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|Agassiz's desert tortoise, G. agassizii|
|Agassiz's desert tortoise, G. agassizii|
The desert tortoises (Gopherus agassizii and Gopherus morafkai) are species of tortoise native to the Mojave desert and Sonoran desert of the southwestern United States and northwestern Mexico and the Sinaloan thornscrub of northwestern Mexico. Gopherus agassizii is distributed in western Arizona, southeastern California, southern Nevada, and southwestern Utah. The species name agassizii is in honor of Swiss-American zoologist Jean Louis Rodolphe Agassiz. Recently, on the basis of DNA, geographic, and behavioral differences between desert tortoises east and west of the Colorado River, it was decided that two species of desert tortoises exist: the Agassiz's desert tortoise (Gopherus agassizii) and Morafka's desert tortoise (Gopherus morafkai). Gopherus morafkai occurs east of the Colorado River in Arizona as well as in the states of Sonora, Sinaloa, Mexico. This species may be a composite of two species.
The desert tortoises live approximately 50 to 80 years; they grow slowly and generally have low reproductive rates. They spend most of time in burrows, rock shelters, and pallets to regulate body temperature and reduce water loss. They are most active after seasonal rains and are inactive during most of the year. This inactivity helps reducing water loss during hot periods, whereas winter hibernation facilitates survival during freezing temperatures and low food availability. Desert tortoises can tolerate water, salt, and energy imbalances on a daily basis, which increases their lifespan.
These tortoises may attain a length of 10 to 14 inches (25 to 36 cm), with males being slightly larger than females. Male tortoises have a longer gular horn than females, their plastron (lower shell) is concave compared to female tortoises. Males have larger tails than females do. Their shells are high-domed, and greenish-tan to dark brown in color. Desert tortoises can grow up to 4–6 in (10–15 cm) in height. They can range in weight from 11 to 23 kilograms (24 to 51 lb). The front limbs have sharp, claw-like scales and are flattened for digging. Back legs are skinnier and very long.
Desert tortoises can live in areas with ground temperatures exceeding 140 °F (60 °C) because of their ability to dig underground burrows and escape the heat. At least 95% of their life is spent in burrows. There, it is also protected from freezing winter weather while dormant, from November through February or March. With its burrow, this tortoise creates a subterranean environment that can be beneficial to other reptiles, mammals, birds and invertebrates.
Scientists have divided the desert tortoise into two types: the Agassiz's and Morafka's Desert tortoises, with a possible third type in northern Sinaloan and southern Sonora, Mexico. An isolated population of Agassiz's desert tortoise occurs in the Black Mountains of northwestern Arizona. They live in a different type of habitat, from sandy flats to rocky foothills. They have a strong proclivity in the Mojave desert for alluvial fans, washes and canyons where more suitable soils for den construction might be found. They range from near sea level to around 3,500 feet (1,100 m) in elevation. Tortoises show very strong site fidelity, and have well established home ranges where they know where their food, water and mineral resources are, and who their neighbors are.
Desert tortoises inhabit elevations from below mean sea level in Death Valley to 5,300 feet (1,615 m) in Arizona, though they are most common from around 1,000 to 3,500 feet (300-1,070 m). Estimates of densities vary from less than 8 individuals/km² on sites in southern California to over 500 individuals/km² in the western Mojave Desert, although most estimates are less than 150 individuals/km². The home range generally average from 10 to 100 acres (4–40 ha). In general, males have larger home ranges than females, and home range size increases with increasing resources and rainfall.
Desert tortoises are sensitive to the soil type, owing to their reliance on burrows for shelter, reduction of water loss, and regulation of body temperature. The soil should crumble easily during digging and firm enough to resist collapse. Desert tortoises prefer sandy loam soils with varying amounts of gravel and clay, and tend to avoid sands or soils with low water-holding capacity, excess salts, or low resistance to flooding. They may consume soil to maintain adequate calcium levels and therefore may prefer sites with higher calcium content.
