The desert tortoise (''Gopherus agassizii''), is a
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of
tortoise
Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: ''tortoise''). Like other turtles, tortoises have a turtle shell, shell to protect from predation and other threats. The shell in tortoises is generally hard, ...
in the
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Testudinidae. The species is native to the
Mojave and
Sonoran Desert
The Sonoran Desert ( es, Desierto de Sonora) is a desert in North America and ecoregion that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the southwestern United States (in Arizona ...
s of the southwestern
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
and northwestern
Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
, and to the Sinaloan thornscrub of northwestern Mexico.
[ ''G. agassizii'' is distributed in western Arizona, southeastern California, southern Nevada, and southwestern Utah.][ The specific name ''agassizii'' is in honor of Swiss-American zoologist ]Jean Louis Rodolphe Agassiz
Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history.
Spending his early life in Switzerland, he rec ...
. The desert tortoise is the official state reptile in California and Nevada.
The desert tortoise lives about 50 to 80 years; 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
Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clos ...
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.[
]
Taxonomy
In 2011, 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: Agassiz's desert tortoise (''Gopherus agassizii'') and Morafka's desert tortoise (''Gopherus morafkai'').[Genetic Analysis Splits Desert Tortoise into Two Species](_blank)
. US Geological Survey (2011-06-28). Retrieved on 2019-08-22. 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
Sonora (), officially Estado Libre y Soberano de Sonora ( en, Free and Sovereign State of Sonora), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is d ...
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 60C (140 F) 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 two types: 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 ]canyon
A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...
s where more suitable soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
s for den construction might be found. 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
Death Valley is a desert valley in Eastern California, in the northern Mojave Desert, bordering the Great Basin Desert. During summer, it is the Highest temperature recorded on Earth, hottest place on Earth.
Death Valley's Badwater Basin is the ...
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.[
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 be firm enough to resist collapse. Desert tortoises prefer sandy ]loam
Loam (in geology and soil science) is soil composed mostly of sand (particle size > ), silt (particle size > ), and a smaller amount of clay (particle size < ). By weight, its mineral composition is about 40–40–20% concentration of sand–sil ...
soils with varying amounts of gravel
Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone.
Gravel is classifi ...
and clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
, 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, so may prefer sites with higher calcium content.[
With the creation of off-road vehicles more humans are making their way in and out of the Desert Tortoises home environment.
]
Shelters
Desert tortoises spend most of their lives in burrow
An Eastern chipmunk at the entrance of its burrow
A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of sh ...
s, 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.[
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.][
Each desert tortoise uses about 5 to 25 burrows 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 squirrel
The white-tailed antelope squirrel (''Ammospermophilus leucurus'') is a diurnal species of ground squirrel, scientifically classified in the order Rodentia and family Sciuridae, found in arid regions of the southwestern United States and the B ...
s (''Ammospermophilus leucurus''), woodrat
A pack rat or packrat, also called a woodrat or trade rat, are any species in the North and Central American rodent genus ''Neotoma''. Pack rats have a rat-like appearance, with long tails, large ears, and large, black eyes. Pack rats are notice ...
s (''Neotoma''), collared peccaries
The collared peccary (''Dicotyles tajacu'') is a species of artiodactyl (even-toed) mammal in the family Tayassuidae found in North, Central, and South America. It is the only member of the genus ''Dicotyles''. They are commonly referred to as ...
(''Dicolytes tajacu''), burrowing owls (''Athene cunicularia''), Gambel's quail (''Callipepla gambelii'' ), rattlesnake
Rattlesnakes are venomous snakes that form the genera ''Crotalus'' and ''Sistrurus'' of the subfamily Crotalinae (the pit vipers). All rattlesnakes are vipers. Rattlesnakes are predators that live in a wide array of habitats, hunting small anim ...
s (''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.[
]
Lifecycle
Reproduction
Tortoises mate in the spring and autumn. Male desert tortoises grow two large white glands around the chin area, called chin glands, that signify mating season. A male circles around female, biting her shell in the process. He then climbs 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 cloaca
In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...
of a female, which is located around the tail. The male may make grunting noises once atop a female, and may move his front legs up and down in a constant motion, as if playing a drum.
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;[ some eggs may overwinter and hatch the following spring. In a laboratory experiment, temperature influenced hatching rates and hatchling sex. Incubation temperatures from resulted in hatching rates exceeding 83%, while incubation at resulted in a 53% hatching rate. Incubation temperatures less than resulted in all-male clutches. Average incubation time decreased from 124.7 days at to 78.2 days at .][
The Desert Tortoise is one of the few known tortoises in existence that has been observed engaging in homosexual intercourse][Biol. Exuberance: Desert Tortoise - Bagemihl (1999 AD), pages 232, 664] 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.[
Desert Tortoises reach their reproductive maturity at ages 15 to 20, when they become longer than 18 cm (7 in). However, it is possible for them to mature faster as 10-year-old females that are able to reproduce have been observed.][
]
Activity
Their activity depends on location, peaking in late spring for the Mojave Desert and in late summer to fall in Sonoran Desert; some populations exhibit two activity peaks during one year. Desert tortoises brumate
Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clos ...
