Desert
Tortoise Description and Natural History
The desert
tortoise (Gopherus agassizii) is one of four tortoises endemic to
North America. The other three endemic species are the gopher tortoise (G.
polyphermus), the Berlandier's tortoise (G. berlandieri),
and the Bolson tortoise (G. flavomarginatus). The natural range of
the desert tortoise encompasses both the Sonoran and Mojave deserts of the
southwestern United States, and includes parts of Arizona, California,
Nevada, Utah, and Mexico (Fig. 1). There are two genetically distinct
populations of the desert tortoise; Sonoran tortoises are found south and
east of the Colorado River, while Mojave tortoises are found north and west
of the Colorado (USFWS, 1989). Desert tortoises have a highly domed,
distinctly ridged carapace ranging in length from 8 to 15 inches (20 to 38
cm). The carapace is brown or horn colored while the plastron is yellow hued
and without a hinge. Male tortoises have extended gular shields used in
combat with other males during the breeding season (Stebbins, 1985). Both
sexes have stout, elephant-like limbs with allows these reptiles access to
an amazing range of microhabitats, from shallow desert washes to extremely
steep mountainous slopes. Like many desert reptiles, desert tortoises
thermoregulate by moving between areas that provide a wide range of
temperatures such as open ground, temporary rest areas, and permanent
sheltersites. Desert tortoise activity at any time of the year is driven by
temperature and forage availability. During the spring months (March to
May), temperatures and forage availability are the most favorable, and
desert tortoises spend a significant portion of their day foraging and
searching for mates. As temperatures rise with the onset of the summer
months (June to late July), desert tortoises begin to aestivate; spending
more and more time inactive underground in sheltersites. In the Sonoran
desert, summer monsoon rains during the months of July to October bring the
tortoises out of aestivation to build up their energy reserves before winter
hibernation. Mojave tortoises do not have access to the summer monsoon and
mainly feed in the spring. When temperatures begin to decrease in the late
fall and winter months (October to February), desert tortoises hibernate in
sheltersites until spring.
Threatened
Status and Desert Tortoise Law
The decline of
desert tortoise populations in the last few decades spurred the emergency
listing of the desert tortoise as an endangered species on August 4, 1989.
The final ruling however, listed the Mojave population as threatened on
April 2, 1990 (USFWS, 1990). There are many causes contributing to the
population decline of the desert tortoise such as collection of tortoises
for pets, raven predation of eggs and juveniles, habitat disturbance and
loss, and the spread of chronic upper respiratory tract disease (URTD) (Luckenbach,
1982). Although a combination of the above factors has lead to population
declines of the desert tortoise, two causes outweigh the others in both
severity and impact. As with many other threatened or endangered reptiles,
the largest threat facing desert tortoise populations today is habitat
disturbance and loss. Land development over the natural range of the desert
tortoise has not only resulted in the direct removal of habitat, but has
also resulted in increased on and off-road vehicular activity, and livestock
grazing. Increased vehicular activity increases the numbers of tortoises
killed while attempting to cross roads, and is a significant cause of
tortoise mortality (Berry, 1984). Off-road vehicle usage and livestock
grazing have made the way for the introduction and spread of non-native
plants. When habitat is disturbed, invasive, non-native plants can
outcompete native plants causing a shift in the vegetational ecology of an
area. Jacobson (et al., 1991) suggested that desert tortoises depend upon
the superior nutrition they receive from a diet composed of native
vegetation to maintain their health and resistance to disease. The other
major threat facing desert tortoises is the incidence and spread of upper
respiratory tract disease (URTD). Over the last ten years, URTD has spread
into wild populations of desert tortoises with devastating effects. Although
the mortality rate of URTD is not as high as researchers once feared it
would be, it is, and will continue to be, a significant cause of desert
tortoise mortality. Researchers have found that the pathogenic agents
responsible for this disease can occur naturally in the respiratory tracts
of healthy desert tortoises with no apparent ill effects. It is currently
believed that stress, caused by such factors as overcrowding, poor diet, or
incorrect environmental conditions increases a tortoise's susceptibility to
URTD (Brown et al., 1992). Any tortoise that is subject to stress can
develop or carry URTD, therefore, it is imperative that captive desert
tortoises are NEVER released back into the wild, as they may carry URTD
which could possibly infect healthy wild populations. It is also inadvisable
to keep desert tortoises in inappropriate habitats, and therefore captive
desert tortoises should only be kept by individuals residing in or close to
the natural range of the desert tortoise.
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