Desert tortoises spend most of their lives in burrows, rock shelters, and pallets to regulate body temperature and reduce water loss. Burrows are tunnels dug into soil by desert tortoises or other animals, rock shelters are spaces protected by rocks and/or boulders, and pallets are depressions in the soil. The use of the various shelter types is related to their availability and climate. The number of burrows used, the extent of repetitive use, and the occurrence of burrow sharing are variable. Males tend to occupy deeper burrows than females. Seasonal trends in burrow use are influenced by desert tortoise gender and regional variation. Desert tortoise shelter sites are often associated with plant or rock cover. Desert tortoises often lay their eggs in nests dug in sufficiently deep soil at the entrance of burrows or under shrubs. Nests are typically 3 to 10 inches (8–25 cm) deep.
Shelters are important for controlling body temperature and water regulation, as they allow desert tortoises to slow their rate of heating in summer and provide protection from cold during the winter. The humidity within burrows prevents dehydration. Burrows also provide protection from predators. The availability of adequate burrow sites influences desert tortoise densities.
The number of burrows used by desert tortoises varies spatially and temporally, from approximately 5 to 25 per year. Some burrows are used repeatedly, sometimes for several consecutive years. Desert tortoises share burrows with various mammals, reptiles, birds and invertebrates, such as white-tailed antelope squirrels (Ammospermophilus leucurus), woodrats (Neotoma spp), collared peccaries (Pecari tajacu), burrowing owls (Athene cunicularia), Gambel's quail (Callipepla gambelii), rattlesnakes (Crotalus spp.), gila monsters (Heloderma suspectum), beetles, spiders and scorpions. One burrow can host up to 23 desert tortoises – such sharing is more common for desert tortoises of opposite sexes than for desert tortoises of the same sex.
Tortoises mate in the spring and autumn. Male desert tortoises will grow two large white glands around the chin area, called chin glands, that signify mating season. A male will circle around female, biting its shell in the process. It will then climb upon the female and insert his penis (a white organ, usually only seen upon careful inspection during mating, as it is hidden inside the male and can only be coaxed out with sexual implication) into the vagina of a female, which is located around the tail. The male may make grunting noises once atop a female, and may move front legs up and down in a constant motion, as if playing a drum.
Months later, the female will lay a clutch of 4–8 hard-shelled eggs, which have the size and shape of ping-pong balls, usually in June or July. The eggs will hatch in August or September. Wild female tortoises produce 0–3 clutches a year depending on the climate. Their eggs incubate from 90 to 135 days; some eggs may overwinter and hatch the following spring. In a laboratory experiment, temperature influenced hatching rates and hatchling gender. Incubation temperatures from 81 to 88 °F (27–31 °C) resulted in hatching rates exceeding 83%, while incubation at 77 °F (25 °C) resulted in a 53% hatching rate. Incubation temperatures less than 88 °F (31 °C) resulted in all male clutches. Average incubation time decreased from 124.7 days at 77 °F to 78.2 days at 88 °F (31 °C).
The desert tortoise grows slowly, often taking 16 years or longer to reach about 8 inches (20 cm) in length. The growth rate varies with age, location, gender and precipitation. It can slow down from ca. 12 mm/year for ages 4–8 years to ca. 6.0 mm/year for ages 16 to 20 years. Males and females grow at similar rates; females can grow slightly faster when young, but males grow larger than females.
Desert tortoises generally reach reproductive maturity at age 15 to 20 years, when they are longer than 7 inches (18 cm), though 10-years-old reproductive females have been observed.
The activity depends on location, peaking in late spring for Mojave desert and in late summer to fall in Sonoran desert; some populations exhibit two activity peaks during one year. Desert tortoises hibernate during winters, roughly from November to February–April. Females begin hibernating later and emerge earlier than males; juveniles emerge from hibernation earlier than adults.
Temperature strongly influences desert tortoise activity level. Although desert tortoises can survive body temperatures from below freezing to over 104 °F (40 °C), most activity occurs at temperatures from 79 to 93 °F (26–34 °C). The influence of temperature is reflected in daily activity patterns, with desert tortoises often active late in the morning during spring and fall, early in the morning and late in the evening during the summer, and occasionally becoming active during relatively warm winter afternoons. The activity generally increases after rainfall.
