Desert tortoise

The desert tortoise (Gopherus agassizii) is a species of tortoise in the family Testudinidae. The species is native to the Mojave and Sonoran Deserts of the southwestern United States and northwestern Mexico, and to the Sinaloan thornscrub of northwestern Mexico. The desert tortoise is the official state reptile in California and Nevada.

The desert tortoise lives 70 to 80 years on average; it grows slowly and generally has a low reproductive rate. It spends most of its time in burrows, rock shelters, and pallets to regulate body temperature and reduce water loss. It is most active after seasonal rains and is inactive during most of the year. This inactivity helps reduce water loss during hot periods, whereas winter brumation 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 lifespans. The new species name is in honor of the late Professor David Joseph Morafka of California State University, Dominguez Hills, in recognition of his many contributions to the study and conservation of Gopherus. G. morafkai occurs east of the Colorado River in Arizona, as well as in the states of Sonora and Sinaloa, Mexico. The acceptance of G. morafkai reduced the range of G. agassizii by about 70% In 2016, based on a large-scale genetic analysis, ecological and morphological data, researchers proposed a split between the Sonoran and Sinaloan populations. This southernmost member of the Gopherus genus was named G. evgoodei, Goode's thornscrub tortoise.

Description

These tortoises may attain a length of , with males being slightly larger than females. A male tortoise has a longer gular horn than a female, his plastron (lower shell) is concave compared to a female tortoise. Males have larger tails than females do. Their shells are high-domed, and greenish-tan to dark brown in color. The high domes of their shells allow for space for their lungs, which helps them maintain thermoregulation, also known as maintaining internal temperature. Desert tortoises can grow to in height. They can range in weight from 8 to 15 pounds, or 3.5 kg to 7 kg. The front limbs have sharp, claw-like scales and are flattened for digging. Back legs are skinnier and very long.

Habitat

Desert tortoises can live in areas with ground temperatures exceeding because of their ability to dig burrows and escape the heat. At least 95% of their lives are spent in burrows. There, they are also protected from freezing winter weather while dormant, from November through February or March. Within their burrows, these tortoises create a subterranean environment that can be beneficial to other reptiles, mammals, birds, and invertebrates.

Scientists have divided the desert tortoise into three species: Agassiz's and Morafka's desert tortoises, They range from near sea level to around 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.

Desert tortoises inhabit elevations from below mean sea level in Death Valley to in Arizona, though they are most common from around . Estimates of densities vary from less than on sites in southern California to over in the western Mojave Desert, although most estimates are less than . The home range generally consists of . In general, males have larger home ranges than females, and home range size increases with increasing resources and rainfall.

Shelters

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 sex 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 deep.

thumb|Hatching baby desert tortoise

Months later, the female lays a clutch of four to eight hard-shelled eggs, which have the size and shape of ping-pong balls, usually in June or July. The eggs hatch in August or September. Wild female tortoises produce up to three clutches a year depending on the climate. Their eggs incubate from 90 to 135 days; Same-sex intercourse happens in many species, There is no one answer as to why this occurs. One possible explanation for this could be the social component of gaining and establishing dominance.

Maturation

The desert tortoise grows slowly, often taking 16 years or longer to reach about in length. The growth rate varies with age, location, gender and precipitation. It can slow down from 12 mm/year for ages 4–8 years to about 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. Adult tortoises can survive a year or more without access to water. This can leave the tortoise in a very vulnerable condition in dry areas, since the tortoise will no longer have a backup water supply. If a tortoise is seen in the wild, you should not handle, or pick them up unless they are in imminent danger. Handling of tortoises may have consequences for the animal, such as the development of upper respiratory tract infections. 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 under the Endangered Species Act of 1973 in 1990, after being listed as endangered, or threatened in the case of the Beaver Dam Slope, Utah population, under an emergency rule in 1989. All desert tortoises are protected under law of the United States based on similarity of appearance to tortoises of the protected Mojave population. 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 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 backs 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 expanded into an area that was habitat for about 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 Desert Tortoise Preserve Committee protects roughly of desert tortoise habitat from human activity. This area includes in Kern County, in San Bernardino County, and in Riverside County. As a result of 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 .

While tortoises are made to withstand tough conditions and high temperatures, they are unable to cope with the dangers of human development, such as the use of off-roading vehicles. These vehicles that come along at high speeds have the potential to crush and kill tortoises, running over their eggs and burrows and significantly impacting their population.

Human development

Ivanpah solar power project

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 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 Bureau of Land Management (BLM) expects that most of the juvenile tortoises on the project will be killed.

Lawsuits

In the summer of 2010, Public Employees for Environmental Responsibility filed a lawsuit against the National Park Service for not having taken measures to manage tortoise shooting in the Mojave National Preserve of California. Biologists discovered numerous gunshot wounds (holes) on dead tortoise shells which could likely have been caused long after natural death as these shells can take five years to disintegrate and make useful targets for well intentioned target shooters. These shells left behind by nature, droughts, roadkill, or vandals may have attracted ravens and threatened the healthy tortoises as any predator bird need only feed once on a small tortoise to remember it as a viable food source. The National Park Service did not take the measures they were urged to. They responded with "We simply do not believe that such regulations are warranted at this time." and no further action has been taken.

Diseases

Reptiles are known to become infected by a wide range of pathogens, which includes viruses, bacteria, fungi, and parasites. More specifically, the G. agassizii population has been negatively affected by upper respiratory tract disease, cutaneous dyskeratosis, herpes virus, shell necrosis, urolithiasis (bladder stones), and parasites. suggests that further research into Mycoplasma is needed to better understand its role in disease in Desert Tortoise populations. Recommendations are given for further research into populations of tortoises that are uninfected, those only recently infected, and those from endemically infected populations. 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.

External links

The Desert Tortoise
The Arizona-Sonora Desert Museum
Desert Tortoise, National Park Service
Desert Tortoise data and information portal, Mojave Desert Ecosystem Program
Desert Tortoise documentary video: The Desert Tortoise: A Delicate Balance, NASA Dryden
Desert Tortoise Council