Gopherus agassizii - (Cooper, 1861)
Agassiz's Desert Tortoise
Other English Common Names: Desert Tortoise - Mohave Population
Synonym(s): Gopherus agassizii (Mohave Population)
Taxonomic Status: Accepted
Related ITIS Name(s): Gopherus agassizii (Cooper, 1863) (TSN 173856)
Unique Identifier: ELEMENT_GLOBAL.2.102027
Element Code: ARAAF01012
Informal Taxonomy: Animals, Vertebrates - Turtles
Kingdom Phylum Class Order Family Genus
Animalia Craniata Chelonia Cryptodeira Testudinidae Gopherus
Genus Size: B - Very small genus (2-5 species)
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Concept Reference
Concept Reference: Murphy, R. W., K. H. Berry, T. Edwards, A. E. Leviton, A. Lathrop, and J. D. Riedle. 2011. The dazed and confused identity of Agassiz's land tortoise, Gopherus agassizii (Testudines, Testudinidae) with the description of a new species, and its consequences for conservation. ZooKeys 113:39-71.
Concept Reference Code: A11MUR01NAUS
Name Used in Concept Reference: Gopherus agassizii
Taxonomic Comments: The Sonoran population of the desert tortoise (east and south of the Colorado River) has been recognized as a distinct species (Gopherus morafkai). The Mohave population (west and north of the Colorado River) retained the name G. agassizii (Murphy et al. 2011).
Conservation Status

NatureServe Status

Global Status: G3
Global Status Last Reviewed: 06Sep2013
Global Status Last Changed: 23Jan2003
Ranking Methodology Used: Ranked by calculator
Rounded Global Status: G3 - Vulnerable
Reasons: Occurs in the Mohave Desert, west and north of the Colorado River; has declined in abundance in many areas as a result of widepsread habitat loss, degradation, and fragmentation, and human-associated factors that cause mortality.
Nation: United States
National Status: N3 (06Sep2013)

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Arizona (S2), California (S2S3), Nevada (S2S3), Utah (S2)

Other Statuses

U.S. Endangered Species Act (USESA): LT: Listed threatened (02Apr1990)
Comments on USESA: This record represents the USFWS Mojave population of the desert tortoise, listed as a threatened species on April 2, 1990. In a 90-day finding on a petition requesting that the threatened Mojave population be emergency reclassified as endangered under the Act, USFWS (2017) found that the petition is not warranted (??not-substantial??).

NatureServe Global Conservation Status Factors

Range Extent: 20,000-200,000 square km (about 8000-80,000 square miles)
Range Extent Comments: Range extends from Inyo County, California (north to Death Valley National Park and about 10 miles south of Lone Pine), southern Nevada (Clark, Nye, and Lincoln counties, north to Yucca Mt. and Coyote Springs), and extreme southwestern Utah (Washington County: Beaver Dam slope and north St. George) south throughout most of the Mohave Desert to the eastern Colorado Desert of Los Angeles, Kern, San Bernardino, Riverside, and Imperial counties, California (i.e., areas west and north of the Colorado River) (Murphy et al. 2011). Elevational range is mainly below 1,677 meters (5,500 feet) but extends from below sea level (Death Valley) to 2,225 meters (7,300 feet) (USFWS 2011).

Area of Occupancy: 2,501-12,500 4-km2 grid cells
Area of Occupancy Comments: A habitat model for desert tortoises in the Mohave Desert and part of the Sonoran Desert found that area in which tortoises were most likely to occur (model scores of 0.8-1.0) totaled not more than 59,196 square kilometers (Nussear et al. 2009). The vast majority of this area was west and north of the Colorado River. The total included anthropogenically altered areas that in reality had lower potential as suitable habitat.

Number of Occurrences:  
Number of Occurrences Comments: The number of distinct occurrences (subpopulations) and locations has not been determined using standardized criteria. The number of occurrences likely would have to be based on arbitrary separation criteria, so the number would have relatively little conservation significance (e.g., occurrences likely would be relatively few but quite large). Instead, population size and area of occupancy are more meaningful measures of conservation status.

Population Size: 100,000 - 1,000,000 individuals
Population Size Comments: total adlut population size is unknown, but based on the estimated area of occupancy and a conservative estimate of average density, the total probably exceeds 100,000.

Overall Threat Impact: High
Overall Threat Impact Comments: Declines have resulted from myriad factors, including habitat loss, degradation, and fragmentation caused by urbanization, agricultural development, livestock and feral burro grazing, invasion of exotic annuals (which fuel local fires), energy and mineral development, and ORV use; mortality on roads; disease; vandalism (illegal shooting); and collecting. These factors vary regionally in their severity.

USFWS Status Review, 2010

In a review of desert tortoise status, USFWS (2010) found that habitat loss, degradation, and fragmentation continue to impact desert tortoises. "In particular, human populations, paved and unpaved roads, non-native invasive plants and the associated threat of wildfire, and prospective energy development (especially renewable energy development and associated utility corridors) have increased. These threats result in continued habitat loss, population fragmentation, nutritional compromise, soil erosion, and indirect impacts associated with increased human presence, including illegal dumping, human-subsidies for predators, and introduction of toxins. Since the time of listing, off-highway vehicle areas and trails have been formally designated, but unauthorized use continues to be a significant source of habitat degradation. Many grazing allotments within Critical Habitat have been retired; however large areas are also still grazed."

"Little quantitative evidence regarding collection and deliberate maiming and killing of desert tortoise by humans has been obtained since time of listing, and the relative significance of this threat remains unknown" (USFWS 2010).

"The available evidence indicates that upper respiratory tract disease is probably the most important infectious disease for desert tortoises, and external factors, such as environmental contaminants and drought, may increase susceptibility. However, additional research is needed to clarify the role of disease in desert tortoise population dynamics relative to other threats. Ravens and coyotes have dramatically increased in the desert southwest over the past 25 years due to anthropogenic subsidization and have been commonly implicated in tortoise predation. Instances of isolated, very intense predation suggest predation comes to the forefront as a management concern, especially where landscapes have been altered and intensive human use occurs or in times of extreme drought. The population-level effects of these or other predators, however, are unknown." Source: USFWS (2010).

"There are Federal and State regulatory mechanisms which provide discretionary protections for the desert tortoise based on current management direction, but with the exception of the California Fish and Game Code, none guarantee protection absent the Endangered Species Act. While many land use plans completed since time of listing include language specific to protection of the tortoise, land management agencies frequently do not have sufficient funding to enforce their land use regulations, and personnel are often spread across vast landscapes with multiple resource responsibilities." Source: USFWS (2010).

"Captive releases continue to have the potential to introduce disease and genetic contamination into wild populations of desert tortoises, although the magnitude of such releases and their effects on tortoise populations remains unknown" (USFWS 2010).

"Since the time of listing, it has become apparent that the combined effects of global climate change (i.e., increased ambient temperatures and altered precipitation patterns) and drought may become significant factors in the long-term persistence of the species. Little is known regarding direct effects of climate change on the desert tortoise and its habitat, although increased drought will likely affect desert tortoises, directly through habitat loss and indirectly through decreased availability/quality of food and increased predation and possibly disease." Source: USFWS (2010).

Earlier Information on Threats

Lovich and Daniels (2000) studied habitat use in an area of wind energy development and concluded that this disturbance, with proper planning, may be compatible with tortoise conservation.

Heavy grazing, especially by sheep, alters forage availability and protein content, and it also removes the desert soil crust, inducing soil compaction and increasing erosion. Tracy (1992) pointed out that a high fiber/low protein diet can retard the age of first reproduction by more than five years, which would have significant demographic consequences; this relationship bears significantly on the debate about the importance of competition between tortoises and cattle and sheep.

