Information about Desert Tortoises
Prepared by Kristin H. Berry
for the Desert Tortoise Preserve Committee and used with permission
1. What is the difference between a turtle and a tortoise?
In the United States the following distinction is made between the terms turtle and tortoise:
♦ A tortoise is a land dwelling turtle with high domed shell and columnar, elephant-shaped hind legs. Tortoises go to water only to drink or bathe.
♦ In contrast, the word "turtle" is used for other turtles: pond turtles, river turtles, box turtles, and sea turtles.
This close-up view of the desert tortoise's face and beak reveals several
indicators of health. The tortoise has eaten fresh succulent green vegetation recently,
its nares (nostrils) are clear and free of discharge, and the eyes are clear with no
drainage. The lower eyelids show mild signs of swelling or edema, a potential sign of
upper respiratory tract disease.
Photo by Mark Massar.
2. How many different kinds of tortoises occur in North America?
Three species of tortoises occur in the United States and a fourth is found in Mexico. The desert tortoise (Gopherus [Xerobates] agassizii) is found in the Mojave and
Colorado/Sonoran deserts of California, southern Nevada, Arizona, southwestern Utah, and in northern Mexico. The Texas tortoise (Gopherus [Xerobates] berlandieri) occurs in southern Texas and northeastern Mexico. Some can be found in California, where they were sold in the past for pets. The third U.S. species is the gopher or Florida tortoise (Gopherus polyphemus), which lives in southwestern South Carolina, Florida, Georgia, Alabama, Mississippi, Louisiana, and extreme southeastern Texas. The fourth species is the bolson tortoise (Gopherus flavomarginatus), which is found in a very small area in Chihuahua and Durango, Mexico.
3. What is the habitat of the desert tortoise in the southwestern U.S.?
Tortoises occupy a wide variety of habitats in the United States. Two generalizations can be made. Tortoises living north and west of the Colorado River-Grand Canyon complex (California, southern Nevada, southwestern Utah, and extreme northern Arizona) generally occur in valleys, flat areas, fans, bajadas and washes. These tortoises live in the Mojave and Colorado deserts and are generally found below 4,000 feet in tree yucca (Joshua tree and Mojave yucca) communities, creosote bush and saltbush scrub habitats, and in some ocotillo-creosote habitats. They occupy a wide variety of soil types, ranging from sand dunes to rocky hillsides, and from caliche caves in washes to sandy soils and desert pavements. Tortoises must have suitable soils and terrain for constructing a burrow and must have adequate annual and perennial plants in the spring and/or summer for forage. Tortoises are usually absent from or rare in the low, very hot, and dry areas, such as the Death Valley sink.
Tortoises living in the Sonoran Desert of Arizona occupy entirely different habitats. They are primarily found on the steep, rocky slopes of hillsides. The slopes may be littered with granitic or volcanic boulders and are often covered with dense vegetation. The palo
verde-saguaro cactus is the most frequently occupied habitat, although some tortoises are found in oak woodlands and dense stands of bunch grass.
4. When can one see tortoises in the deserts? When are they active?
In general, tortoises hibernate from October through February and are underground in burrows during that time. On warm, sunny days an occasional animal may be found near the mouth of its burrow in late fall or winter.
In the Mojave and Colorado deserts, the prime activity period is late winter and spring, from mid to late March through May. In early spring, tortoises are out from mid-morning to mid-afternoon, during the warm part of the day. As air temperatures rise, tortoises emerge from burrows earlier and retreat earlier. By May, tortoises may be out by 6:00 a.m. and back in burrows by 9:00 a.m. In late spring, tortoises may also be active in late afternoon.
After May, when daytime air and soil temperatures are over 90 degrees Fahrenheit and the food supplies of herbaceous plants and grasses have dried, a large percentage of tortoises become inactive and remain underground in burrows. Some will emerge a few times a week or once every two or three weeks, especially in early morning or late afternoon. Others will not come out of burrows until summer thundershowers trigger a brief flurry of above-ground activity. With summer rains, the tortoises will emerge from burrows to drink, travel, or eat. Adults engage in courtship behaviors during summer in the early morning, late afternoon, and evening.
In the Sonoran Desert, tortoises may be more active in summer and early fall than in late winter and spring. The tortoises respond to summer rainstorms, emerging from their burrows and eating the plants that germinate and grow in response to the summer rains.