Although desert tortoises spend the majority of their time in shelter, movements of up to 660 feet (200 m) per day are common. The common, comparatively short-distance movements presumably represent foraging activity, traveling between burrows, and possibly mate-seeking or other social behaviors. Long-distance movements could potentially represent dispersal into new areas and/or use of peripheral portions of the home range.
Desert tortoises can live well over 30 years, with estimates of lifespan varying from 32 to 80 years. Causes of mortality include predation, disease, human-related factors, and environmental factors such as drought, flooding, and fire.
The annual death rate of adults is typically a few percent, but is much higher for young desert tortoises. Only 2–5% of hatchlings are estimated to reach maturity. Estimates of survival from hatching to 1 year of age for Mojave desert tortoises range from 47 to 51%. Survivorship of Mojave desert tortoises from 1 to 4 years of age is 71–89%.
The desert tortoise is an herbivore. Grasses form the bulk of its diet, but it also eats herbs, annual wildflowers, and new growth of cacti, as well as their fruit and flowers. Rocks and soil are also ingested, perhaps as a means of maintaining intestinal digestive bacteria as a source of supplementary calcium or other minerals. As with birds, stones may also function as gastroliths, enabling more efficient digestion of plant material in the stomach.
Much of the tortoise’s water intake comes from moisture in the grasses and wildflowers they consume in the spring. A large urinary bladder can store over 40% of the tortoise's body weight in water, urea, uric acid and nitrogenous wastes. During very dry times they may give off waste as a white paste rather than a watery urine. During periods of adequate rainfall, they drink copiously from any pools they find, and eliminate solid urates. The tortoises can increase their body weight by up to 40% after copious drinking. Adult tortoises can survive a year or more without access to water. During the summer months and dry seasons, they rely on the water contained within cactus fruits and mesquite grass. To maintain sufficient water, it will reabsorb water in its bladder. It moves to its underground burrow in the morning to prevent water loss by evaporation where it is humid and moist compared to the arid above-ground.
Emptying the bladder is one of the defense mechanisms of this tortoise. This can leave the tortoise in a very vulnerable condition in dry areas, and they should not be alarmed, handled or picked up in the wild unless they are in imminent danger. If they must be handled, and their bladder is emptied, then water should be provided to restore the fluid in their body.
Ravens, gila monsters, kit foxes, badgers, roadrunners, coyotes, and fire ants are all natural predators of the desert tortoise. They prey on eggs, juveniles, which are 2–3 inches long with a thin, delicate shell, or in some cases adults. Ravens are hypothesized to cause significant levels of juvenile tortoise predation in some areas of the Mojave Desert – frequently near urbanized areas. The most significant threats to tortoises include urbanization, disease, habitat destruction and fragmentation, illegal collection and vandalism by humans, and habitat conversion from invasive plant species (Brassica tournefortii, Bromus rubens and Erodium spp.).
Desert tortoise populations in some areas have declined by as much as 90% since the 1980s, and the Mojave population is listed as threatened. It is unlawful to touch, harm, harass or collect wild desert tortoises. It is, however, possible to adopt captive tortoises through the Tortoise Adoption Program (TAP) in Arizona, Utah Division of Wildlife Resources Desert Tortoise Adoption Program in Utah, Joshua Tree Tortoise Rescue Project in California, or through Bureau of Land Management in Nevada. When adopted in Nevada, they will have a computer chip embedded on their back for reference. According to Arizona Game and Fish Commission Rule R12-4-407 A.1, they may be possessed if the tortoises are obtained from a captive source which is properly documented. Commission Order 43: Reptile Notes 3: one tortoise per family member.
The Fort Irwin National Training Center of the US Army recently (WHEN?) expanded into an area that was habitat for approximately 2,000 desert tortoises, and contained Critical Desert Tortoise Habitat, (a designation by the US Fish and Wildlife Service). In March 2008, about 650 tortoises were moved by helicopter and vehicle, up to 35 km away. The second phase of the translocation project will occur in the near future (WHEN?), despite legal challenges filed by the Center of Biological Diversity.
Another potential threat to the desert tortoise's habitat is a series of proposed wind and solar farms. As a result of the recent (WHEN?) legislation, solar energy companies have been making plans for huge projects in the desert regions of Arizona, California, Colorado, New Mexico, Nevada, and Utah. The requests submitted to the Bureau of Land Management total nearly 1,800,000 acres (7,300 km2).
