The Threatened Desert Tortoise
The Desert Tortoise (Gopherus Agassizii) in the Mojave desert (north and west of the Colorado River) was Federally listed under emergency provisions of the Endangered Species Act of 1973 as endangered on August 4, 1989 and permanently listed as a threatened species on April 2, 1990. The tortoise was listed because of direct losses and threats to tortoise populations and habitat. Desert tortoises are directly impacted by increased raven predation on juveniles, collected by human, vandalism, losses on roads and to off highway vehicle activities, and the Upper Respiratory Disease Syndrome. Tortoise habitat is lost directly to urbanization, agriculture, road construction, military activities, and other uses. Off highway vehicle use, rights-of-way, and grazing degrade habitat. All of these activities fragment tortoise habitat which may reduce a tortoise population below the level necessary to maintain a minimum viable population.
The U.S. Endangered Species Act makes it illegal to harass, collect, or harm tortoises and provides for penalties up to $50,000 in fines and one year in prison for each count. Nevada State law 503.080.1a also affords protection to the desert tortoise.
The Endangered Species Act allows for individuals of an endangered or threatened species to be taken incidentally to an otherwise lawful activity; as long as the conditions of the Fish and Wildlife Service’s (Service) Biological Opinion are followed. “Take” includes harassing, harming, pursuing, hunting, shooting, wounding, killing, trapping, capturing or collecting, or attempting to engage in any such conduct. Harm includes significant habitat modification or degradation that impacts a listed species by interfering with breeding, feeding, or sheltering behavior. The threatened listing of the desert tortoise occurred because of widespread habitat destruction and degradation, illegal collection, an upper respiratory disease, raven predation, and other factors.
Tortoises in captivity prior to the initial listing of August 4, 1989 are not protected by the Act. If you are interested in having a pet tortoise, you may obtain one from US Fish And Wildlife Service- approved adoption program, which in southern Nevada is Tortoise Group. Those who have a pet desert tortoise are custodians and do not “ own “ the captive desert tortoise. You may also receive a tortoise from another person and register it on the Tortoise group website to make it legally yours.
The desert tortoise is the largest reptile and the only wild land tortoise found in southern Nevada. The tortoise occurs in southern Nevada, western California, southwestern Utah, western Arizona, and northwestern Mexico. In Nevada, tortoises are found in creosote bush, cactus and shadscale scrub, and Joshua tree woodland habitats below 5000’ elevation.
Tortoise populations are patchily distributed and densities range from a few per square mile to 200 per square mile. A tortoise will live in the same general area of less than one square mile during its lifespan of 50 to 100 years. This slow moving desert reptile ranges in size from 2 to 15 inches long and is soil colored. Because of their color and shape, tortoise can be difficult to see.
There are several clues that can be used to tell male and female tortoise apart. However, only tortoise greater than seven inches long can be sexed reliably. Males tend to be larger than females, have longer tail, have longer upward curving gular horns, have larger chin glands, and have a concave plastron (bottom portion of shell).
Tortoises are well adapted to their desert environment and spend up to 98% of their time in burrows they dig. Burrows are crescent shaped and are most often found at the base of desert shrubs or in wash banks.
A tortoise may excavate and use many burrows during the year. Some burrows are used for only a short period of time and others may be used for several years. Some researchers believe that some winter dens on the Beaver Dam Slope in Utah may be 5000 years old. Many mammals, birds, reptiles, and invertebrates utilize tortoise burrows. Burrows and tortoise are most often found on valley floors and slopes, but they may also be found on the less precipitous slopes and ridge of desert mountain ranges.
Besides burrows, and remains; another method that biologists use to determine if tortoises exist in an area is the presence of scat (feces). Fresh scat is dark brown or black, but turns gray as it weathers. Scat length varies, from one half to four inches, depending on the size of the tortoise. Scats usually contain coarse plant fibers.
Tortoise are inactive from mid November until February. The activity period for desert tortoises is from March until late October when they usually spend part of each day above ground. Tortoises are especially active during warm days when it is overcast or raining, when they seek water that collects in natural depressions or in depressions the tortoises dig themselves. Available drinking water is essential to tortoise survival. The diet of tortoises, which are vegetarians, includes a wide variety of herbs, grasses, cacti, and flowers. Since droughts are common in the desert that tortoises inhabit, they rely on the erratic years of good rainfall and the ensuing growth of palatable plants.
Sexual maturity for tortoises occurs at 15-20 years of age. Breeding occurs in March and April and egg laying is from May to july. Nests are almost always located at the entrance of burrows. Clutches 1 to 14 eggs and a mature may lay 0 to 3 clutches annually. The eggs are covered with soil and hatch after 80 to 130 days in August or September.
Predators are usually only a problem for young tortoises. Predation is the greatest cause of mortality for hatchlings. Eggs are eaten by Gila monsters, Foxes, coyotes, snakes, and badgers. The shell of juvenile tortoises does not harden for five or more years and young tortoises may fall prey to ravens, hawks, eagles, coyotes, foxes, bobcats, badgers, skunks, and feral dogs and cats. Up to 200 young tortoise carcasses have been found under raven perches and nests. While successful predation on adults is rare; coyotes, foxes, bobcats, eagles, and feral dogs have been known to prey on tortoises. Habitat quality can affect predation in certain habitats.
The Bureau of Land Management will be actively involved ongoing research projects that are addressing various aspects of tortoise management and physiology. Research is being conducted on the Upper Respiratory Tract Disease. Research will continue on those topics and on livestock grazing, predator-prey relationships, genetics, tortoise translocation/relocation, and habitat restoration.