The desert is a harsh environment; the intense heat, potentially frigid nights, and scarcity of water make for a place seemingly fit only for rocks and cacti. But animal life does survive in the desert, as evidenced by the Desert Tortoise (Gopherus agassizii). Unlike turtles, tortoises water-loving cousins, the Desert Tortoise has various unique water conserving adaptations to cope with the arid climate.
A medium sized tortoise with an average life span of 80–100 years, the Desert Tortoise is commonly considered a gopher tortoise, one of four species of the genus Gopherus. Desert tortoises are in the taxonomic classification of Class: Reptilia; Order: Testudines; Suborder: Cryptodira; Family: Testudinidae1. Its relatives include the famous Galápagos tortoise (Geochelone nigra) that Charles Darwin studied on the Galápagos Islands.
Turtles and tortoises first appeared during the Triassic period, ~250–210 million years ago (mya). Recent mitochondrial DNA research placed the origins of Testudines to 207 (± 20.5) mya. This number agrees with the existing fossil evidence dating their beginnings to ~223–210 mya2. Evolutionary biologists disagree whether Testudines evolved separately from a now extinct anapsid ancestor, or if they are very distantly related to lizards and snakes through descent from a diapsid ancestor3.
Desert Tortoises are identified by having rather plain amber-greenish shells and flattened front limbs adapted for burrowing4. Weighing just 4–7 kg (8–15 lbs), they measure 10–15 cm (4–6 inches) high with a carapace length of 15–38 cm (6–15 inches)5. In fact, the Desert tortoise somewhat resembles a pint-sized version of the Galápagos tortoise, particularly in the shell appearance and coloration6.
These animals are found only in western North America, and their range includes the Mojave and Sonoran deserts of southeastern California, and areas in southern Nevada, south through Arizona into Mexico7. Though currently not classified as distinct subspecies, differences between tortoises of the Mojave and Sonoran regions are apparent in their shell shape and seasonal behavior. According to biologists writing for the Desert USA website:
The Sonoran Desert Tortoise is flat and pear-shaped compared to the Western Mojave tortoise, which is more butterball shape; they are usually active in spring. The Sonoran Desert Tortoise is more active in summer and seeks shade under large rocks and boulders8.
In addition to these two main populations, scientists are currently studying what may be a small Sonoran sub-population of tortoises found only in flat regions of the Black Mountains of northwestern Arizona9.
Desert Tortoises make do with their surrounding habitat, and can be found in thorn scrub, cactus deserts10, rocky hillside slopes, oak woodlands, and bunch grass stands11. The Desert Tortoise Preserve Committee states that the most important considerations for tortoises habitat are:
...suitable soils and terrain for constructing a burrow and... adequate annual and perennial plants in the spring and/or summer for forage12.
These two items are key to enabling the tortoises survival in areas where ground temperature may exceed 60°C (140°F)13. As herbivores with few defenses against predators, Desert Tortoises require burrows for shelter and fresh vegetation (grasses, cacti, succulents, and flowers14) from which to glean water and nutrients.
These tortoises spend most of their life in a burrow, protected from the climatic extremes and from predators15. They construct a number of burrows of varying depths: shallower burrows closer to the surface for the hot months, deeper burrows for warmth during hibernation during the cold months16. Tortoises may construct burrows sized just to fit, or measuring as much as 10 m (33 feet) long17. And burrows are communal property; different tortoises may use neighboring burrows at different times18.
Tortoises have evolved certain physiological adaptations useful to desert survival. Their flattened forelimbs have well-developed muscles for excavating soil19. (However, females still use their hind limbs to dig their nests20, perhaps as a defense method against possible predator attack during egg laying.) Their bodies are extremely efficient at conserving water; so efficient, in fact, that adult tortoises may survive a year or more without access to water21. They can use their bladder as a storage unit, reabsorbing the water as needed during dry times. The bladder water color indicates how long it has been stored, becoming darker and more concentrated over time22. Scientists at Desert USA state:
... during very dry times, tortoises may excrete waste as a white paste rather than watery urine23.
When the tortoises sense imminent rainfall, they dig depressions in the soil to catch water24.
Not surprising for an animal that spends most of its life underground, the tortoises maturity rate and reproductive cycle is somewhat of a mystery, and in disagreement among scientists. Some sources state the Desert Tortoises reach maturity after 15–20 years, mate mainly from May through July, and that the female lays one to three clutches of two to 15 eggs (usually five to six) each year25. Other sources state that tortoises achieve sexual maturity in seven to eight years, mate from the spring through the fall, and have only one to two clutches per year of four to eight eggs26.
Though Desert Tortoises can be considered K-strategists, given their slower growth rate and long life span, their reproductive habits follow r-strategist methods, as they produce many young and do not provide parental care. Once the female has laid the last egg and covered the nest with soil for warmth, the young tortoises are entirely on their own. Desert Tortoise eggs are brittle shelled, elliptical to nearly spherical, and average 40–45 mm (1.6–1.8 inches) by 34–38 mm (1.3–1.5 inches); they require an incubation period of 90–120 days27. A fascinating fact is the effect of incubation temperature on the tortoises gender. According to the Desert USA scientists, experiments show that:
...cooler temperatures [of] 26–30°C (79–87°F) produce all males, [while warmer temperatures] at 31–33°C (88–91°F) [produce] all females28.
The baby hatchlings use an egg tooth to break free and emerge from the nest. Hatchling survival rates are grim, however; one to five of every 100 hatchlings survive to adulthood29.
The required isolation of burrowing for Desert Tortoises seems to make them rather social with each other, unless two males get annoyed with one another. John Iverson states that the tortoises are:
...surprisingly social and head-bob at one another when they meet. Male-to-male encounters begin with head bobbing... escalate into ramming contest using the gular projections on the anterior plastron. This combat is occasionally intense enough to result in the overturning of one of the males30.
Yet even when angry, the turtles exhibit a practical nature; fighting decreases as the desert heat increases, survival taking precedence over asserting dominance31.
Though Desert Tortoises have adapted to survive harsh desert conditions, they have few defenses against animal predators such as ravens, Gila monsters, kit foxes, badgers, roadrunners, and coyotes. Their main defense mechanisms are retreat (into the shell, into the burrow, or into the shell while in the burrow), ramming with the gular projection, or releasing their bladder contents32. The latter defense mechanism, however, can unfortunately be deadly for the tortoise during drought conditions.
Tortoise populations have also been decimated by mycoplasmic bacterium that causes upper respiratory tract diseases33. However, humans and human activities provide the biggest threats to this animals continued existence. Besides the loss and pollution of their native habitat to land development, tortoise populations have suffered due to illegal poaching activities. Poachers hunt the tortoises to sell them as pets and for food in certain ethnic markets34.
Desert Tortoise populations have declined by 90% since the 1980s, and they are listed as vulnerable on the International Union for the Conservation of Nature and Natural Resources Red List35. Given that tortoises have been on Earth for millions of years, humans should do everything possible to ensure their continued existence.