5. What is hibernation and what do tortoises do when they hibernate?
For a tortoise, hibernation is a period of inactivity, generally below ground in a burrow or den. The body temperature is lowered close to that of the air and surface temperatures in the burrow, about 40 to 60 degrees Fahrenheit. The heart rate, respiration rate, and all bodily processes are slowed.
6. What do tortoises eat in the wild?
Tortoises are selective in choice of foods. Food preferences depend on locality and availability of the food items. In general, tortoises feed on herbaceous perennial (perennial plants live for more than one year) and annual wildflowers, such as wishbone bushes, lotus, loco weeds, spurges, blazing stars, lupines, Indian wheat, forget-me-nots, desert dandelions,
gilias, phacelias, coreopsis, and many other species. They also eat annual and perennial grasses and fresh pads and buds of some species of cactus. They do not eat shrubs such as creosote bush and burro bush.
Consumption of dried plants is dependent on the tortoise's state of hydration. Tortoises will eat dried annual plants and grasses throughout late spring and into summer and fall, if, for example, they have recently consumed rain water.
7. Do tortoises drink water in the wild?
Yes. Tortoises drink free water where it collects in pools near rocks or in depressions. Tortoises will dig depressions to collect the water, and such depressions can often be seen on areas of desert pavement.
Tortoises can store water in their bladders, where it can be reabsorbed. During spring, summer, and fall rains, tortoises will drink and "freshen" the water stored in their bladders. Bladder water varies from colorless to dark brown. Clear and colorless bladder fluid is generally fresh, whereas the dark fluid is likely to have been stored for some time and is concentrated with waste products.
8. When do females lay eggs and where?
In the wild, females usually lay one or more clutches of 1 to 12 eggs between mid-April and mid-July. The size of the clutch depends on the size of the female, with small females producing smaller clutches than the larger females. Clutch size and numbers of clutches produced also can depend on the food supply-how much food the tortoise has been able to obtain during the year or two prior to the time the clutch is deposited. Females dig the nests with the hind legs and drop the eggs into the nest, placing them with their hind legs and covering them carefully. The location of an undisturbed nest cannot easily be detected by humans.
Nests are most often associated with the female's burrows. The nest may be in the burrow mound, the mouth of the burrow, or deep inside the tunnel.
9. How much time is required for eggs to hatch?
The eggs, which are the shape and size of ping pong balls, may hatch in 70 to 120 days. The timing depends on the location of the nest and how much warmth it receives, among other factors. Some clutches may over-winter underground and hatch in the spring.
10. How large is the largest known desert tortoise? How are tortoises measured?
The largest known captive desert tortoise in California was a male about 15 inches in length. Known as "Max", the tortoise became the property of the California Department of Fish and Wildlife in the 1970's. Max finally died and has been freeze-dried for exhibits. An even larger captive tortoise is a 17.3-inch male, who lives with his proud owners in Las Vegas.
Two wild desert tortoises vie for record sizes: a 14.5-inch male on the Desert Tortoise Research Natural Area, and a 15-inch female in Lucerne Valley.
Tortoises are measured with calipers, instruments consisting of a pair of movable curved legs fastened together at one end. One end of the caliper is placed at the edge of the carapace (upper shell) immediately above the head and the other end is placed on the carapace edge above the tail. The straight line distance is considered the length of the shell or carapace at the midline of the body.
Tortoises Are Selective Feeders
Just because a plant
grows in the desert does not mean that tortoises will eat it. Tortoise are selective in their choice
of food plants. Availability of preferred food plants also varies from region to region. The
pictures below illustrate a few food plants.
This spring-time photo shows a tortoise with one favored food item, Indian
wheat or Plantago, the long-stemmed annual plant with dry-appearing tops. The reddish-colored
grass in the front lower left is red brome, a noxious alien grass that has contributed
to desert fires.
Photo by Tim Duck
Annual plants in the legume (bean or pea) family are often favored as forage by the tortoise.
The lotus, a spring annual, was the most common species consumed during a good wildflower year
in the central Mojave Desert.
Photograph by Andrea Demmon
Some species of spring blooming locoweeds (genus Astragalus) are favored forage for the
tortoise. This species is only a few inches tall, and is commonly eaten by tortoises in the western Mojave desert whenever
winter rains are sufficient to cause germination.