during winters, roughly from November to February–April. Females begin brumating later and emerge earlier than males; juveniles emerge from brumation earlier than adults.[
Temperature strongly influences desert tortoise activity level. Although desert tortoises can survive body temperatures from below freezing to over , most activity occurs at temperatures from . 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 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.][
]
Lifespan
The lifespan of a Desert Tortoise can vary from 50 to 80 years. The main causes of mortality in Desert Tortoises include predators, human-related causes, diseases, 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%. Survival of Mojave Desert tortoises from 1 to 4 years of age is 71–89%.][
]
Diet
The desert tortoise is an herbivore
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
. Grasses form the bulk of its diet, but it also eats herb
In general use, herbs are a widely distributed and widespread group of plants, excluding vegetables and other plants consumed for macronutrients, with savory or aromatic properties that are used for flavoring and garnishing food, for medicinal ...
s, 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
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
as a source of supplementary calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
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
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
can store over 40% of the tortoise's body weight in water, urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important r ...
, uric acid, and nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
ous wastes. During very dry times, they may give off waste as a white paste rather than a watery urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excretion, excreted from the body through the urethra.
Cel ...
. 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 and dry seasons, they rely on the water contained within cactus fruits and mesquite grass. To maintain sufficient water, they reabsorb water in their bladders, and move to humid burrows in the morning to prevent water loss by evaporation.][
A desert tortoise emptying their bladder is one of their defense mechanisms. 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. If you do pick them up and their bladder is voided, then you must provide them with water to replenish the fluid in their body. If action is not taken this could have “life threatening consequences”. ]
Predation and conservation status
Raven
A raven is any of several larger-bodied bird species of the genus ''Corvus''. These species do not form a single taxonomic group within the genus. There is no consistent distinction between "crows" and "ravens", common names which are assigned t ...
s, Gila monsters, kit foxes, badgers, roadrunners, coyote
The coyote (''Canis latrans'') is a species of canis, canine native to North America. It is smaller than its close relative, the wolf, and slightly smaller than the closely related eastern wolf and red wolf. It fills much of the same ecologica ...
s, and fire ants are all natural predators of the desert tortoise. They prey on eggs, juveniles, which are long with a thin, delicate shell, or, in some cases, adults. Ravens are thought 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
Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...
, disease, habitat destruction and fragmentation, illegal collection and vandalism
Vandalism is the action involving deliberate destruction of or damage to public or private property.
The term includes property damage, such as graffiti and defacement directed towards any property without permission of the owner. The term f ...
by humans, and habitat conversion from invasive plant species (''Brassica tournefortii
''Brassica tournefortii'' is a species of plant known by the common names Asian mustard, pale cabbage, African mustard, and Sahara mustard, and is well known as an invasive species, especially in California.
The plant is generally similar to ot ...
'', ''Bromus rubens
''Bromus madritensis'' is a species of brome grass known by the common name compact brome. The specific epithet ''madritensis'' refers to Madrid, Spain. It has a diploid number of 28.
There are two subspecies:
*''Bromus madritensis'' subsp. ''m ...
'' 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
Threatened species are any species (including animals, plants and fungi) which are vulnerable to endangerment in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of ''critical depensat ...
. 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
The Bureau of Land Management (BLM) is an agency within the United States Department of the Interior responsible for administering federal lands. Headquartered in Washington DC, and with oversight over , it governs one eighth of the country's la ...
in Nevada
Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
. When adopted in Nevada, they will have a computer chip
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
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.
Another potential threat to the desert tortoise's habitat is a series of proposed wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
and solar farm
Solar may refer to:
Astronomy
* Of or relating to the Sun
** Solar telescope, a special purpose telescope used to observe the Sun
** A device that utilizes solar energy (e.g. "solar panels")
** Solar calendar, a calendar whose dates indicate t ...
s. 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 .
Tortoises are made to withstand tough conditions and high temperatures, what they are not meant to withstand are obstacles that come with 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 significantly impacting their population.
Human development
Ivanpah solar power project
Concerns about the impacts of the Ivanpah Solar
The Ivanpah Solar Electric Generating System is a concentrated solar thermal plant in the Mojave Desert. It is located at the base of Clark Mountain (California), Clark Mountain in California, across the state line from Primm, Nevada. The plant h ...
thermal project led the developers to hire some 100 biologists and spend US$
The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official ...
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 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
The National Park Service (NPS) is an agency of the United States federal government within the U.S. Department of the Interior that manages all national parks, most national monuments, and other natural, historical, and recreational propertie ...
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.Jarvis Mojave letter 2011
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
Dyskeratosis is abnormal keratinization occurring prematurely within individual cells or groups of cells below the stratum granulosum.Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Pathologic Basis of Disease'' (8th ed.). Saunde ...
, herpes virus, shell necrosis
Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...
, urolithiasis
Kidney stone disease, also known as nephrolithiasis or urolithiasis, is a crystallopathy where a solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the body in the urine s ...
