Sonoran Desert Tortoise, Gopherus morafkai

Sonoran Desert Tortoise, Gopherus morafkai. Photograph taken within a residential community in the greater Alamos area, Alamos, Sonora, December 2018. Photograph and identifications courtesy of David F Smith, Alamos, Sonora.

The Sonoran Desert Tortoise, Gopherus morafkai, is a member of the Testudinidae family of Tortoises. The Testudinidae family consists of approximately fifty members placed in seventeen genera. There are six species in the Gopherus genus. They are known in Mexico as Tortuga del Desierto de Sonora.

Sonoran Desert Tortoises have a high domed shell with a black to tan or brown carapace and plastron. The center of their scutes are often lighter in color. Their head is tan to reddish brown. They have a gular shield, which is a prominent median forward projection from the plastron. Their legs are stout, brown, heavily scaled in the front, and lack webbing in the digits. Sexual dimorphism is displayed with males growing to a larger size, having longer and thicker tails, a concave plastron, and larger claws than females. Males mature at a carapace length of 25 cm (10 inches), while females mature at a carapace length of 22 cm (9 inches). Adults can weigh up to 7.0 kg (15 lbs).

Sonoran Desert Tortoises are diurnal and most active during the summer rainy season from July into early September. They dig their own burrows, which are less than 2 m (7 feet) in length, and use caves and natural rock shelters to spend the night and periods of inactivity. This inactivity helps to reduce water loss during hot periods, while winter hibernation facilitates survival during freezing temperatures and low food availability. Mating occurs during the summer rainy season and courtship encounters may last one or more hours. During courtship, the male trails the female and exhibits nodding, repeated circling, butting, ramming, and biting of the female. Females may mate with several males and store sperm internally for up to two years, meaning that one season’s mating could produce the following season’s clutch of eggs. Sonoran Desert Tortoises lay a clutch of one to twelve eggs in June and July when the rainy season begins. However, some individuals may not produce a clutch every year. Eggs are buried in the soil of burrows and hatch in September and October. Although, some eggs may overwinter and hatch in the spring. Males in managed care settings have been observed helping females dig nests, however, it is unclear if this behavior is also present in the wild. The hatchlings resemble the adults and have a carapace length of 30 to 40 mm. They may take fifteen years to reach sexual maturity. Females that survive to reproductive age can produce up to eighty-five eggs over the course of their lives. To increase the success of their offspring, females will actively defend their nests. They have lifespans of forty to fifty-five years in the wild.

Sonoran Desert Tortoises are herbivores and consume a variety of plants including herbs, woody plants, grasses, and succulents. In addition to meeting energy and nutritional needs, they use vegetation for predator avoidance, thermal protection, and in social behaviors. In addition to herbivory, Sonoran Desert Tortoises are geophagous. This means they consume bones, stones, and soil for additional nutritional and mineral supplements, mechanical assistance in grinding plant matter in the stomach, or to expel parasites in the intestinal tract. They are highly attracted to sites with exposed calcium carbonate and have even been observed congregating in these sites year after year to consume the soil. They are preyed upon by Coyotes, Foxes, Gila Monsters, Badgers, and Roadrunners. However, these predators are not likely to eat an adult Tortoise, as they are heavy and difficult to get to when they go inside of their shell. Sonoran Desert Tortoises have a home range that varies with precipitation levels, contracting during wet years and expanding during dry years in response to food availability. They often use a group of relatively closely located shelters as focal areas in their home ranges, exploiting areas where resources are most plentiful.

The Sonoran Desert Tortoise was once classified along with Mojave Desert Tortoise under the species name Gopherus agassizii. It was not until 2011 that these species were formally split in two, with the Sonoran Desert Tortoise under the name Gopherus morafkfai. These species are primarily distinguished geographically along the Colorado River. Gopherus agassizii is found north and west of the Colorado River, while Gopherus morafkai is found south of the Colorado River.

From a conservation perspective, the Sonoran Desert Tortoise is considered to be Vulnerable based on population reduction and habitat loss of over 30% during three generations, including past reductions and predicted future declines. Despite evidence that several subpopulations have stabilized or increased, survival rates are predicted to decline with future drought conditions, which are expected to intensify. Based on the IUCN criteria, this species nearly qualifies as Endangered. The Sonoran Desert Tortoise is threatened by increasing drought, conversion of habitat to fire-prone grassland, invasive species, road mortality, and widening urban and suburban effects of habitat loss and degradation. There is some local consumption in Mexico, some poaching of animals from subpopulations in Arizona, and occasional illegal marketing of captive individuals on the internet. The Sonoran Desert Tortoise is frequently kept as pets within its range, and individuals may be illegally obtained from the wild or through unregulated backyard breeding. Thousands of individuals have been legally adopted through the Arizona Game and Fish Department’s Tortoise adoption program, and the number of households in Arizona with captive tortoises could potentially be in the tens of thousands.