In 2006, there was a proposal in California to build a landfill in Kern County, a site near the Desert Tortoise Natural Area (DTNA), to dump trash for Los Angeles residents. A landfill would attract many of the tortoise’s predators – ravens, rats, roadrunners and coyotes – which would threaten their population.
Concerns about the impacts of the Ivanpah Solar thermal project led the developers to hire some 100 biologists and spend US$22 million caring for the tortoises on or near the site during construction. Despite this, in a 2011 Revised Biological Assessment for the Ivanpah Solar Electric Generating System, the Bureau of Land Management anticipated the loss or significant degradation of 3,520 acres of tortoise habitat and the harm of 57–274 adult tortoises, 608 juveniles, and 236 eggs inside the work area, and 203 adult tortoises and 1,541 juvenile tortoises outside the work area. The BLM expects that most of the juvenile tortoises on the project will be killed.
The Desert Tortoise Preserve Committee protects approximately 5,000 acres of desert tortoise habitat from human activity. This area includes 4,340 acres in Kern County, 710 acres in San Bernardino County, and 80 acres in Riverside County.
In the summer of 2010, Public Employees for Environmental Responsibility (PEER) filed a lawsuit against the National Park Service for not having taken measures to manage tortoise hunting in the Mojave National Preserve of California. Biologists discovered numerous gunshot wounds on dead turtle shells. These shells left behind by hunters attracted ravens and threatened the healthy tortoises.
Reptiles are known to become infected by a wide range of pathogens, which includes viruses, bacteria, fungi and parasites. More specifically, the Gopherus agassizii population has been negatively impacted by upper respiratory tract disease (URTD), cutaneous dyskeratosis (CD), herpes virus, shell necrosis, urolithiasis (bladder stones) and parasites.
Upper respiratory tract disease (URTD) is a chronic, infectious disease responsible for population declines across the entire range of the desert tortoise. It was identified in the early 1970s in captive desert tortoise populations, and later identified in the wild population. URTD is caused by the infectious agents Mycoplasma agassizii and Mycoplasma testudineum, which are bacteria in the class Mollicutes and characterized by having no cell wall and a small genome. Mycoplasma appears to be highly virulent (infectious) in some populations, while it is chronic, or even dormant in others. The mechanism (whether environmental or genetic) that is responsible for this diversity is not understood. Infection is characterized by both physiological and behavioral changes: nasal and ocular discharge, palpebral edema (swelling of the upper and/or lower palpebra, or eyelid, the fleshy portion that is in contact with the tortoises eye globe) and conjunctivitis, weight loss, changes in color and elasticity of the integument, and lethargic or erratic behavior. These pathogens are likely transmitted by contact with an infected individual. Epidemiological studies of wild desert tortoises in the western Mojave Desert from 1992 to 1995 shows a 37% increase in M. agassizii. Tests were conducted on blood samples, and a positive test was determined by the presence of antibodies in the blood, defined as being seropositive.
Cutaneous dyskeratosis (CD) is a shell disease of unknown origin and has unknown implications on desert tortoise populations. Observationally, it is typified by shell lesions on the scutes. Areas infected with CD appear discolored, dry, rough and flakey, with peeling, pitting and chipping through multiple cornified layers. Lesions are usually first located on the plastron (underside) of the tortoises, although lesions on the carapace (upper side) and forelimbs are not uncommon. In advanced cases, exposed areas become infected with bacteria, fungus, and exposed tissue and bone may become necrotic. CD was evident as early as 1979 and was initially identified on the Chuckwalla Bench Area of Critical Environmental Concern in Riverside County, California, US. Currently, the means of transmission are unknown, although hypotheses include auto-immune diseases, exposure to toxic chemicals (possibly from mines, or air pollution), or a deficiency disease (possibly resulting from tortoises consuming low-quality invasive plant species instead of high-nutrient native plants).