In some areas, urbanization and agriculture have eliminated tortoise populations in the western and northern parts of the range (though tortoises remain in undeveloped parts of the Antelope, Indian Wells, and Searles valleys; USFWS 2010), and this process is being repeated in the Victor Valley (San Bernardino County, California,) the valley of Las Vegas, and along the Virgin River (St. George, Utah) (USFWS 1994, Appendix D). Natural droughts from 1986-1992, and in 1994 have exacerbated anthropogenic impacts on the Mohave population.

During the 1980s and 1990s, severe population losses in the western Mohave Desert resulted from Mycoplasma-caused (Jacobson et al. 1995) disease (upper respiratory tract disease [URTD], possibly introduced through release of captive tortoises) and possibly from raven predation (15-fold increase in raven population between 1968 and 1988) (California Dept. of Fish & Game 1990). Ravens, along with coyotes, feral dogs, and cats are "subsidized" predators that have semi-urban populations enlarged by feeding on the refuse and rodents associated with human garbage dumps and backyards. They may be significant predators on young (< 7 years old or 120 mm plastron length) tortoises. However, in the south-central Mohave Desert, Bjurlin and Bissonette (2004) found that neonatal desert tortoises are less susceptible to predation than was previously thought, perhaps because of their cryptic coloration and secretive habits. The common raven was not found to be a source of neonate mortality. Decline at Chuckwalla Bench in the eastern Colorado Desert was due to an unidentified shell disease, probably caused by toxicants or mineral deficiencies, in addition to shooting, vehicle kills, and the effects of drought (Berry 1992). Perhaps the most widespread, and recent cause of increased mortality has been URTD (Jacobson 1994). The causal agent is a mycoplasma, Mycoplasma agassizii. Drought and concomitant poor nutrition may immunocompromise tortoises making them more vulnerable to infection. However, even healthy, well-fed tortoises may become infected (USFWS 1994). See USFWS (1994) for a review of factors affecting Mohave and Colorado desert populations.

Releases of non-native desert tortoises into areas occupied by native populations pose a potential threat because of the possible introduction of disease, competition between released and native tortoises for limited resources, and possible outbreeding depression (Bury et al. 1994).

This species is relatively tolerant of nondestructive intrusion. Interventions that cause voiding of tortoise bladder contents or that deny tortoises access to surface water or burrows could result in unfavorable water balance, especially in dry seasons.

Short-term Trend: Decline of 10-30%
Short-term Trend Comments: Populations are declining in several areas throughout the range (USFWS 2010).

A major long-term decline in abundance and distribution often has been reported since the 1980s, but Bury and Corn (1995) concluded that existing data and historical reports do not support the validity of such a decline. During the past two decades annual declines in individual populations have varied between 3% and 59%. More important, many of these losses are adults which otherwise would reproduce and incur a natural attrition of only 2% annually.

Despite protective measures, the Beaver Dam Slope population in Utah probably was at an all-time low as of 1990 (Glenn et al. 1990). Estimates made 30-40 years after the baseline study by Woodbury and Hardy (1948) indicate an 80% decline in population densities and similar decline in total population size (Minden 1980).

USFWS (1990) categorized the status of the Mohave Desert population as "declining." In California, habitat has been reduced 50-60% since the 1920s. In California's western Mohave, populations may have declined nearly 90% since 1940, and as much as 70% locally between 1976-1984 (Berry 1984; however, see Bury and Corn 1995). Demographic analyses agree with field censuses in showing rapid population decline in the western Mohave Desert (Doak et al. 1994). At the Desert Tortoise Natural Area (Kern County, California), the past ten years decline has reduced the tortoise population by 88%; a similar 84% decline has been reported for Johnson Valley (USFWS 1994). At Joshua Tree National Park (then a Monument) populations appear to have remained stable and locally robust (up to 200 tortoises/sq mi). For the Mohave Desert threatened population, the overall estimated rate of decline for the past fourteen years is 4.6% annually (USFWS 1994).

At Chuckwalla Bench in the eastern Colorado Desert, a population decline began in the early 1980s and continued through 1990 (Berry 1992), culminating in a 60-70% population loss. However, adjacent Colorado Desert Chemehevi and Ward Valley populations remain the largest and most robust in the entire range (USFWS 1994).

Long-term Trend: Decline of 30-70%
Long-term Trend Comments: The overall range has not changed very much, but populations in many areas have substantially declined, though the degree of decline is not well known (USFWS 2010).

Other NatureServe Conservation Status Information

Global Range: (20,000-200,000 square km (about 8000-80,000 square miles)) Range extends from Inyo County, California (north to Death Valley National Park and about 10 miles south of Lone Pine), southern Nevada (Clark, Nye, and Lincoln counties, north to Yucca Mt. and Coyote Springs), and extreme southwestern Utah (Washington County: Beaver Dam slope and north St. George) south throughout most of the Mohave Desert to the eastern Colorado Desert of Los Angeles, Kern, San Bernardino, Riverside, and Imperial counties, California (i.e., areas west and north of the Colorado River) (Murphy et al. 2011). Elevational range is mainly below 1,677 meters (5,500 feet) but extends from below sea level (Death Valley) to 2,225 meters (7,300 feet) (USFWS 2011).

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map
Endemism: endemic to a single nation

U.S. & Canada State/Province Distribution
United States AZ, CA, NV, UT

Range Map
No map available.

U.S. Distribution by County Help
State County Name (FIPS Code)
AZ Mohave (04015)
CA Imperial (06025), Inyo (06027), Kern (06029), Los Angeles (06037), Riverside (06065), San Bernardino (06071)
NV Clark (32003), Esmeralda (32009), Lincoln (32017), Nye (32023)
UT Washington (49053)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
15 Lake Mead (15010005)+, Grand Wash (15010006)+, Upper Virgin (15010008)+, Fort Pierce Wash (15010009)+, Lower Virgin (15010010)+, White (15010011)+, Muddy (15010012)+, Meadow Valley Wash (15010013)+, Las Vegas Wash (15010015)+, Havasu-Mohave Lakes (15030101)+, Piute Wash (15030102)+, Sacramento Wash (15030103)+, Imperial Reservoir (15030104)+, Lower Colorado (15030107)+
16 Sand Spring-Tikaboo Valleys (16060014)+, Ivanpah-Pahrump Valleys (16060015)+
18 Owens Lake (18090103)+, Upper Amargosa (18090202)+, Death Valley-Lower Amargosa (18090203)+, Indian Wells-Searles Valleys (18090205)+, Antelope-Fremont Valleys (18090206)+, Coyote-Cuddeback Lakes (18090207)+, Mojave (18090208)+, Southern Mojave (18100100)+, Whitewater River (18100201)+, Salton Sea (18100204)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
General Description: A typical land-dwelling tortoise with all the diagnostic external features: head is roofed with small unevenly sized scales; front feet are club shaped, scaled, and terminate in unwebbed toes with broad, thick claws; the hindlegs are columnar and elephantine, again with unwebbed broad claws; the carapace is highly domed, steep sided and flattened dorsally, brown (dull yellow to light brown in young), and has prominent growth lines; unhinged plastron is yellowish and generally has prominent growth lines; limbs are stocky; tail is short; adult carapace length 20-36 cm.

Compared to females, adult males average larger in size, have longer gular shields, a larger lump (chin gland) on each side of the lower jaw (especially in the spring), and a concave rather than flat plastron, especially in the posterior/femoral area (Stebbins 1985). Males have broader and thicker tails and thick toenails. Sexing individuals less than 15 years old and/or less than 200mm straight carapace length may be difficult by external morphology alone. See Rostal et al. (1994) for information on the use of plasma testosterone and laparoscopy to identify the sex of neonates (hatchlings) and immatures.

