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Desert Tortoise
According
to the fossil record, the desert tortoise (gopherus agassizii) has lived in the arid
southwestern US for the last ten to twelve thousand years. It now faces its greatest
challenge. Once protected by its austere desert environment, the tortoise is threatened by
human population pressures and environmental reduction here in Nevada.
The Desert Tortoise is an herbivore that may attain a length of 9 to 15 inches in upper shell (carapace) length. The tortoise is able to live where ground temperature may exceed 140 degrees F because of its ability to dig underground burrows and escape the heat. At least 95% of its life is spent in burrows. There, it is also protected from freezing while dormant, November through February or March.
The presence of soil suitable for digging burrows is a limiting factor to distribution. Some of these burrows just extend beyond the shell of the tortoise inside. Others extend for several feet. A single tortoise may have a dozen or more burrows distributed over its home range. These burrows may be used by different tortoises at different times.
The tortoise's hind limbs differ markedly from the forelimbs. Whereas the hind limbs are elephantine, the forelimbs are flattened with well-developed muscle. They are used for digging burrows. The females use their hind limbs to dig their nests.
Both sexes have a gular horn -- an anterior extension of the plastron (lower shell). The horn is longer in males and often upturned. Males use these in fighting with other males, attempting to insert the horn under the anterior edge of the carapace and by twisting to the side, flip the other male on its back. The opponent attempts to stand as high as possible to prevent this from happening.
Fighting may occur at anytime that males encounter one another and usually ends in the subordinate male running away from the other. Where there are cavities in partially consolidated gravels with room for several tortoises, males and females will share theses cover sites. The males may begin to fight upon emerging each day but the importance of adequate cover for protection against extreme heat seems to be greater than maintaining the dominance hierarchy.
Tortoises grow at varying rates depending upon forage availability. The number of growth rings in a given year may be zero to several; hence, one cannot determine a tortoise s exact age by counting those rings. Sexual maturity is a function of size rather than age, approximately 7-8 inches mid- carapace length in females.
Courting and copulation may occur at any time that the tortoises are above ground; however, there seems to be more of this behavior in late summer and early fall when the testosterone levels peak in males. Females store sperm and egg laying occurs May, June, and July. The number of eggs varies. Female size seems to be one factor. A mature female might lay 4-8 white, hard-shelled eggs in a clutch and produce two, sometimes three clutches in a season.
Nests are often dug near the burrow opening early in the season and farther inside, late in the season. Some nests are dug away from the burrow but usually under a shrub. After laying, the female leaves the nest and the soil temperatures support growth of the embryos. Incubation periods of 90 to 120 days are typical. Data from experiments using controlled incubation temperatures show that cooler temperatures, 79-87 °F produce all males; at 88-91°F all females.
Diet composition varies throughout the tortoise's range. If winter rainfall has been sufficient to result in germination of annuals, these are used heavily when the tortoises emerge from winter torpor (brumation). Other herbs, grasses, some shrubs and the new growth of cacti and their flowers comprise a major portion of the diet. If there is summer rain, tortoises will utilize dry forage.
To maximize the utilization of infrequent rainfall, tortoises dig catchment basins in the soil, remember where these are, and may be found waiting by them when rain appears imminent. Water that reaches the bladder is not lost to the system but can be drawn upon as needed.
The Desert Tortoise of the Mojave Desert is federally listed as a threatened species. It is unlawful to touch, harm, harass, or collect a wild tortoise. There are programs run by tortoise clubs in Arizona, California and Nevada through which legally acquired captives may be adopted. State and federal wildlife and land management agencies and local jurisdictions are actively involved in conservation programs to help the recovery of the Desert Tortoise throughout the Mojave Desert.
For more information about adoption, care, and feeding of Desert Tortoise’s, contact the following organizations: