 2% survive to adulthood. Tortoises grow at varying rates depending upon forage availability; sexual maturity is a function of size rather than age, though sexual maturity usually occurs at 15-20 years of age. Tortoises can make a variety of sounds from hisses, pops and poink sounds, perhaps as fear and distress calls. Males grunt when mating. Surprisingly, tortoises can run fast for short distances when trying to escape other males.

The population of the Sonoran Desert tortoise has been steadily dwindling since the 1950s, when the human population in the Southwest began to grow so rapidly. Habitat destruction due to urban area expansion, home construction, canals, roads and off-highway vehicles, and competition from grazing cattle (they both eat the same grasses) are the main cause of the population decrease, but people capturing wild tortoises as pets also contribute to their decline.

Most captive tortoises don't live long, due to improper care (do not feed them lettuce, please, feed them grass). It is now illegal to remove a tortoise from the wild; it is also illegal to release a captive tortoise into the wild as it may spread diseases (mainly an upper respiratory tract disease) throughout the wild populations. Captive-bred tortoise can be adopted through several regulated sources, though.

That is how I came to have Tonka. We've been together since the early 1980s. My xeriscape backyard is designed to provide suitable desert tortoise habitat and Tonka thrives in it. Tonka seeks me out when I'm gardening, and I oblige with soothing head scritches and the occasional treat of a strawberry or grape. © Tanna Thornburg 2014