The age of juvenile tortoises up to approximately 20-25 years old may be determined by counting concentric annual rings radiating outward from the areolar center of each shell scute. The second right costal scute is recommended for age accounts. After 25 years shell wear and shedding of juvenile rings may obscure rings previously accrued (Germano 1988). However, such "growth rings" are annular only in localities in which plant forage growth is confined to a single season (Miller 1932). Areas with multiple peaks in primary production driven by rainfall may exhibit multiple rings for a given year.

The eggs are pale, elliptical to spherical, brittle shelled, and relatively large (averaging 30-40 mm in diameter, and 20-40 g). Fertile egg shells usually become an opaque chalky white within the week following deposition, but become increasingly pink or gray and translucent if they are infertile or dead.

Diagnostic Characteristics: This species differs from box turtles (genus Terrapene) in lacking a hinged plastron and in having columnar hindlegs with flattened, rather than pointed and tapered, nails. Differs from the Texas tortoise (G. berlandieri) in having a single axillary scute on each side (rather than paired axillary scutes) and the 5th vertebral scute the broadest (rather than the 3rd). Differs from both the gopher tortoise (G. polyphemus) and the Mexican bolson tortoise (G. flavomarginatus) in having relatively larger hind feet (in the desert tortoise, the distance from the base of the first claw to the base of the fourth claw on the forefoot is approximately equal to the same measurement on the hind foot; in the bolson tortoise the measurement is smaller on the hind foot) (Ernst and Barbour 1989).

Box turtles and Texas tortoises have commonly been brought into the Southwest as pets. The southwestern box turtle (Terrapene ornata luteola) is the one terrestrial chelonian which now overlaps geographically and ecologically with the desert tortoise in the vicinity of Tucson, continuing east through Cochise County, Arizona. In recent years the Russian desert tortoise, Agrionemys (Testudo) horsfieldi has been imported by the pet trade in large numbers. While these tortoises resemble the North American species, their maximum size is a smaller 8" straight midcarapace length, their carapaces tend toward olive-gray rather than brown, and the forelimb toes number four rather than five. Other Eurasian tortoises are occasionally imported, but most have vivid carapace blotches of yellow, brown, or black, and/or a high domed rather than flattened top of the carapace.

Gopherus morafkai differs from G. agassizii in having a relatively narrower shell, shorter gular scutes, shorter projections of the anal scutes and in having a flatter, pear-shaped carapace (Murphy et al. 2011). Note that reliable identification of captive tortoises can be impossible due to hybridization or abnormalities resulting from poor nutrition.

Reproduction Comments: The endocrine-reproductive physiology of eastern Mohave tortoises is as follows (Rostal et al. 1994):

Males: increase production of testosterone and begin spermatogenesis in July; from August through October mating takes place; during winter, blood testosterone levels continue the decline which began in September and testes regression probably also begins; March-April emergence initiates a spring mating season which continues though May, paradoxically while testosterone levels continue to drop and testes remain regressed; during the spring mating, males use stored sperm from the epididymis to fertilize females.

Females: ovulation and mating occur in April and May, and presumably some fertilization takes place at this time, both from spring matings and from sperm stored from the prior fall and from prior years of mating, though fertility declines as time since mating increases (Gist 1989); in May and June, grown follicles become hard shelled and are deposited in egg nests; from July through October, the most recent follicles continue to grow by vitellogenesis (yolk enlargement) until they are mature; during the fall, female blood testosterone level begins to increase toward the April peak.

At least in the better studied northern/eastern Mohave populations, fall mating may be particularly important. Access to mates is determined both by male-male dominance hierarchies and by selective female receptivity (Niblick et al. 1994, Burge 1994). A subset of the adult males account for most mating. Male-male encounters may result in agnostic behaviors ranging from head bobbing to ramming, partially to establish social dominance. Greater size, longer residency at a particular site, and past social interactions favor the dominance of one male over others.

Courting is initiated by males through the following series of behaviors (Ruby and Niblick 1994): approach > headbob > trailing > biting, raming, sniffing, and circling > mounting > shell scratch, hops, grunts, head in and out > copulation. Female acquiescence is indicated by pulling her head into her shell and lying down (withdrawing limbs). Rejection is expressed when females walk away. Mounting is facilitated by the concave plastron (undershell) of the males. Copulation is achieved by the insertion of true penis into the cloaca.

Egg laying occurs mainly from May to early July. Clutch size is up to 15 (often 3-7). Number of clutches per year (0 to 3) may depend on environmental conditions, including those of the year prior to oviposition (Turner et al. 1984, 1986). Double clutching is common in the Mohave Desert in or following a wet year, with the second clutch following the first by about one month. After several continuing years of drought in California, desert tortoises continued to produce a single clutch, averaging 3 eggs (USFWS 1994).

Experimental evidence from Nevada tortoises (Spotila et al. 1994) indicate an incubation period of 125 days at 26 C, 68-73 days at 33 C, and 85 days at 35 C (a largely lethal temperature treatment). Best results were at temperatures between 28 and 33 C. Cooler incubations generally facilitated yolk reabsorption and resulted in larger hatchlings. Spring emergence of hatchlings (neonates), and hatchlings overwintering in their egg nest has been recorded, but whether embryogeneis may be suspended over winter is unknown.

Embryogenesis begins only after eggs are deposited. Rotation of eggs after deposition may reduce hatching success rates (Turner et al. 1986). Vascularization of the embryo and its membranes is apparent 22 days into an 82-day incubation. A 9.5-mm embryo is well formed after day 35, and movement occurs after day 37. The embryo is well formed by 66 days (Booth 1958).

Hatching requires the neonate to pip the shell with its egg tooth and reabsorb a residual yolk sac while straightening its embryologically concave plastron and hunched carapace. This process requires 48-72 hours and is followed by excavation of a path to the surface. Hatchling behavior does not appeared to be synchronized within clutches.

Growth of juveniles is much more vigorous than that of adults. Size-age classes were defined by Berry et al. (1990). With recent modifications, Berry's age-size classes are as follows: juvenile 1: less than 60 mm straight midline plastron length; juvenile 2: 60-99 mm; immature 1: 100-139 mm; immature 2: 140-179 mm; adults greater than 180 mm (young adults less than 207 mm, medium adults less than 240 mm). This subjective categorization has been questioned by Germano (1994b).

Individuals attain sexual maturity in 13-20 years. Estimates of mean age of sexual maturity 14.4 years in the western Mohave Desert and 15.4 years in eastern Mohave Desert (Germano 1994). Gravid females with plastron lengths (PL) as small as 186 mm (Joyner-Griffith 1991) have been found in the central Mohave Desert. Tortoises with straight plastron midline lengths larger than 200 mm are generally sexually mature, including males in which plastron concavity is not conspicuous.

Survivorship from hatchling to adult varies by site and by generation, but probably averages about 2% for healthy populations (USFWS 1994). In California, survivorship of eggs to hatching was 0.24 (see Iverson 1991); annual survivorship of adults was 0.98 (Turner et al. 1984). Maximum life span is greater than 50 years in eastern Mojave populations, but tortoises often survive for only 20-25 years of adulthood (Germano 1994b).

Miscellaneous reproductive information: Sex determination is temperature dependent, with a (Nevada) pivotal temperature of approximately 31.8, with mostly males produced at lower temperatures and mostly females at higher temperatures. Unlike leather shelled eggs of turtles, tortoise eggs do not respond to drier conditions by hatching earlier with larger residual yolks.

