Gopherus agassizii

Geographic Range

Desert tortoises are found in the southwestern United States and northwestern Mexico. They range from northern Sinaloa up through Sonora and western Arizona to southeastern California, southern Nevada, and the southwestern tip of Utah.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Desert tortoises live in different habitats in different parts of their range. In the south, (northern Sinaloa and southern Sonora) they inhabit thornscrub and tropical deciduous forests, further north, this habitat gives way to foothills thornscrub and Sonoron desertscrub, and in the northenmost part of their range (California, Nevada, and Utah), Mohave desertscrub. There are three distinct populations of desert tortoises, which are adapted to these different habitat types. Sonoron desert tortoises inhabit the Sonoron desertscrub of western Arizona, where paloverdes, saguaros, and ironwoods are the most prominant plants. At about 1,100 to 1,400 meters in elevation, these plants give way to desert grassland communities, which mark the upper elevational limit of desert tortoises. Tortoises in the Sonoron desert inhabit mountain slopes strewn with large boulders at densities of about 40 tortoises per square kilometer, and occur at much lower densities in intermountain valleys. On the other hand, Mohave desert tortoises of southeastern California, southern Nevada, and southern Utah live primarily in valleys where large rocks are absent, and may occur in densities of more than 150 tortoises per square kilometer. The Mohave desertscrub that these tortoises call home is characterized by creosotebush, white bursage, and galleta grass. Finally, to the south, Sinaloan desert tortoises inhabit the thornscrub and tropical deciduous communities of southern Sonora and northern Sinaloa. The thornscrub habitat is dominated by plants such as desert feather tree, papelio, torotes, tree ocotillo, and organpipe cactus, and the tropical deciduous forests are dominated by mauto, amapa, brasil, torotes, kapok, and tree morning glory.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• forest forest
• scrub forest scrub forest
• mountains mountains

Physical Description

The head of a desert tortoise is scaly, and the body has thick skin. Desert tortoises also have extremely long nails, which are used in digging through the desert sand to find shelter. The upper shell of a desert tortoise ranges in length from 15 to 36 centimeters, and its color varies from dull brown to a dull yellow. Males are typically larger than females. An adult male desert tortoise averages around 20 kilograms in weight, and an adult female averages 13 kilograms. Males and females are also shaped differently. Male tortoises have long, curved, gular horns that are used for leverage in male-male combat, heavier claws, and longer tails that facilitate copulation. Each male tortoise also has a depression in the plastron that fits around the carapace of a female, and an inward curve at the rear portion of the carapace that allows him to achieve the upright postion needed for mating. Females, on the other hand, have carapaces that curve outward and flat plastrons.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes shaped differently

Reproduction

Desert tortoises have multiple mates during their lifetime. During the breeding season, males fight one another for access to receptive females. During combat, males posture, bob their heads, ram into one another, and chase one another. Often, they attempt to flip one another over by using their large gular shields as levers. If one succeeds, the loser eventually rights himself and escapes. Meanwhile, the winner courts the female using behaviors similar to those used in male-male combat. He bites the female, hisses at her, and butts her in the flank until he is able to mount her. Mating ends when the female eventually wanders away.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Desert tortoises breed from spring to fall. Females store sperm in their reproductive tracts, and stored sperm can remain viable for eighteen months after copulation. Thus, females often mate in late summer and hibernate before laying eggs in spring. Each female desert tortoise lays 1 to 14 (most commonly, 3 to 5) round, off-white eggs in a deep nest that is scooped out of the desert floor. Females in the Mohave desert often lay a second clutch in late summer, which may not hatch until the next spring. Usually, however, the incubation period is 90 to 135 days, and the eggs hatch in September or October. The length of the incubation period is determined by temperature (as is the sex of the offspring). The shell of a newly hatched tortoise is extremely soft and remains soft up during the first five years of life. It eventually hardens as the tortoise matures. Sexual maturity is reached at about 14 to 21 years.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous
• sperm-storing sperm-storing

