Sonoran Desert Tortoise by John R. Leeper
On a hike within the Rincon Mountain District’s Saguaro National Park, one of the longest-lived yet most newly named residents crossed my trail (Photo 1.). It was a mature Sonoran Desert tortoise.
The tortoise was considered to be Gopherus agassizii (Agassiz’s desert tortoise) prior to it being renamed Gopherus morafkai (Sonoran Desert tortoise or Morafka’s desert tortoise) in 2011. The renaming followed extensive studies that separated the latter from the former based on DNA, geography and behavior. In making the species separation, a tortoise from Tucson became the type species used in describing and differentiating G. morafkai from G. agassizii.
G. agassizii has gone through a litany of scientific name changes since first being described in 1863. The name Gopherus refers to the ability of species in the genus to excavate burrows which have been reported to be as long as 40 feet. The specific name agassizii was given in honor of a Swiss-American zoologist, Jean Louis Rodolphe Agassiz. The specific name morafkai followed similar suit and was named in honor of Professor David Joseph Morafka for his studies and conservation efforts for tortoises in the desert southwest.
G. agassizii and G. morafkai are geographically separated by the Colorado River. With G. agassizii on west side and G. morafkai are found on the east in the Sonoran Desert of Arizona and the Mexican states of Sonora and Sinaloa.
Although the Department of the Interior, Fish and Wildlife Service presented a finding in the Federal Register in 2015 that G. morafkai was neither threatened nor endangered, the Arizona Game and Fish Department classify it as a Tier 1b “Species of Greatest Conservation Need.” Under Arizona State law, it is unlawful to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, collect, or release captives of G. morafkai into the wild.
The reasons for Arizona’s stringent regulations meant to protect the Sonoran Desert tortoise go beyond protecting it from purposeful harm by humans. A number of diseases have been documented effecting Gopherus population declines and human handling and release of pet tortoises into the wild have been contributing factors. Additionally, tortoises will relieve themselves as a fright response when even handled casually. This puts the tortoise under greater risk of death due to excessive water and electrolyte loss.
Incursion into tortoise habitat by human land development and associated infrastructure has also been identified as causal factors affecting tortoise populations. Even wind and solar farms are recognized as posing significant threats to the habitat and survival of desert tortoises. For their own safety and preservation, leave a tortoise be. That, in fact, is a good rule to follow with all wild creatures who share their desert with us.
The Sonoran Desert tortoise can live up to 80 years and grow to having a shell 15 inches in length. The size of the specimen seen in the Park was close to that size and must have been nearly that old (Photo 2) and appeared to be in excellent health. Males tend to be slightly larger than females and have a longer gular horn (the forward extension of the lower shell below the head and neck). The carapace, or upper shell, of G. morafkai is high domed with distinct growth rings and can range in color from tan and various shades of brown to dark gray and nearly black. The lower shell, or plastron, is tan to yellow in color in both sexes. The plastron on males is concave and flat on females. The forelimbs of the Sonoran Desert tortoise are paddle-like with strong claws and sharp, flat scales (Photo 3), both used in digging their burrows. The forelegs are larger than the short, stocky hind legs. Many of the anatomical characteristics described above would require handling to see and were therefore not observed in the specimen photographed.
The Sonoran Desert tortoise occupies a diverse range of habitats from sea level to about 5,000 feet and appears to do particularly well in rugged rocky uplands associated with hillsides, mountain slopes, and canyons.
It has been estimated that desert tortoises spend at least 95% of their lives in their burrows or in rock shelters or depressions in the ground to escape both the summer and winter extremes in temperature. Burrows also have higher humidity which helps prevent dehydration. The environmental conditions within burrows dug by desert tortoises are also favorable to many other desert animals who take up habitation within them.
The Sonoran Desert tortoise has low fecundity, taking between 15 and 20 years before reaching sexual maturity. And, the survival rate for hatchling to adulthood is estimated to only be between 2 and 5%.
Mating occurs during the early monsoon season, typically between June and early August during which mature males develop two large white glands around their chins. A female G. morafkai can retain viable sperm from mating one cycle to fertilize eggs the following season. She will lay a single clutch of between 1 and 12 eggs, buried within a burrow. It is estimated a female can lay up to 85 eggs in her lifetime.
Hatchlings emerge at the end of the monsoon season. It takes up to five years for their shells to fully harden and it is in this period they are most vulnerable to predation. While Gila monsters are thought to be the primary predator of young, soft shelled Sonoran Desert tortoises other reptiles, arthropods, birds and mammals also contribute to their low survival rate.
Sonoran Desert tortoises hibernate in there burrow during the winter months but may emerge on warm winter afternoons. Male begin hibernation sooner and emerge later than females. Juvenile tortoises emerge before adults. They are most active foraging for food at temperatures between 79 and 93 F° and following rain.
The Sonoran Desert tortoise is a herbivore with grasses composing the bulk of their diet. It is thought that displacement of native grasses with introduced species has placed a strain on tortoise populations due to differences in nutritional value. G. morafkai also consume herbs, tree shoots and roots, wildflowers, and cactus pads, flowers and fruit. They are also known to consume rocks and soil. It is believed that this practice provides the animal with calcium and other minerals, supplements intestinal bacteria, and act as grind stones in the stomach; aiding in the digestion of their forage.
The Sonoran Desert tortoise is another of our unique desert neighbors we should appreciate from a distance without disruption. For those interested in more information on the Sonoran Desert tortoise, conduct a web search on Gopherus morafkai.