Ecology Comments: Density in different areas ranges from less than 8 to 184 per sq km (Berry 1986, Freilich et al. 2000). Densities in several Colorado Desert populations ranged between 50-250/sq mi (Berry et al. 1983). In the eastern Mohave Desert of northern Arizona, southwestern Utah, and southern Nevada, more than 75-95 percent of the populations now average less than 50 tortoises/sq mi. In this region only Piute Valley, Cottonwood Valley, 40 Mile Canyon and Coyote Springs, Nevada, and the Paradise Canyon-St. George Hills area of Utah supported tortoises densities in the 100/sq mi range and absolute population sizes that were favorable to long term viability. In California, densities are lowest in the far western (Antelope Valley) Mohave Desert, and highest in the west/central (Superior-Cronese) and eastern Mohave Desert and locally in the northern Colorado Desert.

The Desert Tortoise Recovery Plan (USFWS 1994, Appendix F) provided a detailed regional account of local population densities for the threatened Mohave "population," summarized here as follows by Recovery Units: 1-Northern Colorado Desert, 10-275 adults/sq mi; Eastern Colorado Desert, 5-175/sq mi; Upper Virgin River DWMA, up to 250/sq mi; eastern Mohave Desert, 10-350/sq mi (formerly up to 440/sq mi at Goffs, San Bernardino County, California); Northeastern Mohave Desert, 5-90/sq mi; Western Mohave Desert, 5-250/sq mi.

In the eastern Mohave desert, depressed survival rates were associated with drought conditions during three of four years (Longshore et al. 2003). "If periods of drought-induced low survival are common over relatively small areas, then source-sink population dynamics may be an important factor determining tortoise population densities" (Longshore et al. 2003).

A number of organisms are intimately associated with desert tortoise burrows (summarized by Grover and DeFalco 1995): ground squirrels, Peromyscus and pocket mice, kangaroo rats, woodrats, jackrabbits, desert cottontail, domestic cat, spotted skunk, kit fox, burrowing owl, Gambel's quail, poorwill, roadrunner, desert gecko, desert iguana, desert spiny lizard, western whiptail, gopher snake, coachwhip, night snake, Mohave rattlesnake, sidewinder, western rattlesnake, antlion larvae, ground beetles, roaches, silverfish, blackwidow spider, tarantula, and ticks.

Ectoparasites include ticks (Ornithodoros turicata, O. parkeri), trombicula mites, and dipteran maggot larvae (include those of the botfly). Potential endoparasites and pathogens include intestinal protozoa, bacteria, and the oyurate nematode (Tachygonetria). Some of the bacteria actually may be mutualists that facilitate hemicellulose digestion, while high nematode loads may serve a shredders of high fiber fragments, increasing surface areas for digestion without inducing pathological symptoms in the host (Morafka et al. 1986).

As a result of their unfavorable surface to volume ratio and the high metabolic rate, smaller tortoises are more vulnerable to dehydration (and starvation) than are older/larger individuals. The younger age classes are particularly vulnerable to short-term habitat degradation (occasional overgrazing by livestock) and drought. Immatures lack the lipid reserves and the proportionately larger urinary bladder that allow adults to endure several years of drought with very little effect on physiological homeostasis and reproduction.

Tortoises are effective in retaining water under desert conditions. They have some capacity to switch from water-demanding urea to more conserving uric acid for nitrogen waste elimination when such conservation is needed. In addition, they are more vulnerable to water loss during surface activity when their eyes are open, pulmonary gas exchange is rapid, and the head is extended than when resting or hibernating in a burrow (see Cloudsley-Thompson 1971, Minnich 1977, Schmidt-Nielsen and Bentley 1966, and Nagy and Medica 1986).

Habitat Type: Terrestrial
Non-Migrant: Y
Locally Migrant: Y
Long Distance Migrant: N
Mobility and Migration Comments: Home ranges (or paths), rather than defended territories, characterize the behavioral ecology of desert tortoises. In southern Nevada, minimum convex polygon (MCP) home ranges overlapped; MCP home ranges varied from 6 to 46 ha; area estimates corrected for number of sightings ranged from 13 to 72 ha (O'Connor et al. 1994). However, individuals may move several kilometers over several weeks or years (Auffenberg and Iverson 1979, Berry 1986). Home ranges of desert tortoises in Utah averaged 3.7-12 ha, with a lifetime home range estimate of 180 ha (USFWS 1994). MCP-estimated home ranges may include significant areas that are not used. O'Connor et al. (1994) suggested that home ranges be considered as indicators of movement scales and patterns rather than as estimations of the areas actually used. Home ranges as actual measures of habitat use (White and Garrott 1990) may be better conceptualized as home paths, narrow linear routes, sometimes involving several burrows (see Wilson et al. 1994 for juvenile gopher tortoises, and Auffenberg 1969). These linear routes may traverse several hundred meters (>1000 ft) between outlying points, at least for adults (USFW 1994, Appendix C). Fidelity to these routes is often incomplete, seasonal, and temporary, but they may minimize predation, maximize the use of a favorable thermal mosaic, and optimize foraging by stimulating new growth along a repeatedly grazed route (see Bjorndal 1982).

Home ranges of adults are typically larger than those of juveniles but do not exhibit a simple positive relationship with body size (O'Connor et al. 1994). Estimates of male home ranges (25 ha) average about double those of females, though individual and seasonal variation may be considerable. In Joshua Tree National Park, multi-year home ranges (MCP) were 27-61 ha (mean 44 ha) for males and 3-14 ha (mean 10 ha) for females (Freilich et al. 2000). Production of spring annuals (kg dry mass/ha) correlates negatively with home range size (USFWS 1994). Increase from one to 20 kg/ha reduces mean home range size from about 30 ha to about 5 ha. In its lifetime a tortoise may use 1.5 square miles of habitat and make forays of more than 7 miles at a time.

In Utah, tortoises migrated relatively short distances from winter hibernacula to summer feeding grounds.

Terrestrial Habitat(s): Desert
Special Habitat Factors: Burrowing in or using soil
Habitat Comments: This tortoise is almost entirely confined to warm creosote bush (Larrea tridentata) vegetation characteristic of the Upper Sonoran life zones of the Mohave and Colorado deserts. Specific habitat associations vary geographically, as do substrate preferences. In the Mohave Desert, the tortoise occurs in creosote scrub, creosote bursage (Ambrosia dumosa), shadscale (Atriplex) scrub, Joshua tree (Yucca brevifolia) park, and, more rarely (in the northern periphery of their range), in mixed blackbush scrub between 3,500-5,000 feet elevation. In the warmer and lower Colorado Desert, tortoises generally are confined to creosote scrub and wash woodland habitats. Often native desert grasses, especially galleta (Hilaria/Plueraphis) and indian rice grass are associated with high tortoise densities, and the former species provides significant forage for adults. Exotic Mediterranean weed grasses (Schizmus and Bromus) are abundant across the Mohave Desert.

Most often tortoise habitats are associated with well drained sandy loam soils in plains, alluvial fans, and bajadas, though tortoises occasionally occur in dunes, edges of basaltic flow and other rock outcrops, and in well drained and vegetated alkali flats. In the Mohave Desert, sandy loam soils may be obscured by a surface of igneous pebbles or a veneer of desert pavement. Tortoise burrows are most often proximate to washes and arroyos in this Mohave Desert habitat (Woodbury and Hardy 1948, Luckenbach 1982). However, north of St. George, Utah, they occur in burrows excavated directly into cliffs of red sandstone.

