The oblong carapace (to 38 cm) is rather flat topped, highest behind the center, has the marginals above the hindlimbs flared, and the rear margin serrated. Its cervical scute is about as broad as long. Vertebrals are broader than long; the 1st is narrowest, and the 5th broadest and laterally expanded. Raised growth annuli surround the slightly raised vertebral and pleural areolae. Eleven marginals lie on each side, and the single supracaudal scute is undivided and downturned. The carapace is black to tan, and often the areolae are yellow or orange. The plastron is large and well-developed; the paired elongated gulars are forked, upturned, and project anteriorly past the carapacial rim. Its forelobe is longer but slightly narrower than the hindlobe, which bears a deep posterior notch. The plastral formula is: abd > hum > fem >< gul > an >< pect. Only a single axillary scute occurs on the broad bridge. The plastral scutes are black to tan; some may have yellow centers. The head is somewhat rounded with a nonprojecting snout and a slightly hooked upper jaw. The angle between the upper triturating ridges is less than 65°. Dorsal head scales are small and irregularly shaped. Well-developed integumentary glands lie beneath the chin; these become prominent in males during the mating season. The head usually is tan but may be reddish brown; the iris is greenish yellow. Skin of the limbs is brown, that of the sockets and neck yellow. The anterior foreleg surfaces are covered with large, slightly overlapping scales in eight or more rows. The thighs contain numerous conical scales.
Diploid chromosomes number 52 (26 macrochromosomes and 26 microchromosomes): 20 metacentric and submetacentric, 10 subtelocentric, and 22 acrocentric and telocentric (Stock, 1972; Dowler and Bickham, 1982).
Males are larger than females and have longer, thicker tails, longer gular projections, concave plastra, better developed chin glands, and more massive claws.
In the United States, Gopherus agassizii ranges through southern Nevada, extreme southwestern Utah, southwestern California, and western Arizona. In Mexico it is found in western Sonora (including Tiburón Island in the Gulf of California), and northwestern Sinaloa; it is also found in the cape region of Baja California Sur, where it may have been introduced.
No subspecies have been designated, but populations vary in allozymes, plasma protein markers, and mitochondrial DNA (Rainboth et al., 1989; Lamb et al., 1989; Glenn et al., 1990; Britten et al., 1997).
A desert tortoise from Baja California was described as a different taxon, Xerobates lepidocephalus by Ottley and Velázques Solis (1989), but Crumly and Grismer (1994) have shown the adult female holotype is a Gopherus agassizii.
Gopherus agassizii inhabits desert alluvial fans, washes, canyon bottoms, and rocky hillsides in drylands having sandy or gravely soil; it reaches an altitude of at least 1070 m. The particular habitat types used vary geographically, gradually changing to rocky slopes in the eastern part of the range (Schamberger and Turner, 1986; Barrett, 1990). Vegetation present includes creosote bush, cheese bush, black bush, salt bush, hop sage, paloverde, ironwood, smoke tree, grasses and cacti.
Woodbury and Hardy (1948) estimated that sexual maturity in Utah tortoises is reached in 15-20 years, when females are 23.0-26.5 cm in carapace length and males are 25.0-31.6 cm, but in California, Berry (1978b) estimated females first reproduced when 21.5-22.0 cm at ages 15-20, and Turner et al. (1986) reported that females in the Mojave Desert reproduce at 18.9 cm and probably matured between 18-19 cm. Miller (1955) reported that male secondary sexual characters begin to appear at 16 years, are definite at 17 years, and are complete at 20 years.
Rostal et al. (1994) studied the reproductive cycles of both sexes in tortoises from the eastern Mojave Desert. In May, the male seminiferous tubules are completely regressed and contain only spermatogonia and Sertoli cells. By July, however, spermatogenesis has progressed, and spermatocytes and spermatids are abundant and a few spermatozoa present. The diameter of the seminiferous tubules and spermatogenesis are maximal in late September and October. The spermatozoa pass into the epididymides after this when the male is in his winter quarters, by the time he emerges again in April spermatozoa are no longer present in the seminiferous tubules (although large numbers of spermatogonia and spermatocytes are present in the walls). In females, vitellogenesis and ovarian follicular growth begin in July after the nesting season and continue to October. Ovarian follicles reach mature diameter (mean 2.4 cm) prior to hibernation. Shelled oviductal eggs are first present in mid-April (90% of females ovulate by 30 April). Oviposition occurs in May-July.
Courtship and mating begin shortly after spring emergence, possibly as early as late March, and continue through the summer and into the fall to as late as October. Observations by Weaver (1970) and Black (1976) indicate that the male initiates mating behavior. The male approaches the indifferent female who moves away. He trails her (if she remains stationary, this trailing stage is unnecessary) and bobs his head at her, increasing the frequency of head bobs with duration. When she is caught, the male continues high-intensity head bobbing while circling her, usually in a counterclockwise direction. The female may still try to move away, or avoid the male by circling around him, but once she stops, the male starts biting her head and forelimbs, and occasionally her carapace. He continues to circle to keep in front of her, and sometimes rams her gular projections with his while giving an open mouth threat. Finally she withdraws into her shell and remains in place. The male then moves behind, always ready to bite at her head, and mounts. Hissing and grunting sounds are produced, but Weaver (1970) thought these are a product of copulatory effort, not auditory signals. Intromission soon follows.
