A NATURAL HISTORY
Produced by the Arizona-Sonora Desert Museum's Tortoise Adoption Program
TORTOISES IN GENERAL
The tortoises comprise the turtle family Testudinidae consisting
of thirty-nine living species in ten genera. Today, tortoises are found in Asia,
Europe, Africa, oceanic islands and the Americas. Included among these are two
gigantic forms, the well-known Galapagos tortoise (Geochelone elephantopus) of
the Galapagos Islands in the Pacific Ocean and the Aldabran tortoise (G. gigantea)
of the Seychelles Island in the Indian Ocean. Given the ling evolutionary history
of tortoises in North America and the dynamic environment of the last thirty million
years, it is indeed remarkable that the form of the four living North American
tortoises have changed so little from their ancestors. One of these, the desert
tortoise, (Gopherus Tortoises
may be distinguished from other turtle families by the following characteristics:
the hindlegs are cylindrical and elephantine in shape: the feet are short, broad
and club-shaped. In some genera, the forelimbs are flattened and adapted for digging
and burrowing and the toes are completely unwebbed. The carapace (upper shell)
is usually high and domed with the ribs modified in alternating triangular wedges.
All are terrestrial and basically herbivorous, although they may occasionally
eat invertebrates and carrion. CLASSIFICATION OF NORTH AMERICAN TORTOISES
North American tortoises are grouped in the genus Gopherus although two
distinctive groups, considered by some authorities to represent subgenera, are
now recognized. The primitive gopher tortoises, including the desert tortoise
(G. NATURAL
HISTORY OF THE DESERT TORTOISE The desert tortoise, an ancient denizen
of western North America, occurs today in the Mohave and Sonoran deserts in southwestern
Utah, southern Nevada, southeastern California and western Arizona in the United
States. In Mexico,"la tortuga de tierra" occurs throughout most of Sonora, including
Isla Tiburón in the Gulf of California and south into northwestern Sinaloa.
Considerable variation in its ecology, behavior, morphology, and DNA has been
noted in different portions of its range. Three distinctive subspecific populations
have been defined in the Mohave Desert, Sonoran Desert, and tropical deciduous
forest. The desert tortoise lives in a variety of habitats form sandy flats to
rocky foothills, with a strong proclivity in the Mohave Desert for alluvial fans,
washes and canyons where more suitable soils for den construction might be found.
It is found from near sea level to around 3,500 feet in elevation. The desert
tortoise reaches an average length of 6 to 14.6 inches, with males growing larger
than females. A gigantic specimen, allegedly from Mexico, at the San Diego Natural
History Museum, has a shell of 15.9 inches long. Other large individuals have
been found in the Mohave Desert in California. The desert tortoise occurs
in a number of plant communities ranging from sparse creosote bush desertscrub
in the winter rainfall Mohave Desert to palo verde-saguaro desertscrub in the
bi-seasonal Sonoran Desert and eventually to summer rainfall tropical thornscrub
and deciduous forest in Sonora and Sinaloa. In the Sonoran Desert, tortoise density
seems to be related to the density of perennial plants and plant species composition
which are controlled by the amount of rainfall and winter freeze frequency. Prior
to the early 1950's, many populations reached densities of several hundred tortoises
per square mile. Today, most populations contain no more than five to fifty tortoises
per square mile. Native and introduced grasses comprise the bulk of the desert
tortoise diet. Otherwise, they eat any available edible plants including spring
and summer annual wildflowers, forbs and cactus fruit. Tortoises forage selectively,
often sniffing or sampling various plants before consumption. Rocks and soil are
also ingested, perhaps as a means of maintaining intestinal digestive bacteria
and as a source of supplementary calcium or other minerals. Stones may function
as gastroliths enabling more efficient digestion of plant material in the stomach.
The extent of the home range (total habitat area used to fulfill life functions)
of the desert tortoise depends upon various factors such as the densities of food
plants, and the age, size, sex of the tortoise. These factors and presumably the
size of the home range vary throughout the species' range. There is some evidence
that tortoises utilize their feces in making home ranges, dens and burrows. They
may be detecting secretions from cloacal glands. It has been suggested that tortoises
rarely move more than two miles from their natal nest in their entire lives.
Well-adapted physiologically and behaviorally to live in dry desert environments,
desert tortoises derive almost all their water intake from the plants they eat.
A large urinary bladder can store over forty percent of the tortoise's body weight
in water, urea, uric acid and nitrogenous wastes. Water conservation is further
aided by an ability to precipitate solid urates in the bladder, allowing water
and ions to be reabsorbed while uric acid is eliminated in semi-solid form. During
periods of sufficient rainfall tortoises drink copiously from temporary rainpools
and eliminate solid urates. A common defensive behavior when molested or handled
is to empty the bladder, leaving the tortoise at a considerable disadvantage in
drier conditions. For this reason, desert tortoises should not be handled when
encountered in the wild. Other avenues of water loss include respiration, defecation,
and evaporation. Activity patterns of the desert tortoise help in water conservation.