Landscape features affect distribution and dispersal. "Gene flow among desert tortoise populations is at least partially restricted by large topographic features such as high-elevation mountain ranges (e.g., Spring Mountains, New York Mountains, Providence Mountains) and very low elevation regions (e.g., Death Valley, Cadiz Valley)" (Hagerty et al. 2011). "Elevation appears to be an important determinant of these partial barriers, but it is an indirect measure of several variables, including thermal environment, soil type, and vegetation assemblages...areas with extremely high or low elevations likely impose thermal constraints..., provide suboptimal vegetative cover, and physically impair movements" (Hagerty et al. 2011).

Tortoises are often subterranean when inactive, which is about 98% of their total life span (Nagy and Medica 1986). Typically they utilize and/or excavate shelters of four different types: burrows, dens, pallets, and nonburrows (Burge 1978). "Summer" burrows of adults are subterranean excavations usually constructed by the tortoises themselves; typically the openings face north (or NE, NW) and the burrows are larger and longer than the tortoise, often extending one to eight feet with a mean floor declination of 15 +/- 4.0 degrees, and having a single opening. Burrows tend to be longest and deepest in the northern part of the range (Burge 1978, Woodbury and Hardy 1948). Most often they open under a shrub; in the Mojave Desert, creosote bushes provided cover for 58.5-77.2% of the burrow apertures, while bursage cover accounted for another 21%. In the Mohave Desert, most burrows of juveniles were under large shrubs (Wilson et al. 1999).

Winter burrows, or more properly, dens, are generally more extensive, up to 30 feet long, and are more often subject to communal use (by up to 17 tortoises, Woodbury and Hardy 1948). In southern Nevada, December temperatures taken 7 feet deep into the passage varied only a few degrees and the lowest temperature recorded was about 3 C (Burge 1978). These dens typically open to a southern exposure and, in upland northern tortoise range, dens typically are excavated beneath caliche or sandstone rock shelves along wash banks. These same dens may hold air masses with stable, high relative humdities reaching 40% (Woodbury and Hardy 1948). These northern dens, unlike summer burrows, often are enhanced by chambers and even interconnections between dens.

Pallets are shallow excavations which often barely cover the tortoise. They provide common summer shelters throughout most of the range (Aufffenberg 1969). Pallets generally are placed under the cover of shrubs. Summer burrows and pallets are particularly fragile and vulnerable to the erosive effects of livestock hoofs, rodent excavation, wind, and rain (Coombs 1977).

Nonburrows are often in shaded rest areas, formed in depressed or compressed vegetation and soil. Mormon tea (Ephedra) often is utilized by Mohave Desert tortoises for such shelters.

Multiple burrow use is common in tortoises, with several burrows utilized by one individual in a single week. In southern Nevada, semicaptive and/or relocated individuals used an average of nine different burrows and 35% of these were used by other tortoises (Bulova 1994). Likewise, winter dens commonly are shared, and even summer burrows temporarily may be cohabited by a mating pair (Bulova 1994). Juveniles also commonly share burrows when confined to enclosures (Spangenberg, pers. comm.) but rarely do so when free ranging. Therefore, a count of active burrows may not accurately represent local population numbers or densities.

In spring, juveniles are in or near burrows with westerly to southeasterly openings, especially under bushes. During July-October in the lower Mohave Desert, tortoises occupied burrows during the hot midday period but typically did not sleep in burrows at night when surface air temperatures were cooler than the soil; at those times most tortoises rested on the surface under bushes at night (McGinnis and Voigt 1978, Zimmerman et al. 1994).

Eggs are laid in shallow depressions, often 3-4 inches deep. Mohave desert tortoises commonly construct egg nests inside the most superficial two feet of the burrow floor, in the soil apron immediately surrounding the burrow aperture, or under the shade of a shrub adjacent to the burrow. Near St. George, Utah adults descended from sandstone cliff burrows to excavate egg nests in sandy soil washes below (M. Coffen, pers. comm.). Nests usually are placed in well-drained, friable soils. The range of tolerance for nesting conditions for eastern Mohave Desert tortoises has been ascertained experimentally (Spotila et al. 1994). Incubation temperatures should be above 26 C and below 35 C. Temperatures beyond this range usually prove lethal regardless of humidity. Natural nest sites in Nevada often exhibit as much as a 5 C range over the incubation period (Muller, pers. comm.). Incubation soil moisture of 4% is lethal at temperatures of 26 C and probably at 33 C as well, whereas dry soils (0.4% moisture) do not compromise hatching success at 33 C. Eighty-three percent of neonatal tortoises excavated new burrows or enlarged pre-existing rodent burrows during the first weeks following their September emergence from nests.

Habitat Models

Schamberger and Turner (1986) presented a habitat suitability model that showed suitability (1) increasing with yearly mean net production of annuals and grasses, (2) maximized in sandy loam, light gravel, and heavy gravel soils, (3) maximal at an annual rainfall of approximately 20 cm, and (4) maximal in creosote vegetation and becoming progressively less suitable in cactus scrub, shadscale, Joshua tree, and alkali scrub vegetation types.

Nussear et al. (2009) also modeled desert tortoise habitat. Sixteen environmental data layers representing four major categories (landscape, climate, biotic, and soils) were converted into a grid covering the study area (Mohave Desert and part of the Sonoran Desert in CA, NV, UT, and AZ) and merged with the desert tortoise presence data. Final environmental data layers used in the model included: mean dry season (May-October) precipitation, mean wet season November-April) precipitation, elevation, average surface roughness, percent smooth, average soil bulk density, depth to bedrock, average percentage of rocks > 254 mm B-axis diameter, and perennial plant cover. These data were input into the Maxent habitat-modeling algorithm. The model provided output of the statistical probability of habitat potential and was used to map potential areas of desert tortoise habitat. The analysis was robust in its predictions of habitat, but did not account for anthropogenic changes that may have altered habitat with relatively high potential into areas with lower potential.

Adult Food Habits: Herbivore
Immature Food Habits: Herbivore
Food Comments: Tortoises forage primarily on native winter and summer annuals (dicots and grasses), perennial grasses, cacti, and other vegetation, including a few perennial shrubs. Insects also may be eaten, and caterpillars and other insect larvae may occasionally provide rich lipid and protein supplements to an otherwise vegetarian diet; these may be especially valuable to juvenile growth (Avery, pers. comm.).

Annual grasses important in the diet are largely exotic species, part of the Mediterranean "weedland" that dominates spring growth in much of the western Mohave Desert (Berry 1984). Perennial grasses, largely native, contribute more to shelter, soil retention, and a longer growing season. One of the few shrubs regularly ingested is the herbaceous Sphaeralcea ambigua (Berry 1978). Succulent buds, flowers, and fruit are also ingested.

Hatchling or neonate (first year) congeneric gopher tortoises conserved about half of the lipid energy in their residual yolk at the time of hatching (Linley and Mushinsky 1994). In most cases these reserves are significant and could provide energy for early growth and first burrow construction or elaboration, even during a dry fall hatching season when little or no palatable forage is available. Hatchlings have been known to overwinter in their egg nests before emergence, sustained by residual yolk lipids. Another source of hatchling nutrition (in the absence of fresh forage) is feces of conspecific adults. Coprophagy of adult tortoise feces typified the first feedings of hatchlings (Joyner Griffith, pers. comm.). Such coprophagy may provide bacterial protein, inoculation of mutualistic fermenting anaerobes (Clostridium) that assist in later cellulose digestion (Dezfulian et al. 1994), and may provide rich supplies of calcium, magnesium, and vitamin B complex as they do for small coprophagous mammals. In the eastern and Mexican portions of tortoise range, fall forage is available to emergent hatchlings as a result of late summer monsoonal rains. More general proclivities toward coprophagy have also been reported for adults; these cases may involve ingestion of tortoise feces and wild and domestic mammal feces as well (Hohman and Omart 1980, Grover and DeFalco 1995).