Most nesting takes place from May through July, but females have been known to lay eggs as late as September and October and to nest more than once a season (Luckenbach, 1982; Murray et al., 1996). Early morning and late afternoon, possibly because of their cooler temperatures, seem to be the favorite nesting times. The eggs normally are deposited in cavities dug in sandy or friable soil but are also laid in the mouths of burrows or deposited singly at random. Funnel-shaped nests are dug with the hind feet to depths of 8-25 cm; a typical nest is 15 cm deep, 23 cm wide at the top, and 18 cm wide at the bottom.
One to three (typically 1-2) clutches of 2-15 eggs (normally 4-6) are laid a season. Murray et al. (1996) reported that no female in the Sonoran Desert population they studied laid more than one clutch a year. First clutches tend to contain more eggs than subsequent ones (Rostal et al., 1994). Clutch size is correlated to female body size; larger females lay more eggs per clutch. The eggs vary from elliptical to almost spherical in shape; elliptical eggs measured by Miller (1932) were 41.6-48.7 mm long, 36.7-39.6 mm in maximum diameter, and 34.9-38.2 mm in least diameter. More spherical eggs may have diameters of 32-40 mm (Murray et al., 1996). Hatching occurs from mid-August to October, with peak emergence in September and early October. Natural incubation periods usually last 90-120 days. Hatchlings are nearly as wide as long and are 36-48 mm in carapace length and 35-43 mm in carapace width.
Hatchlings are pale yellow to brownish, with darker brown areas on each of the scutes. In the center of the plastron is a yolk sac about one-third the size of the young tortoise. The yolk sac is resorbed over a period of about two days.
Daily activity is determined by ambient temperature, and in the summer they avoid the hottest period of the day by remaining within their burrows, the only place with temperatures at ground level below the tortoise's lethal range, emerging only in the early morning and at dusk. In the winter, they hibernate in deeper burrows, to which some tortoises make lengthy migrations from their feeding areas.
The serrated jaws are well-suited for shredding vegetation. The species is mostly herbivorous, subsisting on various grasses, cacti, and the blossoms of desert Compositae (lists of plants eaten are presented by Burge and Bradley, 1976; Hansen et al., 1976; and Luckenbach, 1982). The most important foods seem to be desert annuals, plants that often have a life span of less than 30 days and are generally available only from April to June. Both flowers and vegetative parts are eaten, but when available, flowers are preferred (Luckenbach, 1982; Carl H. Ernst, pers. obs.), and peak tortoise activity corresponds to the period of spring blooming. In northern Arizona and southern Utah, the grasses Aristida, Tridens, and Bromus may amount to 61% of the diet, and in Nevada 61% of the diet consists of desert mallow (Sphaeralcea) and plaintain (Plantago) (Hansen et al., 1976). Food quality decreases dramatically after June, so the tortoises must eat enough to carry them through summer aestivation and winter hibernation, and females must accumulate extra energy reserves for egg yolking. Grasses and other desert scrub plants have been thought to be mostly secondary foods used only to support limited summer activity, but they may be more important, especially as a factor in the annual energy budget. Opuntia cacti may become important secondary foods and water sources in dry years (Turner et al., 1984). Other materials reported consumed by desert tortoise are sand, stones, bird feathers, vulture pellets, mammal hair, snake and lizard skins, animal bones, and arthropod parts (Hansen et al., 1976; Esque and Peters, 1994; Murray, 1997). Captives eat radishes, dandelions, watermelon, cantaloupe, lettuce, cabbage, spinach, bananas, figs, peaches, clover, grasses, grapes, apples, bread, cheese, snails, insects, and bird eggshells.
The desert tortoise spends most of its life underground. It constructs a burrow which provides a special microhabitat: the humidity is higher and the temperature is lower and more constant in the burrow than outside it. The turtle digs by scraping alternately with the forelimbs, and when the hole becomes deep enough the turtle turns around and pushes the dirt out with its shoulders. The burrows are dug in dry, gravelly soil and are located under a bush either in an arroyo bank or at the base of a cliff. In cross section they are somewhat oval; they may be straight, curved, or forked; and many have enlarged chambers. Although sometimes just long enough to admit the tortoise, they are occasionally over 10 m in length. Woodbury and Hardy (1948) reported that two kinds of burrows— dens and summer holes—are used in southwestern Utah. Dens are horizontal tunnels dug in banks of washes, usually for distances of 2.4-4.6 m but occasionally for 6-9 m. Summer holes are scattered over the flats and benches and are dug downward at angles of about 20-40° for a distance of about 1-1.5 m. In Sonora, Mexico, Auffenberg (1969) found that the summer retreat was most commonly a shallow hollow dug into the base of an arroyo wall. Several tortoises may use the same shelter during a single season. Individual tortoises may move about quite a bit, and those in Nevada have used as many as 12-25 cover sites (dens, burrows, pallets, and non-burrows) in one year (Burge, 1978).
IUCN Red List Status (1996)
Vulnerable (A1acde+2cde, E). Populations of desert tortoises have declined from a variety of reasons: habitat destruction, vehicular deaths, pet trade collection, upper respiratory tract disease (introduced into some populations by the release of sick captives), and livestock competition. Gopherus agassizii is federally protected as a threatened species under the United States Endangered Species Act.