Our Desert Tortoise

Turtles, tortoises, terrapins are members of the order “Testudines” – what’s the difference? It seems to be a matter of semantics that varies throughout the world. In the United States, Testudines that live in freshwater, oceans, and on land are called turtles. Those that inhabit brackish waters of marshes and river inlets along the coast are called terrapins. The Testudines that are wholly terrestrial are called tortoises.

The Desert Tortoise (Gopherus agassizii aka Xerobates agassizii) occurs from tropical areas in northern Sinaloa, through Sonora, Mexico, in Arizona throughout the Sonoran Desert, and in the Mohave Desert in southeastern California, and southwestern Utah and southern Nevada.

Desert tortoise

 

The Arizona-Sonora Desert Museum (ASDM) describes the Desert Tortoise as follows:

“Tortoises differ from other turtles in having cylindrical and elephantine hind legs and short, broad, club-shaped feet. The genus Gopherus also has flattened forelimbs for digging. The adult desert tortoise can measure up to 14 inches (35.5 cm) in length; the hatchlings are only about 2 to 2½ inches (5 to 6.5 cm) long. The carapace [shell on top] is brown to gray and rounded. The yellowish plastron [shell on bottom] is not hinged and is connected to the carapace at the sides. The male’s plastron is concave at the posterior to accommodate the rounded carapace of the female during copulation. The female’s plastron is flat. Like all turtles it is toothless; the large tongue helps push food back in the mouth. There are no visible ears.”

Turtles are the most ancient of reptiles. They first appeared about 200 million years ago and have changed little. The Desert Tortoise lineage began about 50 million years ago in the Tertiary tropical forests that existed here before advent of the desert.

Desert tortoises in Nevada and Utah generally live in valleys. They dig extensive burrows and live in groups. In Arizona, by contrast, desert tortoises occupy rocky hillsides and generally are loners.

According to ASDM:

“Desert tortoises are well-adapted to withstand the extended dry periods typical of deserts. Although they extract much of their water from the plants they eat, tortoises drink prodigiously from temporary rain pools. They have large urinary bladders that can store over 40 percent of their weight in water and urinary wastes. Urea is precipitated as solid uric acid in the bladder, freeing additional water and useful ions. During periods of inactivity in winter (hibernation) or summer (estivation), metabolic rates, digestion, and water loss from defecation and urination are greatly reduced. As soon as fresh water is available, the solid urates are eliminated from the bladder. Well-hydrated tortoises are able to eat dried plants and store fats in the body. Dehydrated tortoises are physiologically stressed and cannot digest dry plant foods.”

“Tortoises are generally active in times when water stress is reduced (early morning or evening); they can be diurnal or crepuscular depending on temperature and season. Mohave tortoises are mostly active in the spring months from February to May. Sonoran tortoises are primarily active in the summer monsoon months from July to October. Estivation [slowing or ceasing activity] during the hottest, driest parts of the summer helps conserve water because burrows or rock shelters are relatively cool and relative humidity of up to 40 percent reduces evaporation. Without extensive burrows it is unlikely that tortoises could survive the dry Mohave Desert summers. The common defensive behavior of emptying the bladder when molested or handled can have serious consequences in drier periods.”

Desert tortoises are herbivores, but they will eat carrion and insects. They can digest cellulose.

ASDM notes:

“The social behavior of tortoises is relatively straightforward — males fight all adult males and court all adult females they encounter. Mating occurs throughout the summer, although females lay eggs in late June or early July in the Sonoran Desert. Mohave Desert tortoises typically lay a second clutch of eggs at the end of the summer. Retention of viable sperm in the cloaca of the female for at least two years is an excellent survival strategy in non-colonial animals: it ensures fertilization during extended droughts when males are less active or populations crash. The eggs contain all of the water and nutrients necessary for complete development of the hatchlings. Hard egg shells retard desiccation. The normal incubation period of about 90 days in the summer can be much longer later in the year as temperatures fall.”

Desert tortoises can live up to 35 to 40 years. In Arizona, they are protected. They cannot be collected, killed, transported, bought, sold, bartered, imported, or exported from Arizona without authorization by the Arizona Game and Fish Department. ASDM has a tortoise adoption program where you can acquire or drop off a tortoise. Captive tortoises should not be released into the wild because they may carry diseases and introduction may upset the local order of tortoises.

To find out more about ASDM’s tortoise adoption programs go to http://www.desertmuseum.org/programs/tap.php
That site also has more information on tortoise natural history.

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