Two case studies outlined the spread of disease in desert tortoises. The Daggett Epidemiology of Upper Respiratory Tract Disease project, which provides supporting disease research for the Fort Irwin translocation project, lends an example of the spread of disease. In 2008, 197 health evaluations were conducted, revealing 25–45.2% exposure to M. agassizii and M. testudineum, respectively, in a core area adjacent to Interstate 15. The spread of disease was tracked over two years, and clinical signs of URTD spread from the core area to adjacent, outlying locations during this time. Overlaying home ranges and the social nature of these animals, suggests that disease-free individuals may be vulnerable to spread of disease, and that transmission can occur rapidly. Thus wild tortoises that are close to the urban-wildlife interface may be vulnerable to spread of disease as a direct result of anthropogenic influence.
The second study indicated that captive tortoises can be a source of disease to wild Agassiz's desert tortoise populations. Johnson et al. (2006) tested blood samples for URTD (n = 179) and herpesvirus (n = 109) from captive tortoises found near Barstow, CA and Hesperia, CA. Demographic and health data were collected from the tortoises, as well from other reptiles housed in the same facility. Of these, 45.3% showed signs of mild disease, 16.2% of moderate disease, and 4.5% of severe disease, and blood tests revealed that 82.7% of tortoises had anti-mycoplasma antibodies, and 26.6% had anti-herpesvirus antibodies (which means the tortoises were seropositive for these two diseases, and indicate previous exposure to the causative agents). With an estimated 200,000 captive desert tortoises in California, their escape or release into the wild is a real threat to uninfected wild populations of tortoises. Projections from this study suggest that approximately 4400 tortoises could escape from captivity in a given year, and with an 82% exposure rate to URTD, the wild population may be at greater risk than previously thought.
Edwards et al. report that 35% of desert tortoises in the Phoenix area are hybrids between either Gopherus agassizii and G. morafkai, or G. morafkai and the Texas tortoise, G. berlandieri. The intentional or accidental release of these tortoises could have dire consequences for wild tortoises.
Before obtaining a desert tortoise as a pet it is best to check the laws and regulations of the local area and/or state. Desert tortoises may not be moved across state borders or captured from the wild. They may, however, be given as a gift from one private owner to another. Desert tortoises need to be kept outdoors in a large area of dry soil and with access to vegetation and water. An underground den and a balanced diet are crucial to the health of captive tortoises.
Wild populations of tortoises must be managed effectively to minimize the spread of diseases, which includes research and education. Despite significant research being conducted on desert tortoises and disease, there is still a considerable knowledge gap in understanding how disease affects desert tortoise population dynamics. It is not known if the population would still decline if disease was completely absent from the system; are tortoises more susceptible to disease during draught conditions? How does non-native diet impact a tortoise’s ability to ward off pathogens? What are the causes of immunity exhibited by some desert tortoises? The 2008 USFWS draft recovery plan suggests that populations of tortoises that are uninfected, or only recently infected should likely be considered research and management priorities. Tortoises are known to show resistance to disease in some areas, an effort to identify and maintain these individuals in the populations is essential. Furthermore, increasing research on the social behavior of these animals, and garnering a greater understanding of how behavior facilitates disease transmission would be advantageous in understanding rates of transmission. Finally, translocation of tortoises should be done with extreme caution; disease is typically furtive and moving individuals or populations of tortoises across a landscape can have unforeseen consequences.
Corollary to research, education may help preventing captive tortoises from coming in contact with wild populations, as outlined in the case study by Johnson et al. (2006). Education campaigns through veterinarians, government agencies, schools, museums and community centers throughout the range of the desert tortoise could limit the spread of tortoise diseases into wild populations. Strategies may include encouraging people to not breed their captive tortoises, ensure that different species of turtles and tortoises are not housed in the same facility (which would help to prevent the spread of novel diseases into the desert tortoise population), ensure captive tortoises are adequately housed to prevent them from escaping into the wild, and to ensure that captive turtles and tortoises are never released into the wild.
Desert tortoises have been severely impacted by disease. Both upper respiratory tract disease and cutaneous dyskeratosis have caused precipitous population declines and die-offs across the entire range of this charismatic species. Both of these diseases are extremely likely to be caused by people, and URTD is easily linked with people releasing captive tortoises into the wild. The combination of scientific research and public education are imperative to curb the spread of disease and aid the tortoise in recovery.
The desert tortoise is the state reptile of California and Nevada.
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