Female desert tortoises provide their young with yolk, which not only sustains them while the eggs incubate, but which provides the hatchlings with enough energy for six months--enough to sustain them in case they are not able to feed before hibernating. Before laying their eggs, females select nest sites in sheltered spots near their burrows or resting sites. Each female digs a hole with her hind legs in the spot she has chosen, urinating throughout the process--perhaps to ward off predators or provide the eggs with moisture. After deposting her eggs in the hole she has dug, each female covers the eggs with soil and urinates again. Females may guard their eggs for some time after laying, fending off predatory Gila monsters ( Heloderma suspectum ) and curious humans. Before long, however, females wander off and leave their young to fend for themselves.

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

Mortality for young desert tortoises is very high. For every 15 clutches of eggs that are laid, only one individual is likely to live to age 20. However, once a desert tortoise makes it to age 20, it has a very high chance of living at least 20 more years. Thus, the life expectancy of desert tortoises that live past age 20 is 50 to 80 years.

Behavior

Desert tortoises are active spring to fall, hibernating during winter, though the timing of activity varies by habitat. Tortoises in the Mohave desert concentrate their activity in the spring, aestivating during the hot, dry summers. Those in the Sonoran desert are most active in August and September, when late summer monsoonal rains bring an abundance of food. Finally, tortoises living in the thornscrub and tropical deciduous forests of northwestern Mexico seem to be active throughout the late summer and fall. The timing of daily activity changes with the season; on cooler days, desert tortoises are diurnal, but they become crepuscular if the midday heat is too intense. With their front legs, desert tortoises dig burrows where they can rest when it is either too hot or too cold for foraging. Burrow construction varies among the three desert tortoise populations. Tortoises in the Mohave desert dig extensive burrow systems, sometimes more than ten meters in length, which are used year after year. Up to 25 tortoises (though usually no more than five) may share a burrow. The burrows are usually placed in washes, under shrubs or under rocks. Sonoran desert tortoises are more likely to dig simple burrows, sometimes enlarging burrows made by other animals, such as packrats ( Neotoma albigula ), and sometimes just digging shallow depressions, or pallets. The sheltersites of Sonoran desert tortoises are usually on rocky slopes under boulders or shrubs. Desert tortoises in Mexico also dig shallow burrows, enlarge those made by other animals, or simply hide under overhanging rocks or vegetation. Tortoises in all three populations use multiple sheltersites each year, often ten or more. It is estimated that desert tortoises spend 98% of their lives resting in burrows or other sheltersites.

Desert tortoises are not highly social, though two or more tortoises may share a burrow. Opposite-sex tortoises are more tolerant of one another than same-sex tortoises, and loose dominance hierarchies may form among males.

• Key Behaviors
• terricolous
• fossorial fossorial
• diurnal diurnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• hibernation hibernation
• aestivation
• daily torpor
• solitary solitary
• dominance hierarchies dominance hierarchies

Home Range

Some individual desert tortoises have been recorded moving large distances. One marked female was recovered 30 km from where she was originally released two years before. However, this tortoise was moving through an urban area, and such long distance movement is probably atypical. In general, desert tortoises are fairly sedentary, occupying the same basic home range throughout their lives. The size of the home range varies by sex, age, habitat, and season. In several telemetry studies (summarized by Averill-Murray et al. 2002), the average home range size for females was 2.6 to 23.3 hectares, and the average home range size for males was 9.2 to 25.8 hectares. These tortoises are not territorial, and home ranges of several individual tortoises may overlap. Desert tortoises use a larger portion of their overall home range during the spring or summer months when they are most active.

Communication and Perception

Desert tortoises perceive the world using visual, chemical, tactile, and auditory senses. They communicate with one another by vocalizing and posturing, and they may use feces and anal gland secretions to mark their burrows and home ranges.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• scent marks scent marks

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Desert tortoises are herbivorous, surviving on low-growing plants and freshly fallen leaves. The species of plants eaten vary widely by season and geographic region, but overall the desert tortoise diet consists of the leaves, bark, stems, fruits, and/or flowers of trees, shrubs, woody vines, succulents, perennial and annual grasses, herbaceous perennials, and annuals. During rainy seasons, desert tortoises drink large amounts of water from temporary pools.