11. What are tortoise burrows like?
Tortoise burrows vary considerably in length and type. The style of burrow appears to be dependent upon the region, geologic formations, soil type, and vegetation in which it is found. For example, burrows in the eastern Mojave Desert of Utah are of two basic types on the Beaver Dam Slope: deep winter dens in natural caves in washes, some of which are 30 feet in length; and shallower summer burrows three to six feet in length in the flat areas. In the western Mojave Desert, tortoises have a variety of burrows. They use burrows about 2.5 to 10 feet in length for summer estivation and winter hibernation. They may also use shallower burrows or pallets that just barely cover the shell in spring, summer, and fall. These temporary burrows or pallets can be fragile and may be used for shelter for a few days while a tortoise is foraging in a particular area. A temporary burrow usually lasts from a few weeks to a season and disintegrates.
In the Sonoran Desert, tortoises frequently use rocks and boulders as sites for burrows or shelter. They may excavate a space beneath a boulder or squeeze into rock crevices.
Each tortoise usually has more than one burrow. The number of burrows the tortoise uses may depend on age and sex, as well as on the season. When burrows are constructed in soil, they are the size and shape of the tortoise -- half moon for the roof and flat on the bottom. Small tortoises have small burrows and large tortoises have large burrows.
12. Why are tortoise burrows important?
The tortoise burrow provides protection from the extremes of heat, cold, lack of moisture, and too much moisture. The burrow is especially important because it provides a cool place for the tortoise during the dry hot days in late spring and summer when water and food are unavailable and a relatively "warm" site for winter hibernation. The tortoise spends most of its life in the burrow.
Burrows also serve as protection from predators, such as common ravens, bobcats, coyotes, kit foxes, golden eagles, and roadrunners.
13. Do tortoises migrate?
Migration refers to movement to a particular place for a particular purpose, such as feeding or breeding, and then return to the former site. Migration is not an appropriate term to use for tortoise movements. Each tortoise has a home range or activity area. A home range is the area in which a tortoise travels, feeds, sleeps, courts, and has its burrows. This is the area with which the tortoise is familiar. In general, large tortoises have large home ranges and small tortoises have small home ranges. Females are more sedentary than males, so they probably have smaller home ranges. Large males are known to occupy home ranges over 0.75 square mile.
Tortoises appear to have a good sense of compass direction. They are also very familiar with local landmarks. They can travel to find their burrows in a straight line. They also know locations of other tortoises (e.g., males know the location of females), drinking depressions, mineral licks (sites with deposits of calcium, sodium, magnesium and other salts), and particular food sources.
Some people, upon seeing tortoises cross roads in spring, think that tortoises are "migrating". Actually the tortoises are merely living in close proximity to the highways and roads and will travel across them during the course of moving about the home range.
14. When do tortoises court and mate?
Male tortoises generally court female tortoises whenever the opportunity presents itself, e.g., in spring, summer, or fall. There does not appear to be a well defined "mating season".
Male tortoises may court and mount the females, but not actually copulate. Don't assume that mating is occurring because you see a male mounted on a female.
However, the reproductive organs of male tortoises are active (have motile sperm) in late summer and fall. At that time, courting is more frequent than in spring.
15. How does one distinguish a male from a female tortoise?
Sex is difficult to determine until the tortoise is about seven inches in carapace length. The shape of the shell differs between male and female adults. The shell is composed of a domed roof or back called the carapace, and a flattened under portion called the plastron. The portion of the plastron immediately beneath the extended head and neck is called the gular horn. Adult male tortoises have a longer and up-curved gular horn, a concave plastron (a dish-shaped depression on the
underside of the shell near the tail), a longer tail, and chin glands or knobs on the chin. Females may have longer toenails for digging nests, a small gular horn, a flat plastron, and no obvious chin glands.
The differences in shell shape become more pronounced with age.
16. What predators eat tortoises?
The type of predator varies depending on the age and size of the tortoise. There are egg predators such as the gila monster, kit fox, coyote, and badger. Predators of juveniles include ravens, roadrunners, some snakes, kit foxes, bobcats, badgers, coyotes, and probably the spotted skunk.
The larger the tortoise the more likely it will be able to resist predation. Large tortoises may be eaten by kit foxes, badgers, bobcats, coyotes, and golden eagles. The large mammalian predators are not likely to eat adult tortoises unless other food sources, such as rabbits and rodents, are in short supply. Coyotes and kit foxes may dig tortoises out of their burrows to eat. These predators can eat the tortoise without breaking open the shell.