Diet may change in response to the changing abundance of food items with different seasons. Tortoise diets also are influenced by the differences in available forage between wet and dry years. Wet springs and by extension, wet years, provide a greater biomass of annuals for a longer period of time. Variation in diet is also related to location and habitat (USFWS 1994:24-25). Generally, annuals dominate spring diets, while dry grasses (Oftendal et al. 1995; Oftedal 2002) and cactus dominate the summer diet. Dietary potassium affects the choice of food items, seasonal shifts in choices, seasonality of feeding, nitrogen retention, growth, with high loads inhibiting many of these processes.

Adult Phenology: Diurnal, Hibernates/aestivates
Immature Phenology: Diurnal, Hibernates/aestivates
Phenology Comments: Activity period varies by region, sex, and age class. Adults typically are active March through fall (Behler and King 1979). Some may estivate during dry periods in summer. In the Mohave Desert, the total active period for adults is about 4-5 months/year, mainly in spring and fall.

In spring in Nevada, tortoises were active about 3 hours every fourth day (Nagy and Medica 1986). Some tortoises did not feed for several weeks following spring emergence (Nagy and Medica 1986). Morning activity may begin as early as 0500 h (Berry 1975), but more often after 0700 h. Morning activity typically is more frequent and extensive than at other times. Surface activity, when evidenced between October and early April, is typically unimodal and may extend from 0800 to 1700 hrs in the Mohave Desert of Nevada (Ruby et al. 1994). From late April through September in the Mohave Desert, activity tends to be bimodal, bouts being punctuated by a 1100-1500 or 1600 h retreat from the hot surface soils into cooler burrows. Individual foraging bouts may run from one to seventy-five minutes.

Individuals may emerge from burrows when temperatures and precipitation are favorable. Emergence may occur during the heat of late summer in response to a thunderstorm. Sometimes emergence occurs at night in response to rain. Given suitable conditions, opportunistic activity may occur in winter as well. Late January emergence of juveniles has been observed regularly in the central Mohave Desert at Fort Irwin (Joyner Griffith, Spangenberg, pers. comm.). Perhaps the surface to volume ratio of small juveniles is more favorable to rapid heating of short-day warming opportunities in winter; this also may facilitate activity relatively early in the morning in summer.

Though precipitation and temperature largely govern the timing and extent of tortoise surface (epigean) behavior, tortoises do not exhibit the finely tuned thermoregulatory shuttling behavior around an eccritic temperature that is characteristic of heliothermic lizards. Rather, desert tortoises tend to operate within 25-35 C range of body temperatures (Zimmerman et al. 1994). Supercooling (Lowe and Halpern 1969) has been experimentally established for this species, indicating that they share the widespread reptilian tolerance level of about 20 F (-6 C).

Economic Attributes Not yet assessed
Management Summary
Stewardship Overview: The recovery actions for each strategic element in the recovery plan (USFWS 2011) are as follows: "1. Develop, Support, and Build Partnerships to Facilitate Recovery. 1.1. Establish regional, inter-organizational Recovery Implementation Teams to prioritize and coordinate implementation of recovery actions. 2. Protect Existing Populations and Habitat. 2.1. Conserve intact desert tortoise habitat. 2.2. Minimize factors contributing to disease (particularly upper respiratory tract disease). 2.3. Establish/continue environmental education programs. 2.4. Increase law enforcement. 2.5. Restrict, designate, close, and fence roads. 2.6. Restore desert tortoise habitat. 2.7. Install and maintain urban or other barriers. 2.8. Sign and fence boundaries of sensitive or impacted areas. 2.9. Secure lands/habitat for conservation. 2.10. Restrict off-highway vehicle events within desert tortoise habitat. 2.11. Connect functional habitat. 2.12. Limit mining and minimize its effects. 2.13. Limit landfills and their effects. 2.14. Minimize excessive predation on tortoises. 2.15. Minimize impacts to tortoises from horses and burros. 2.16. Minimize impacts to tortoises from livestock grazing. 3. Augment Depleted Populations through a Strategic Program. 3.1. Develop protocols and guidelines for the population augmentation program, including those specific to head-starting and translocation. 3.2. Identify sites at which to implement population augmentation efforts. 3.3. Secure facilities and obtain tortoises for use in augmentation efforts. 3.4. Implement translocations in target areas to augment populations using a scientifically rigorous, research-based approach. 4. Monitor Progress toward Recovery. 4.1. Monitor desert tortoise population growth. 4.2. Monitor the extent of tortoise distribution in each recovery unit. 4.3. Track changes in the quantity and quality of desert tortoise habitat. 4.4. Quantify the presence and intensity of threats to the desert tortoise across the landscape. 5. Conduct applied research and modeling in support of recovery efforts within a strategic framework. 5.1. Determine factors that influence the distribution of desert tortoises. 5.2. Conduct research on the restoration of desert tortoise habitat. 5.3. Improve models of threats, threat mitigation, and desert tortoise demographics. 5.4. Conduct research on desert tortoise diseases and their effects on tortoise populations. 5.5. Determine the importance of corridors and physical barriers to desert tortoise distribution and gene flow. 6. Implement an Adaptive Management Program. 6.1. Revise and continue development of a recovery decision support system. 6.2. Develop/revise recovery action plans. 6.3. Amend land use plans, habitat management plans, and other plans as needed to implement recovery actions. 6.4. Incorporate scientific advice for recovery through the Science Advisory Committee."
Restoration Potential: Veterinary treatment of illness, translocations, restocking, and artificial watering of habitats for forage enrichment all appear to be experimental, costly, often ineffective, and, in some cases, detrimental. Although this species is easily maintained and bred in captive and/or confined circumstances (Booth and Buskirk 1988, Spotila et al. 1994), and the appropriate husbandry is available to develop restocking programs without the risk or disruption of translocating wild tortoises, and large pools of captive tortoises are available for relocation into depleted natural habitats, pilot studies in California and Utah have not been encouraging (St. Amant and Hoover 1978, Minden and Metzger 1981). More importantly, some past translocation/restocking efforts may have introduced or propagated the URTD epidemic through formerly captive hosts. Also, introduced tortoises may compete with native tortoises for limited resources, and interbreeding between native and introduced tortoises may disrupt locally coadapted gene complexes (Bury et al. 1994). Natural population recovery is slow, with best case scenarios projecting 1% population growth per year or a doubling time of 70 years; at the more realistic rate of 0.5%, the doubling time increases to 140 years (USFWS 1994). Germano and Joyner (1988) reported that tortoises in the Piute Basin recovered from short-term high mortality.

Restoration of the degraded desert ecosystems supporting tortoise populations is both a slow and uncertain process. Without proven protocols for effective mitigation, no assurance may be made for re-establishing climax communities. Historical climatic regimes have been altered, water tables lowered irreversibly, and new exotic vegetation may preclude the restoration of native dominants.

No specific densities are required for delisting, but a stable population or one with a growth trend must be confirmed by repeated monitoring. A stable population would be confirmed by the following age class proportions: hatchlings about 22%, small juveniles 22%, larger juveniles 6%, subadults 8%, and adults 42%. Populations consisting entirely of adults or adults and hatchlings/neonatals may not be undergoing progressive replacement. Population viability, above a certain minimum, may be better evaluated by age class (and sex) distribution, than by absolute density.