• Primary Diet
• herbivore herbivore
  • folivore folivore
  • frugivore frugivore
  • lignivore

• Plant Foods
• leaves
• wood, bark, or stems
• fruit
• flowers

Predation

Adult tortoises, with their hard shells, have few natural predators. Only mountain lions ( Puma concolor ) can crush their shells. Eggs, hatchlings and juveniles are more vulnerable, and are preyed upon by kit foxes ( Vulpes velox ), coyotes ( Canis latrans ), feral dogs ( Canis lupus familiaris ), gray foxes ( Urocyon cinereoargenteus ), bobcats ( Lynx rufus ), badgers ( Taxidea taxus ), golden eagles ( Aquila chrysaetos ), common ravens ( Corvus corax ), greater roadrunners ( Geococcyx californianus ), and Gila monsters ( Heloderma suspectum ). Female desert tortoises may deter egg predators by guarding their eggs for some time after laying, but hatchlings and juveniles must rely heavily on camouflage and the use of shelters to keep themselves safe. All desert tortoises, large and small, will urinate as a last resort when handled.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Desert tortoises are primary consumers, and they are prey for various mammalian, avian, and reptilian predators. Desert tortoises are also ecosystem engineers, digging burrows that are used as shelters by snakes ( Serpentes ), lizards ( Sauria ), birds ( Aves ), rodents ( Rodentia ), javelinas ( Pecari tajacu ), and insects ( Insecta ) and other invertebrates. Desert tortoises themselves take advantage of packrat ( Neotoma albigula ) houses for shelter. In one study, desert tortoises were found sheltering with large colonies of Africanized honeybees ( Apis mellifera )--a very effective defense against predators! Finally, desert tortoises have few external parasites, but they are host to intestinal pinworms ( Oxyurida ).

• Ecosystem Impact
• creates habitat

Economic Importance for Humans: Positive

In the past, desert tortoises were used by indigenous peoples of the American Southwest for food and medicine, and the shells were used to make bowls, ladles, and shovels. Desert tortoises were also central figures in the folklore of the region. Desert tortoises are occasionally still used for food in some parts of Mexico. In addition, many people in Arizona keep desert tortoises as pets, though the keeping of captive tortoises is strictly regulated: tortoises must not be collected from the wild, only one tortoise is allowed per family member.

• Positive Impacts
• pet trade pet trade
• food food
• body parts are source of valuable material
• source of medicine or drug source of medicine or drug

Economic Importance for Humans: Negative

There are no known negative impacts of desert tortoises on humans.

Conservation Status

Desert tortoises face many threats from humans, including habitat loss and fragmentation, road mortality, shooting, collection for food and the pet trade, trampling by livestock, and predation by feral dogs ( Canis lupus familiaris ) and ravens ( Corvus corax ), which thrive around human settlements. Desert tortoises have suffered enormous declines in population sizes in recent years--up to 55% in some areas. The Mohave Desert population, ravaged by upper respiratory tract disease, has fared the worst, and Mohave desert tortoises were listed as threatened by the U.S. Fish and Wildlife Service in 1990. Beginning in 1988, Arizona state law has recognized desert tortoises as a threatened species and strictly prohibited their capture. Possession of captive desert tortoises is strictly regulated, and it is illegal for anyone to release captive desert tortoises into the wild, so that released captives do not disturb wild populations. Desert tortoises have been listed as threatened in Mexico since 1994. In addition, desert tortoises appear in the CITES appendix II and as "vulnerable" on the IUCN's 2004 Red List of Threatened Species.

