LIFE HISTORY

The desert tortoise is between 8 and 15 inches long.  It is the largest reptile in the Mojave Desert.  The shell is domed and horn-colored or brown. 

The maximum age of the desert tortoise is typically eighty years, but they may live to reach one hundred years old.  The desert tortoise has existed for around a million years.  They start reproducing between the ages of 12 and 20 when the females lay a clutch of eggs (up to 15) in the spring.  Many desert tortoises never reach the adult stage because of egg predation and the vulnerability of juveniles as a result of their soft shell. 

HABITAT

The desert tortoise is well adapted to life in the desert.  There are many plants in the desert which the desert tortoise eats: cactus, annual forbs, grasses, and wildflowers.  In the spring (from March to June) they forage; they are building up stores of fat and water for the rest of the year.  Desert tortoises live in burrows which they dig to escape summer heat and winter cold.  When it gets very hot in the summer they estivate in these underground burrows.   In the fall, when it is cooler, the desert tortoise will again emerge and eat dried grasses and drink after a thunderstorm.  When there is no water available they are able to absorb the water from their bladders.  Watch out, you do not want to approach a desert tortoise too closely; when frightened, they may release their bladder and lose this vital water supply.  In the winter (October to March) they return to their burrows to hibernate.  Some burrows have been passed down through generations of tortoises.

Critical habitat designation for the Mojave desert tortoise was published in the Federal Register in February 1994 (59 FR 5820).  Areas within the Mojave and Colorado deserts that are essential for the species' recovery were designated critical habitat.  The Mojave population lives primarily on flats and alluvial fans.  Common traits of these areas are scattered shrubs with abundant inter-space for growth of herbaceous plants, with soils ranging from sand to sandy-gravel. There is, however, significant geographic variation in the way desert tortoises use available resources; for, they are also found on rocky terrain and slopes. 

THREATS

The primary reasons for listing the Mojave population include deterioration and loss of habitat, collection for pets or other purposes, elevated levels of predation, disease, and the inadequacy of existing regulatory mechanisms to protect desert tortoises and their habitat.  It is estimated that many desert tortoise populations have declined at rates ranging between 3 and 59% per year. 

Human actions are a major factor in the decline of the desert tortoise. People illegally collect desert tortoises for pets, food, and commercial trade.  Mortality of desert tortoises due to gunshot wounds and off-highway vehicles is common in parts of the Mojave region.  In addition, desert tortoises are often struck and killed by vehicles on roads and highways.

Hatchlings and juveniles are preyed upon by several mammals, reptiles, and birds.  Domestic and feral dogs are probably a significant source of mortality; however, the common raven (Corvus corax) is the major predator of young tortoises.  There has been a large increase in raven populations in the Mojave and Colorado deserts due to increased food supplies (e.g., roadkills, landfills, trash, garbage dumps, agricultural developments), as well as new sites for perches and nests (e.g. fence posts, power poles and towers, signs, buildings, bridges, and freeway access-ramps). 

An upper respiratory tract disease (URTD) is prevalent in captive desert tortoises and has been identified in wild desert tortoises in the Mojave region.  The illegal release of captive desert tortoises that are ill may be spreading URTD to wild populations.  The wild populations of desert tortoise are likely predisposed to URTD because of habitat degradation, poor nutrition, and drought.  A shell disease, characterized by lesions, is also having an impact on the population. 

There has been a substantial decrease in perennial grasses, shrubs, and native annuals and an increase in exotic annuals such as red brome (Bromus rubens).  These vegetational changes can be detrimental to desert tortoises for a number of reasons.  First, they require perennial shrubs for cover from the intense solar radiation in the desert.  Second, perennial grasses are important secondary food sources for the desert tortoise in many areas.  Third, recurrent fires and competition from exotic annuals may reduce the abundance and diversity of native forbs which are the major food source of the desert tortoise.  There is some controversy over the role that introduced exotics play in the desert tortoise diet suggesting that further research is needed. 

Another factor that is contributing to the decline in desert tortoise populations is habitat fragmentation.  Urbanization is a major cause of habitat destruction.  Over its lifetime, each desert tortoise may require more than 1.5 square miles of habitat.  Males have larger habitat ranges especially in drought years when the desert tortoise will forage over larger areas. 

Two of the many activities that are considered to have an impact on habitat quality are livestock grazing and off-road vehicle use.  Livestock can trample desert tortoises and their eggs.  Grazing can also have an indirect impact, through damage to soil crusts, reduction of water infiltration, promotion of erosion, inhibition of nitrogen fixation in desert plants, and providing a favorable seed bed for exotic annual vegitation.  Sheep often bed down and destroy shrubs that are essential for tortoise protection from the sun.  Off-road vehicle use may also destroy, degrade, and fragment considerable areas of desert tortoise habitat. Some feel that further study is necessary before a strong correlation may be made between these land uses and decline in desert tortoise populations.