It is chiefly active in the day (diurnal) or morning and evening (crepuscular),
depending upon temperature and season. Summer estivation during the hottest, driest
periods of the year conserves water already stored in the body. This is especially
important in the hot, dry Mohave Desert summers. Burrow humidity is often as high
as forty percent or more, thus reducing the rate of evaporation. Winter hibernation
also aids in minimizing water loss. Burrows and dens are also used by the desert
tortoise as an aid in regulating body temperature. The flattened forelimbs
of the desert tortoise an other gopher tortoises are capable tools for burrow
construction. They dig with the front legs, stopping intermittently to sniff the
soil. As soil is displaced, the tortoise will frequently exit the burrow and kick
the excavated soil still further from the burrow entrance. The entrance is half-moon
shaped and high enough for the tortoise to comfortably enter without fully extending
the legs. The location, extent and type of burrow or den varies geographically.
Tortoises in the Mohave Desert in California and the northern limits of the range
in Nevada and Utah seem more inclined to construct extensive burrows, up to thirty-five
feet in length. Such burrows stabilize temperature and humidity providing protection
form intense winter freezes. They may be used year after year by one or more tortoises.
As many as twenty-five hibernating tortoises have been found in one den, although
a more typical aggregation would contain no more than five individuals. Some dens
in southern Utah are estimated to be 5000 years old. Burrows are typically located
under rocks or bushes, preferably along sloping terrain, and along washes, either
at the base or elevated from the bottom. In the Sonoran Desert in southern
Arizona and presumably south into Sonora, the desert tortoise hardly burrows.
Refuges merely cover the carapace and are often modified from mammal burrows or
natural refuges in rocky terrain. Sonoran desert tortoise retreats are often on
rocky slopes in mountains, avoiding the deep soiled valley situations favored
by Mohave desert tortoises. Pallets are shallow depressions constructed under
low shrubs at various points within the tortoises' home range, providing temporary
resting sites. They are especially prominent in southern desert tortoise populations
where mild winters mitigate the need for extensive burrows and desert tortoises
may not hibernate. Here, burrows are often dug into the base of packrat houses
rather than in the gravelly soil. Like most other burrowing animals, the desert
tortoise creates a subterranean environment beneficial to other reptiles, mammals,
birds and invertebrates. Animals which share tortoise burrows benefit from permanent
or temporary shelters afforded by the tortoise dens and burrows, although they
offer little or nothing to the tortoise. Many behavioral attributes of the
desert tortoise are well documented. When confronted by a predator, tortoises
typically withdraw their head, feet, and tail, folding their front knees in front
of their head, thus exposing only the shell and heavy scales of the armored forelimbs.
This is an effective defense against most predators except people. If attempts
are made to remove a tortoise from its burrow, it will retreat to the interior
or extend the legs, wedging the carapace against the roof of the burrow. The defensive
behavior of adult tortoises is usually passive while juveniles can be surprisingly
pugnacious. Social behavior consists of a series of head bobs for species
and gender recognition, courtship and threat. Head bobbing normally precedes agonistic
(combative) behavior between males, although females may also be aggressive. Prominent
chin glands in male tortoises produce a secretion which aids in sex recognition
and often evokes combative behavior. Male combat is most intensive in spring and
late summer in the Sonoran Desert. During these encounters, each male stands as
high as possible, making short rushes toward his adversary while attempting to
use the gular horn at the front of the plastron (undershell) to overturn the other
or drive him away. An overturned tortoise can usually right itself using its head
and a forelimb; if not, the tortoise may overheat and die under the desert sun.
The desert tortoise produces a variety of sounds (hisses, grunts, pops, whoops,
huhs, echs, bips, etc.) which seem to be the most important when vocalized to
an unfamiliar tortoise. A loose male dominance hierarchy is apparently established
by aggression. Dominant males court and mate with females more often than other
males. Courtship involves extensive head bobbing as the male attempts to nip and
bite at the edges of the female's carapace and legs while circling her. If the
female is receptive, she will allow the male to mount her from behind. At this
point the female will remain still as the male probes with his tail while grunting
and enthusiastically stamping his hind feet. The nuptial embrace continues until
the female wanders away. The mating posture is facilitated by a strong depression
in the male's plastron that fits neatly onto the convex carapace of the female.