(bladder stones), and parasites.[
]
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.[ 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
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
and a small genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
.[
Mycoplasmae appear to be highly virulent (infectious) in some populations, while chronic, or even dormant in others.][ The mechanism (whether environmental or genetic) 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
An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyeli ...
, or eyelid, the fleshy portion that is in contact with the tortoises eye globe) and conjunctivitis
Conjunctivitis, also known as pink eye, is inflammation of the conjunctiva, outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may ...
, 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
The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily in ...
from 1992 to 1995 showed 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
Cutaneous dyskeratosis
Dyskeratosis is abnormal keratinization occurring prematurely within individual cells or groups of cells below the stratum granulosum.Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Pathologic Basis of Disease'' (8th ed.). Saunde ...
(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 fore limbs are not uncommon. In advanced cases, exposed areas become infected with bacteria, fungi, 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.][ Currently, the means of transmission are unknown, although hypotheses include ]autoimmune disease
An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. At least 80 types of autoimmune diseases have been identified, with some evidence suggesting that there may be more than 100 types. Nearly a ...
s, 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).[
]
Impacts of disease
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 25.0–45.2% 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
A home range is the area in which an animal lives and moves on a periodic basis. It is related to the concept of an animal's territory which is the area that is actively defended. The concept of a home range was introduced by W. H. Burt in 1943. He ...
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 human 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 antibodies to mycoplasma, and 26.6% had antibodies to herpesvirus (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 about 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.][
]
Domestic pets
Edwards ''et al.'' reported 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 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.
]
Management activities and spread of disease
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, a considerable knowledge gap still exists in understanding how disease affects desert tortoise population dynamics. It is not known if the population would still decline if disease were completely absent from the system; are tortoises more susceptible to disease during drought conditions? How does a 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
As a corollary to research, education may help prevent captive tortoises from coming into contact with wild populations.[ 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 affected 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 is imperative to curb the spread of disease and aid the tortoise in recovery.
]
State reptile
The desert tortoise is the state reptile
Twenty-eight U.S. states have named an official state reptile. As with other state symbols, states compare admirable aspects of the reptile and of the state, within designating statutes. Schoolchildren often start campaigns promoting their favo ...
of California and Nevada.
File:Desert tortoise at Red Rock National Conservation Area.jpg, Closeup of desert tortoise at Red Rock Canyon National Conservation Area, 2020
File:Desert tortoise with habitat background at Red Rock National Conservation Area.jpg, Same desert tortoise at Red Rock Canyon NCA showing habitat,2020
File:Wild desert tortoise at Red Rock National Conservation Area.jpg, Front view closeup of desert tortoise at Red Rock Canyon NCA, 2020
File:Wild desert tortoise shown in rocky habitat at or near Red Rock Canyon National Conservation Area.jpg, Photo showing rocky habitat of Desert tortoise at Red Rock Canyon NCA, 2020
References
External links
*
The 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
Further reading
* Behler JL, King FW (1979). ''The Audubon Society Field Guide to North American Reptiles and Amphibians''. New York: Alfred A. Knopf. 743 pp. . (''Gopherus agassizii'', pp. 471–472 + Plate 328).
* Boulenger GA (1889). ''Catalogue of the Chelonians, Rhynchocephalians, and Crocodiles in the British Museum (Natural History). New Edition.'' London: Trustees of the British Museum (Natural History). (Taylor and Francis, printers). x + 311p. + Plates I-III. (''Testudo agasizii'', p. 156).
* Cooper JG (1861). "New California Animals". ''Proc. California Acad. Sci.'' 2: 118-123. (''Xerobates agassizii'', new species, pp. 120–121).
* Goin CJ, Goin OB, Zug GR (1978). ''Introduction to Herpetology, Third Edition''. San Francisco: W.H. Freeman. xi + 378 pp. . (''Gopherus agassizi'', p. 155).
* Smith HM, Brodie ED Jr (1982). ''Reptiles of North America: A Guide to Field Identification''. New York: Golden Press. 240 pp. (paperback), (hardcover). (''Gopherus agassizi'', pp. 62–63).
* Stebbins RC (2003). ''A Field Guide to Western Reptiles and Amphibians, Third Edition''. The Peterson Field Guide Series ®. Boston and New York: Houghton Mifflin. xiii + 533 pp. . (''Gopherus agassizii'', pp. 255–257 + Plate 22 + Map 63).
* Stejneger L, Barbour T (1917). ''A Check List of North American Amphibians and Reptiles''. Cambridge, Massachusetts: Harvard University Press. 125 pp. (''Gopherus agassizii'', p. 121).
{{Taxonbar, from=Q914356
Reptiles described in 1861
Gopherus
Taxa named by James Graham Cooper
North American desert fauna
Reptiles of Mexico
Reptiles of the United States
Fauna of the Mojave Desert
Fauna of the Colorado Desert
Fauna of the Sonoran Desert
Fauna of the Lower Colorado River Valley
Symbols of California
Natural history of the Mojave Desert
Symbols of Nevada