Preserve Selection & Design Considerations: Major protection units (critical habitat blocks or Desert Wildlife Management Areas, DWMAs) should be capable of supporting metapopulations of 50,000 adults, according to some Minimum Viable Population (MVP) models (USFWS 1994). A population viability analysis by Brussard (1992) concluded that preserves or management areas should be large enough to support 20,000 adults. By genetic criteria alone a minimal adult population would require 5,000 adults (assuming an effective population size of 0.1 or 500) for continued viability (see Gilpin's model in USFWS 1994). Optimally these DWMAs should be individually 1,000 square miles in extent. Preserves should contain large uninterrupted and undisturbed blocks of high quality habitat (perennial grasses and native forbs) and should be interconnective, simply shaped polygons spread across representative habitats and regions.

See Britten et al. (1997) for information on the concordance between DWMAs and genetically distinct populations of tortoises in the northeastern Mojave Desert.

Berry (1986) recommended that areas to be restocked should be at least 14 km in diameter to permit dispersal.

Management Requirements: Active management may be required to maintain the viability of relatively small populations. Such management might entail frequent patrols and/or establishment of vehicular barriers to reduce destructive intrusion by humans, plus measures to address abnormally high levels of predation or excessive grazing by livestock.

An important management consideration is maintenance of the integrity of burrow systems, which are important in energy and water balance (Zimmerman et al. 1994); hence limitation of off-road vehicle use in tortoise habitat is warranted. Provision of burrows may facilitate adjustment of relocated individuals to the new area; captive tortoises readily use artificial burrows constructed of PVC pipe (Bulova 1992, 1994). However, release of captive tortoises into the wild is not a recommended conservation measure (Bury et al. 1994).

Studies by Ruby et al. (1994) indicate that fencing could be useful in reducing mortality by keeping tortoises off well-traveled highways. Tortoises readily enter undergound culverts, and these may be effective in allowing tortoises to move safely from one side of a fenced road to another (Ruby et al. 1994). Ruby et al. (1994) recommended 1-cm-mesh hardware cloth as an effective fence material but noted that other barriers also could be used.

In October 1989, BLM declared a special quarantine that closed 15,260 ha of the Mohave Desert southwest of Ridgecrest, California, to human use for one year (see Washington Post, 2 October 1989); this was in response to the epidemic respiratory infection, which may be spread in the wild population through the release of pet tortoises; the closure generated strong objections from developers, ranchers, and recreationists in California and Nevada (Matthews and Moseley 1990).

Conservation efforts that improve availability and abundance of annual plants can benefit juveniles (Nagy et al. 1997).

See Collins (1995) for a review of management and protection actions in Clark County, Nevada.

A population in the Black Mountain of Arizona, east of the Colorado River, is genetically and morphologically most similar to Mohavean populations west of the river and, despite contrary regulatory designation by USFWS, should be managed as such (McLuckie et al. 1999).

Monitoring Requirements: Adequate protection requires repeated and adequate monitoring for densities, age classes, forage, and health. In the past, several techniques involving strip transects (805-2400 m, Turner et al. 1982, Turner and Berry 1984), quadrate and grid systems (Bury and Luckenbach 1977), and permanent plots and mark and recapture estimations have been used to estimate tortoise density. In these surveys, sign (scat) and burrows as well as tortoises were often used to determine local densities. Such estimations may be distorted by weather, season, vegetation, decomposition, and tortoise behavior (especially the tendency to excavate and utilize more than one burrow) (Berry 1986, Fritts 1985). Furthermore, juvenile tortoises are especially refectory to observation (Adest et al. 1989, Morafka 1994) and particularly prone to excavate multiple burrows as well as utilize preformed and abandoned rodent burrows.

USFWS evaluated monitoring protocols that determine tortoise densities using distance weighted-sampling and the Zippen removal method (or maximum likelihood method described by Southwood 1978). In the procedure, a DWMA is divided into 1- sq-km plots using Universal Transverse Mecator coordinates. Plots encompassing disturbed areas or those over 4,000 ft elevation generally are excluded. A minimum of 10 sample plots is examined. At least three control plots 2-10 miles outside the DWMA boundary also are sampled. Then plots constituting 5% of the reserve area are surveyed by "removing"(marking) all tortoises 140 mm in mid- carapace length or greater. Adults also are sexed. This estimation method predicts that the rate at which new captures declines is directly related to the size of the total population and the total number previously removed/marked. Unlike more traditional mark-recapture methods, this approach requires tortoises to be handled only once. Tortoise densities for each sex are estimated along with their standard errors for each plot (USFWS 1994, Appendix A). A schedule implementing the above removal protocol would require sampling a random 5% of each Desert Wildlife Management Area (DWMA) every three years, during the months of February (Morafka recommends surveying no earlier than April 1) through May. At least 20 sample plots would be investigated in tandem with a minimum of three control plots outside the reserve. The control plot comparisons would continue through a minimum of five samples (12 years) in order to establish statistically valid trends in the effects of DWMA management (USFWS 1994). During a twelve-year period with a minimum of five sampling cycles, statistical comparisons of density trends within and outside the DWMA allow evaluation of the hypothesis that DWMA protection significantly increases tortoise densities over unprotected controls. Support of the hypothesis would confirm the efficacy of the reserve, while rejection, and/or the establishment of a negative correlation, might justify more stringent protection for the DWMA and the extension of its protection for a longer period of time.

Freilich et al. (2000) evaluated factors affecting population assessments and found that desert tortoises are likely to be undercounted in dry years.

A serological test has been developed to confirm the presence of blood antibodies to the URTD pathogen, but no effective cure for the disease is available (Schumacher et al. 1993). Proper health monitoring entails the establishment of baseline values (adjusted for age, sex, and season, see O'Connor et al. 1994) for healthy tortoises before blood panels are used for diagnostic monitoring purposes. Seropositive tortoises, even those with nasoepithelial lesions, are sometimes externally asymptomatic (Jacobson et al. 1995). Infections may be suppressed by Baytril treatments, but poorly vascularized regions, such as nasal epithelium often serve as reservoirs for re-infection. Symptoms include clear wet discharges from eyes and nose (the latter often generating nasal bubbles), loss of weight, and wheezing. Translucent, opaque, or colored discharges generally indicate the presence of other, largely bacterial, infections. Infection of mycoplasma may be achieved the transmission of moist discharges from an original host, nose to nose encounters involving aerosol exchanges, or cophrophagy. The origin of this infection is unknown though the concentration of infected individuals around urban areas raises the possibility of its introduction through the field release of former captives, or even by infections from other domesticated species. The capacity of tortoises to develop immunological and genetic resistance is unknown. See Jacobson et al. (1992) for information on blood sampling methods.

Peterson (1994) suggested the following criteria for monitoring and perhaps defining health: the body mass to the cube of carapace length ratio, and blood chemistry panel values for plasma osmolarity, CPK (creatinine phosphokinase), blood potassium, and BUN (blood urea nitrogen).

Germano (1988) reproted that scute annuli can be used to age individuals up to 20-25 years, but Tracy and Tracy (1995) found that this technique may not be accurate and urged caution in the use of scute-ring counts to estimate age.

See Blankenship et al. (1990) for information on a method for tracking tortoises using fluorescent powder.

Management Programs: TNC has been active in acquiring and retiring grazing privileges on BLM-administered lands that include high-quality tortoise habitat.

See End. Sp. Tech. Bull., Sept./Dec. 1991, for information on BLM's proposed licensing of livestock use on public land in tortoise habitat in southern Nevada; one prescription (for 726,390 ha) restricts grazing from March 1 to June 14, in order to reduce trampling and forage competition, whereas the other prescription (for 557,085 ha) includes no seasonal restriction on grazing (USFWS issued a no-jeopardy biological opinion).