The males's longer tail enables the penis to penetrate the cloaca. The neatly
upright copulatory position of the male is further aided by the inward curve at
the rear of the male's carapace. The mature female differs in having a flat plastron,
a shorter tail, and an outward curve at the rear of the carapace which probably
provides a wider space for egg laying. The gular horn of the male is longer and
more curved, the claws more massive. While sexual maturity in the wild is
estimated to take twelve to twenty years, it is a factor of growth and size rather
than age. Tortoises reared in captivity may mature sooner. Mating has been observed
from early spring to fall with the highest frequency in late summer in the Sonoran
Desert. Viable sperm retained in the cloaca of the female has resulted in fertilization
a year and a half after copulation. Other turtle species have laid fertile eggs
as long as four years after mating. Sperm retention is an excellent survival adaption
in non-colonial animals that wander and whose numbers can decline in fluctuating
climates of deserts. Nest sites are often selected in or near dens or pallets.
The female excavates the nest hole using her hind legs. She urinates before, during
and after the nest hole is dug as well as after covering the eggs, possibly to
deter predators by camouflaging the nest and to prevent egg desiccation. In one
observation, a female fought a Gila monster (Heloderma suspectum) attempting to
eat newly laid eggs suggesting that nest sites may be defended for a period of
time following egg deposition. Desert tortoises normally construct nests and lay
eggs in May or June. In the southern portions of the range, a second clutch may
be produced in late summer. These late summer clutches may undergo extended incubation
periods hatching in the fall or next spring. The normal incubation period is ninety
to one hundred thirty-five days depending upon incubation temperature. The clutch
size varies from two to fourteen eggs with an average of three to five although
some eggs may not be fertile. The eggs are hard-shelled, moisture proof, white
and nearly spherical to ellipsoid in shape. Mortality of both eggs and juveniles
is extremely high due to predation and environmental conditions. Probably no more
than one hatchling from every fifteen to twenty nests will reach sexual maturity
in the wild resulting in very low recruitment to the population. The fifty to
eighty year life span estimated for desert tortoises suggests population turnover
is not only low but should be very episodic following fluctuating climates. The
desert tortoise could reach a "point of no return" as more reproducing adults
fall victim to humanity's expanding impact in fragile desert environments.
The desert tortoise has long been utilized by southwestern peoples. The tortoise
was relished as food by the Piman, Paijute and Seri Indians. Shells were used
as cooking vessels and as trade items. The Seris of the coast of the Gulf of California
in Sonora used tortoise parts for medicine and shell rattles as musical instruments
and toys. According to Richard Felger and Rebecca Moser, who have studied the
ethnobiology of the Seris extensively, "If a woman has given birth to only female
offspring, she is said to have eaten the reproductive organs of a female desert
tortoise. If her offspring are all male, it is said that as a child she had been
hit in the small of her back with the reproductive organ of a male desert tortoise
playfully thrown at her by a girlfriend." Seri folklore features the desert tortoise,
called ziix hehet cquiij meaning "thing that sits in bushes." Later, tortoises
were eaten by white settlers and prospectors. Mexican traders carried them alive
as a source of fresh meat and water. Tortoises are occasionally eaten in Sonora
today. The desert tortoise is protected as a Threatened species under the
U.S. Endangered Species Act north and west of the Colorado River in California,
Nevada, and Utah where an upper respiratory tract disease has decimated many populations.
Numerous other factors have contributed to this decline including residential
development, road construction, agricultural and mineral development, use of off-road
vehicles, overgrazing, malicious vandalism, and collection as pets. The status
of the populations in Arizona is apparently less serious but warrants continued
monitoring and research. While remote populations appear to be stable, those near
urban or recreational centers have declined significantly. The desert tortoise
is fully protected in Arizona and collection from the wild is strictly prohibited
without a permit issued by the Arizona Game and Fish Department.
Many Arizonans
are interested in having a desert tortoise for a number of reasons. This interest
can be met through regulatory mechanisms and captive tortoise recycling programs
like the Desert Museums' Tortoise Adoption Program (TAP). Modeled after similar
programs in California, applicants are carefully screened for intent and responsibility
before being assigned a tortoise, which remains the property of the State of
Arizona. All tortoises placed are urban foundlings, unwanted captives, or their
progeny. The purposes of the program, sanctioned by the Arizona Game and Fish
Department, are to provide appropriate care and custody for tortoises already
in captivity while vigorously discouraging the taking of tortoises from the
wild. Thousands of tortoises are held in captivity in Arizona. It is ironic
that people's attraction to the tortoise has become a significant threat to
its future. Unfortunately, release of captive tortoises is considered a high
risk to existing populations because of the potential to introduce disease,
disrupt population structure, and mix genetic stock from different regions.
Management of the captive population separately from those in the wild may actually
aid conservation of wild tortoises. Under Arizona law, one tortoise per family
member may be possessed if the tortoises are obtained from a captive source
and properly documented.
