Search Wildlife
≡
Photo by Lynn Chamberlain
Photo Copyright Lynn Chamberlain
Mojave Desert Tortoise
Gopherus agassizii
NatureServe conservation status
Global (G-rank): G3
State (S-rank): S2
- Reason: This species occurs in Utah only in southern Washington County, mostly an area of about 80 square miles in the southwestern part of that county. The population in Utah is subject to an array of threats and is experiencing high mortality. Population decline in Utah has been great, at least in the past.
External links
General information
The desert tortoise, Gopherus agassizii, is found in southwest Utah, southern Nevada, southeastern California, and western Arizona. Due to genetic, morphometric, and behavioral differences, the desert tortoise in Utah, commonly referred to the Mojave desert tortoise, is a different species than Sonoran desert tortoise (Gopherus morafkai), found south and east of the Colorado River. The Mojave desert tortoise is federally protected under the Endangered Species Act as a threatened species.
Desert tortoise populations are vulnerable to habitat loss or degradation because of their life history. They are long lived vertebrates, with life span estimates of 50 to 80 years, and have delayed sexual maturity, low reproductive rates, and high juvenile mortality. To regulate body temperature and reduce water loss, they spend a majority of time in shelters such as soil burrows, caves, rock shelters, and pallets. Within its range, the desert tortoise can be found in grasslands, canyon bottoms, and rocky hillsides. Females nest under a large shrub or at the mouth of a burrow, and lay one to three clutches of up to ten eggs from May to July; eggs hatch in late summer or fall.
The desert tortoise is an herbivore and its diet consists of perennial grasses, annual wildflowers, shrubs, and cacti including the pads, fruit and flower.
Phenology
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).
Species range
The species is restricted to the southern half of Washington County in the southwestern corner of the state.
Migration
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.
Habitat
Diverse desert scrub habitats are occupied, including those dominated by creosote bush (Larrea tridentata), white bursage (Ambrosia dumosa), blackbrush (Coleogyne ramosissima), and sagebrush (Artemisia spp.). Local distribution is determined in part by the availability of natural recesses suitable for use as den sites.
Food habits
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.
Ecology
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).
Reproductive characteristics
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.
Threats or limiting factors
Threats to population viability are numerous and diverse (see discussion in USFWS 1994c). Livestock (cattle) grazing is a threat to population viability (Coombs 1977, Berry 1978) through competition for food as well as trampling of food resources, dens, and young. Habitat fragmentation and loss through development is also an important threat. Increased predation rates by common ravens (Corvus corax) may be correlated with urbanization and agricultural development in desert habitat. The prevalence of Upper Respiratory Tract Disease is increasing in most populations and may result in a dramatically increased mortality rate. Other serious threats include predation by domestic dogs, road mortality, and illegal collection.
References
- Biotics Database. 2005. Utah Division of Wildlife Resources, NatureServe, and the network of Natural Heritage Programs and Conservation Data Centers.
- U.S. Fish and Wildlife Service. 1994. Desert tortoise (Mojave population) Recovery Plan. Portland, Oregon: U.S. Fish and Wildl. Serv., 73 pp. plus appendices.
- U.S. Fish and Wildlife Service. 2011. Revised recovery plan for the Mojave population of the desert tortoise (Gopherus agassizii). U.S. Fish and Wildlife Service, Pacific Southwest Region, Sacramento, California. 222pp.