Management Research Programs: The following bibliographies and review articles summarize knowledge and technical trends in both desert tortoise biology and management (through the mid-1990s): Auffenberg (1969), Auffenberg and Franz (1978), Auffenberg and Iverson (1979), Beaman et al (1989), Bury (1982), Bury and Germano (1994), Douglass (1975, 1977), Duck (1988), Grover and DeFalco (1995), Hohman et al (1980), Johnson et al (1990).
Management Research Needs: Management still needs considerable information on tortoise habitat needs and climate-specific carrying capacities, the role of both natural and "subsidized" predation (especially raven and coyote on juveniles, the roles of dumps and utility corridors in amplifying predator densities, and predation of eggs and juveniles by foxes), URTD and shell rot disease (including causes, prevention/prophylaxis, and possibly treatments), general survivorship (especially for eggs and juvenile age-size classes), the impact of exotic annuals on both nutrition and fire, and the impacts of feral equines and livestock grazing on tortoise nutrition, health, and reproduction, and the interrelated effects of soil erosion and compaction.

Public education programs for culturally diverse, new and growing human populations interacting with tortoises need to be developed for both local school systems (or integrated into existing programs like Operation WILD) and for the general public information and interpretation programs of the National and state park services, USFWS, state wildlife agencies, and the Bureau of Land Management.

Effective modeling for optimal habitat is still in its pioneer stages, but could become a valuable tool. Such models need to define and locally apply successional processes in desert ecosystems, particularly disclimax and secondary succession. If unstable climates and the invasion of exotic annuals precludes modeling of classical succession, alternative processes of recovery need to be described.

The relationship of tortoise health and demographics to recovery processes needs to characterized. Only when these processes and relationships are understood will it be possible to design programs that restore disturbed and degraded tortoise habitats. This latter endeavor should become a primary mission for management-oriented research. Methods for accurately determining the health status of individual torpoises need to be developed; clinical appearance alone is not enough (Jacobson et al. 1995).

Monitoring of tortoise densities and movements needs to be refined to improve accuracy and cost efficiency. For the former, evaluation of the Zippen removal method and distance-weighted sampling must be completed. For the latter, costly but effective radiotelemetry must be compared to the lower cost, but low- reception-range Passive Integrated Transponders (PITs) or Radio Frequency Tags (RFTs) (Dixon and Yanosky 1993) and the longer range RECCO (a Swedish rescue system, using a transmitter and reflecting diodes, originally to locate humans lost in avalanches). Chemically marked trailing systems (Blankenship et al. 1990) need further evaluation as well.

Biological Research Needs: Future research needs to determine minimum viable population sizes in various habitat types, nutritional forage quantity and quality needs, the juvenile niche, nest microhabitat requirements, TSD as determined by field nest temperature cycles (not fixed incubation values), and mating systems in nature. Futher taxonoic research is also needed.
Population/Occurrence Delineation
Use Class: Not applicable
Minimum Criteria for an Occurrence: Occurrences are based on evidence of historical presence, or current and likely recurring presence, at a given location. Such evidence minimally includes collection or reliable observation and documentation of one or more individuals (including eggs) in or near appropriate habitat where the species is presumed to be established and breeding.
Separation Barriers: Busy highway or highway with obstructions such that turtles rarely if ever cross successfully; untraversable topography (e.g., cliff); major river, lake, pond, or deep marsh; urbanized area dominated by buildings and pavement; ladscapes with elevations higher than 6,000 feet.
Separation Distance for Unsuitable Habitat: 1 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Annual home ranges generally are less than 50 ha; one estimate of lifetime home range size was 180 ha (see Migration/Mobility comments). However, individuals may move several kilometers over several weeks or years (Auffenberg and Iverson 1979; Berry 1986; Barrett 1990; Edwards et al. 2004, Herpetol. Rev. 35:381-382). Separation distance reflects occasional long-distance dispersal but is restricted such that occurrences do not become too large for practical conservation use.

Populations are typically uneven in density and often discontinuously distributed. This is particularly true of the upland "island" populations of the Sonoran Desert (Dodd 1982). Even in relatively undisturbed expanses of good lowland Mojave Desert habitat high density clusters are separated by low densities or even total absence. The minimal population unit, or deme, could be as small as 10-20 adults. Intervening habitat supporting less than 10 adult tortoises/sq mi could effectively isolate, at least behaviorally, such patches. Such patches, estimated by the collective home ranges, and allowing for partial overlap, might cover 500-1,000 hectares. Larger demographic units could be defined in terms of clusters of these demes isolated by topographic barriers, namely uplands higher than 4,000 to 5,200 feet (Yucca Mt., Nevada) in the Mojave Desert and paradoxically, valleys below 2,000 feet elevation in the Sonoran Desert. [This paragraph by D. Morafka.]

Inferred Minimum Extent of Habitat Use (when actual extent is unknown): 1 km
Date: 27Apr2005
Author: Hammerson, G.
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
NatureServe Conservation Status Factors Edition Date: 06Sep2013
NatureServe Conservation Status Factors Author: Hammerson, G.
Management Information Edition Date: 18Aug2011
Management Information Edition Author: Morafka, D. J. (deceased). Edited and updated by G. Hammerson
Management Information Acknowledgments: Much of Morafka's experience with desert tortoise biology was developed since 1989 through professional and financial support of several environmentally responsible agencies and their personnel. He particularly acknowledged the support of Southern California Edison, Inc. (J. Palmer), the Department of Public Works at the National Training Center, Fort Irwin, California (S. Ahmann), the National Biological Service (K. Berry), the U.S. Fish and Wildlife Service Desert Tortoise Recovery Team (P. Brussard, C. R. Tracy, C. Schwalbe, K. Berry, M. Gilpin, E. Jacobson, and F. Vasek, along with USFWS staff, especially J. Hohman and C. Mullen). He also expressed thanks to his present and past graduate students: R. Yates, M. Joyner, E. Trevino, C. Okomoto, and K. Spangenberg. D. J. Germano, T. Esque and H. Avery brought important documents and issues to his attention.
Element Ecology & Life History Edition Date: 26Aug2011
Element Ecology & Life History Author(s): Morafka, D. J. (deceased), and G. Hammerson

Zoological data developed by NatureServe and its network of natural heritage programs (see Local Programs) and other contributors and cooperators (see Sources).

  • Auffenberg, W. and R. Franz. 1978. Gopherus agassizii. Cat. Am. Amph. Rep. 212.1-212.2.

  • Auffenberg, W., and J. B. Iverson. 1979. Demography of terrestrial turtles. Pages 541-569 in M. Harless and H. Morlock, editors. Turtles: perspectives and research. John Wiley & Sons, New York.

  • Berry, K. H. 1975. Desert Tortoise relocation project: status report for 1973. Desert Tortoise Relocation Project. Division of Highways, CA. Contract F-9353.

  • Berry, K. H. 1978. Livestock grazing and the Desert Tortoise. Pp. 505-19 in Transactions of the 43rd North American Wildlife and Natural Resources Conference; Phoenix, AZ; 1978 March 18-22. Wildlife Management Institute, Washington, DC.

  • Berry, K. H. 1984. Status of the desert tortoise in the United States. Report from the Desert Tortoise Council. U.S. Fish and Wildlife Service, Sacramento, CA. Order No. 11310-83-81. 848 pp.

  • Berry, K. H. 1986a. Desert tortoise (Gopherus agassizii) research in California, 1976-1985. Herpetologica 42:62-67.

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