Desert tortoise

Desert Tortoise
Desert Tortoise, G. agassizii
Conservation status
Vulnerable  (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Order:	Testudines
Suborder:	Cryptodira
Superfamily:	Testudinoidea
Family:	Testudinidae
Genus:	Gopherus
Species:	G. agassizii
Binomial name
Gopherus agassizii Cooper, 1863

The desert tortoise (Gopherus agassizii) is a species of tortoise native to the Mojave desert and Sonoran desert of the southwestern United States and northern Mexico. The epithet agassizii is in honor of Swiss-American zoologist Jean Louis Rodolphe Agassiz.

Description

This tortoise 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. Their shells are high-domed, and greenish-tan to dark brown in color. Desert tortoises can grow from 4–6" in height and weigh 8–15 lb (4–7 kg) when fully grown. The front limbs have heavy, claw-like scales and are flattened for digging. Back legs are more stumpy and elephantine.

Habitat

Desert Tortoise in Rainbow Basin near Barstow, California.

The tortoise is able to live where ground temperature may exceed 140 degrees Fahrenheit (60 degrees Celsius) because of its ability to dig underground burrows and escape the heat. At least 95% of its 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 Mojave and Sonoran Desert tortoises, with a possible third type 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 in elevation. It is believed that, in their entire lives, these tortoises rarely move more than two miles from their natal nest. They also live to be 80-100 years old.

Diet

The desert tortoise is a 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 forty percent 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. Adult tortoises can survive a year or more without access to water.

One defense mechanism the tortoise has when it is handled or molested is to empty its bladder. This can leave the tortoise in a very vulnerable condition in dry areas, and they should never be alarmed, handled or picked up in the wild unless they are in imminent danger.

Reproduction

These tortoises will mate anytime above ground, usually in the spring. The female will lay a clutch of 4 to 8 hard-shelled-eggs, and can reproduce 2 to 3 clutches a year.

A young desert tortoise

Predators and conservation status

File:Desert tortoise 2 tds.jpg

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, habitat destruction and fragmentation, illegal collection and vandalism by humans, and invasive species.

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, or through the 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.

As yet another part of the tortoise desert habitat was to undergo "development" - this time, to be converted into combat training grounds at Fort Irwin National Training Center - the US Army expanded into an area that was nearly pristine habitat for approximately 2000 desert tortoises, and contained Critical Desert Tortoise Habitat, a designation by the US Fish and Wildlife. In March of 2008, about 670 tortoises were moved by helicopter and vehicle, up to 35 km away. This project was permitted by the US Fish and Wildlife Service. Most of the translocation sites were located in between a power line corridor and interstate 15, and there was no rigorous habitat assessment conducted before the tortoises were translocated. Thus far, 29% of the translocated individuals have died, as of May 2009, mostly preyed upon by coyotes. The second phase of the translocation project will occur in the near future, 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.

Diseases

Reptiles are known to become infected by a wide range of pathogens, which includes viruses, bacteria, fungi and parasites. More specifically, the desert tortoise (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 (Jacobson et al. 1991, Jacobson et al. 1994, Berry et al. 2002).

Upper Respiratory Tract Disease

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 (Jacobson et al. 1991). 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 (Brown et al. 1994, Berry and Van Abbema 1997, Brown et al. 1999). Mycoplasma appears to be highly virulent (infectious) in some populations, while it is chronic, or even dormant in others (USFWS 2008). 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 (Jacobson et al. 1991, Schumacher et al. 1993, Homer et al. 1998, Berry and Christopher 2001). 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 (Brown et al. 1999). 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

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 (Homer et al. 2001). 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 (Homer et al. 1998, Homer et al. 2001). CD was evident as early as 1979 and was initially identified on the Chuckwalla Bench Area of Critical Environmental Concern in Riverside County, California, USA (Jacobson 1994). 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) (Jacobson et al. 1994, USFWS 2008).

Impacts of disease: Case Studies

There are two case studies outlining 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 excellent example of the spread of disease across a landscape. 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 disease-free individuals may be vulnerable to spread of disease, and that transmission can occur rapidly (Mack and Berry 2009). This study suggests that wild tortoises that are close to the urban-wildlife interface may be vulnerable to spread of disease as a direct result of anthropogenic influence.

A second example outlines a study which indicates captive tortoises can be a source of disease to wild 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 was collected from the tortoises, as well as data on other reptiles housed in the same facility. Of these, 45.3% showed signs of mild disease, 16.2% showed signs of moderate disease, and 4.5% showed signs 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 seropositve 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 (Johnson et al. 2006).

Research

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.

Education

Corollary to research, education campaigns are absolutely critical to prevent 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 be beneficial in limiting 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.

In summary, 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.

References

• Template:IUCN2006 Listed as Vulnerable (VU A1acde+2cde, E v2.3)

• "Gopherus agassizii". Integrated Taxonomic Information System. 6 February. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
• The Desert Tortoise
• The Biogeography of The Desert Tortoise, by Kerrie Bathel
• The Arizona-Sonora Desert Museum