To preserve desert tortoises, the Federal Bureau of Land Management has established a 98 square kilometer sanctuary in California called the Desert Tortoise Natural Area. This preserve is closed to all vehicles, livestock grazing, and mining. In Arizona, the Arizona Interagency Desert Tortoise Team, established in 1985, has produced a management plan for desert tortoises calling for the establishment of management areas that would support healthy tortoise populations, continuous monitoring of tortoise populations, and measures such as tortoise-proof fencing and tortoise overpasses that would keep tortoises off of roads.

Additional Links

Encyclopedia of Life

Contributors

Allison Poor (author), University of Michigan-Ann Arbor.

Gregory Crozier (author), University of Michigan-Ann Arbor.

References

Arizona-Sonora Desert Museum's Tortoise Adoption Program, 2003. "The Desert Tortoise (Gopherus agassizii): A Natural History" (On-line). Arizona-Sonora Desert Museum. Accessed September 27, 2005 at http://www.desertmuseum.org/programs/tap_tortnathistory.html .

Averill-Murray, R., A. Averill-Murray. 2005. Regional-scale estimation of density and habitat use of the desert tortoise (Gopherus agassizii) in Arizona. Journal of Herpetology , 39(1): 65-72.

Averill-Murray, R., B. Martin, S. Bailey, E. Wirt. 2002. Activity and behavior of the Sonoron desert tortoise in Arizona. Pp. 135-158 in The Sonoron Desert Tortoise . Tuscon: The University of Arizona Press.

Averill-Murray, R., A. Woodman, J. Howland. 2002. Population ecology of the Sonoran desert tortoise in Arizona. Pp. 109-134 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Bury, B., D. Germano, T. Van Devender, B. Martin. 2002. The desert tortoise in Mexico: Distribution, ecology, and conservation. Pp. 86-108 in The Sonoran Desert Tortoise . Tuscon: The University of Arizona Press.

Dickinson, V., J. Jarchow, M. Trueblood, J. DeVos. 2002. Are free-ranging Sonoran desert tortoises healthy?. Pp. 242-264 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Edwards, T., E. Stitt, C. Schwalbe, D. Swann. 2004. Gopherus agassizii (Desert tortoise). Movement. Herpetological Review , 35(4): 381-382.

Germano, D., F. Pough, E. Morafka, M. Demlong. 2002. Growth of desert tortoises: Implications for conservation and management. Pp. 265-288 in The Sonoran Desert Tortoise . Tuscon: The University of Arizona Press.

Howland, J., J. Rorabaugh. 2002. Conservation and protection of the desert tortoise in Arizona. Pp. 334-375 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Jarchow, J., H. Lawler, T. Van Devender, C. Ivanyi. 2002. Care and diet of captive Sonoran desert tortoises. Pp. 289-311 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Lamb, T., A. McLuckie. 2002. Genetic differences among geographic races of the desert tortoise. Pp. 67-85 in The Sonoran Desert Tortoise . Tuscon: The University of Arizona Press.

Nabhan, G. 2002. When desert tortoises talk, Indians listen: Traditional ecological knowledge of a Sonoran Desert reptile. Pp. 355-375 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Stitt, E., C. Schwalbe, D. Swann. 2004. Gopherus agassizii (Desert tortoise). Association with Africanized bees. Herpetological Review , 35(4): 381.

Van Devender, T., R. Averill-Murray, T. Esque, P. Holm, V. Dickinson, C. Schwalbe, E. Wirt, S. Barrett. 2002. Grasses, mallows, desert vine, and more: Diet of the desert tortoise in Arizona and Sonora. Pp. 159-193 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Van Devender, T. 2002. Natural history of the Sonoran tortoise in Arizona: Life in a rock pile. Pp. 3-28 in The Sonoran Desert Tortoise . Tucson: The University of Arizona Press.

Campbell, F.T. 1988. "The Desert Tortoise". Audabon Wildlife Report. Academic Press, San Diego.

Ferrara, J. 1984. "Digging In". National Wildlife. 2/22 pgs. 22-28

Klein, Stanley. 1983. The Encyclopedia of North American Wildlife. Facts on File Publications, NY.