COMMONLY ASKED QUESTIONS ABOUT THE DESERT TORTOISE AND ANSWERS
by Kristin H. Berry
Tortoise Tracks, Vol.11, No. 1, 1990.
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, musk turtles, sea turtles, etc.
2. How many different kinds of tortoise occur in North America?
Three species of tortoises occur in the United States and a fourth is found in Mexico. The desert tortoise (Xerobates [Gopherus] agassizii) is found in the Mojave and Colorado/Sonoran deserts of California, southern Nevada, Arizona, southwestern Utah, and in Mexico. The Texas tortoise (Xerobates [Gopherus] 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 (Gopheruspolyphemus), 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. We can make two generalizations about the habitats, however. Tortoises living north and west of the Colorado River-Grand Canyon complex (California, southern Nevada, southwestern Utah, and extreme northern Arizona) occur in valleys, flat areas, fans, bajadas and washes. These tortoises live in the Mojave and Colorado Deserts and are generally found below the 4,000 foot elevation in tree-yucca (Joshua tree and Mohave 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. The tortoise 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 living in the Sonoran Desert of Arizona occupy entirely different habitats. They are found on the steep, rocky slopes of hillsides. The slopes may be covered 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 stands of heavy bunch grass.
4. When can one see tortoises in the California deserts? When are they active?
In general, tortoises hibernate from October through February and are underground in burrows during that time. On a warm sunny day, an occasional animal may be found near the mouth of a burrow in late fall or winter.
The prime activity period in the western Mojave Desert is the late winter and spring, from March through May. After this time, when daytime air and soil temperatures are elevated and over 90° F and the food supplies of annual plants 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 August or September, or when summer thundershowers trigger a brief flurry of above-ground activity. With summer rains, the tortoises will emerge from burrows to drink and travel.
Thus, if one wants to find tortoises in the western Mojave, one should look in prime habitats in spring. 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. In summer, the best time to see them is during or after a thunderstorm.
In the eastern Mojave and Colorado Deserts, tortoises may also have a summer activity period, which is associated with summer rains and a summer food supply.
5. What is hibernation and what do tortoises do when they hibernate?
Hibernation for a tortoise is a period of inactivity, generally below ground in a burrow or den. The body temperature is lowered and is close to that of the air temperature in the burrow, about 40° F to 60° F. 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 food items. In general, tortoises in California feed on annual wildflowers, such as blazing stars, lupines, loco weeds, lotus, Indian wheat, forget-me-nots, desert dandelions, gilias, phacelias, coreopsis, alkali goldfields and many other species. They also eat annual and perennial grasses and fresh pads and buds of some species of cactus.
After the annual wildflowers have dried in spring, tortoises will eat the dried plants throughout late spring and into summer and fall. Consumption of dried plant material is somewhat dependent on the tortoise’s state of hydration, and whether the tortoise has recently drunk 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 the bladder, where it can be reabsorbed. During spring, summer, and fall rains, tortoises will drink and “freshen” the water stored in the bladder. Bladder water varies in color from clear and colorless to dark brown. Fresh water is clear and colorless; water that has been stored for some time is dark and concentrated.
8. When do females lay eggs and where?
In the wild, females usually lay one or more clutches of 1 to 14 eggs between mid-April and the first week of July. The size of the clutch depends on the size of the female, with small females producing smaller clutches than the larger females.
Females dig the nests with the hind legs and drop the eggs into the nest, placing them with the hind legs and covering them carefully. The location of an undisturbed nest cannot 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 is dependent on the location of the nest and how much warmth it receives, among other factors. Some clutches may overwinter and hatch in spring.
10. How large is the largest known desert tortoise? How are tortoises measured?
The largest known desert tortoise is a captive tortoise about 15 inches in length and is the property of the California Department of Fish and Game. It has been a pet for many years and was turned into the Fish and Game office in Long Beach in the 1970’s.
The largest known wild desert tortoise on the Desert Tortoise Natural Area was about 14.5 inches. The tortoise was a male and was frequently observed on the western portion of the Natural Area in the 1970’s.
Tortoises are measured with calipers. 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.
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, soil type, and vegetation in which they are found. For example, burrows in Utah are of two basic types: deep winter dens in calotte 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 1 0 feet in length for summer estivation and winter hibernation. They may use shallower burrows or pallets that just barely cover the shell in spring, summer, and fall also. 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 then disintegrates.
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. The burrow is usually the size and shape of the tortoise–half moon in shape 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) a cool place for the tortoise during the dry hot days in late spring and summer when water and food are unavailable and (b) a relatively “warm” site for winter hibernation. The tortoise spends most of its life in ‘the burrow.
Burrows serve as protection from predators, such as common ravens, coyotes, kit foxes, golden eagles, and greater roadrunners.
13. Do tortoises migrate?
Migration refers to movement to a particular place for a particular purpose, such as feeding or breeding, and then migrating back to the former site.
I do not think that migration is 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. Large tortoises have large home ranges and small tortoises have small home ranges. Females are more sedentary than males and do not move about as much, 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 also are very familiar with local landmarks. They can travel to find burrows in a straight line. They also know locations of other tortoises (e.g., males know the location of females), drinking sites, mineral licks, 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. The subject is complex and deserves much more study.
15. How does one distinguish a male from a female tortoise?
Sex is difficult to determine until the tortoise is about 7 inches in carapace length. Male tortoises develop chin glands or knobs on the chin, a longer gular horn, have a longer tail, and have a concave plastron (a dish-shaped depression on the posterior, underside of the shell).
Females have longer toenails (maybe for digging nests), a small gular horn, and no obvious chin glands.
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 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.