Gopherus agassizii

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.

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.

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 ricegrass 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.

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).

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.