San Francisco State University
Department of Geography

Geography 316:  Biogeography  In progress 11/14/00

The Biogeography of The Desert Tortoise (Gopherus agassizii)
by Kerrie Bathel, student in Geography 316, Fall 2000
 


(Desert tortoise in it's natural habitat)

Photo credits: Kristin H. Berry, Robert D. Berry, Nathan W. Cohen, Michael J. Connor,
Ralph Crane, Susan Moore, Mary H. Shepherd, Beverly Steveson, Laura Stockton
 

Taxonomy Classification

Kingdom:  Animalia
  Phylum:  Chordata
    Subphylum:  Vertebrata
      Class:  Reptilia
        Order:  Chelonia
          Suborder:  Cryptodira
            Super Family:  Testudinoidea
              Family:  Testudinidae
                Genus:  Gopherus
                     Species:  Gopherus agassizii
 
 

Introduction

The desert tortoise is a member of the reptile family that is composed of snakes, lizards, crocodiles, and other chelonians (turtles).  Turtles are obviously different from other reptiles by the shell or “box” that completely covers the body.  The shell is actually a part of the body and hardens about three years after hatching (Redrobe, 2000).  Tortoises are any of the land-dwelling turtles of the family Testudinidae (Burge, 2000).  The desert tortoise is one of four species of the genus Gopherus, known collectively as gopher tortoises.
 
 
 

Description of Species

Size:

 (Anatomy description of the desert tortoise)

Illustration by The Desert Tortoise Preserve Committee

Gopherus agassizii is terrestrial, with a domed shell and round, stumpy elephantine hind legs.  The front limbs are flattened for digging and heavily scaled without webbed toes.  The carapace (upper shell) is oblong and domed with the sides round due to joining of the carapace and plastron (lower shell).  The scute centers are often yellowish which have grooved concentric rings.  The plastron is also yellowish, with brown along the scute margins.  The head is small and rounded in front with reddish-tan coloring and the iris being greenish-yellow.  The front and hind feet are about equal in size and the tail is of short length (Behler/King, 1979).
 

Evolution
 


What species did turtles derive from?  Could it be from birds?  How about crocodilians or snakes?  Recently, molecular biologists found new evidence from two nuclear genes, and analyses of mitochondrial DNA and 22 additional nuclear genes, join crocodilians with turtles and place squamates at the base of the tree (Rieppel and Hedges SB, 1999).    Morphological and paleontological evidence for this molecular phylogeny is unclear according to Rieppel and that molecular time estimates support a Triassic origin for the major groups of living reptiles.  The time estimates indicate that squamates diverged from the other reptiles at 245 12.2 million years ago (mya), birds diverged from the lineage leading to turtles and crocodilians at 228 10.3 mya, and that turtles diverged from crocodilians at 207 20.5 mya (Rieppel and Hedges SB, 1999).  These divergence times are close to when the first turtles (223 to 210 mya) and crocodilians (210 to 208 mya) appear in the fossil record and earlier than the first birds (152 to 146 mya) and first squamates (157 to 155 mya) (Rieppel 1999).

(A molecular phylogeny of reptiles)

There will always be disputes and disagreements in science, especially when it refers to the origins of life.  These differences are primarily due to the variance in methodologies used and the incomplete fossil record.  The controversy that surrounds the evolution of Testudines is a complex one.  Several researchers have recently proposed that the closest relatives to turtles are Anapsids, which include the Proclolphonids and Parieasaurs.  While other groups suggest that turtles are not derived from an extinct anapsid group but arise from within diapsids and are thus close to the origins of lizards and snakes.  According to Rieppel (1999), phylogenetic analyses are the best ways to determine the origin of turtles.
 
 

Distribution Range

Gopherus agassizii is found in the Mojave and Sonoran Deserts of southeastern California, southern Nevada and south through Arizona into Mexico from near sea level to around 3,500 feet in elevation (Lawler, 2000).  The distribution of Gopherus agassizii is a continuous distribution throughout the Mojave and Sonoran Deserts, while the family, Testudinidae is disjunct, spread out over various continents due to continental drift or climatic factors.  What was once a dominant and widespread family, 220 million years ago, is now an evolutionary relict.  Gopherus agassizii has specific preferences and has a limited tolerance ecologically (stenotopic).  According to Lawler, the range of individual tortoises depends on factors such as density of food plants, size, age and sex of the tortoise.  It is said that a tortoises range is no more than two miles from where it hatched (Lawler, 2000).

Even though the desert tortoise is classified as a single species, scientists have divided the species into two populations, Mojave and Sonoran Desert tortoises.  However, recent discoveries show that there is another population similar to both the Mojave and Sonoran (Blanchard, 1999).  Scientists are in the process of studying them to determine the tortoises characteristics and origin (Blanchard, 1999).  The new and very small population is found in the Black Mountains of northwestern Arizona, north and west of Kingman, and on the Sonoran side of the Colorado River, preferring the topography to be flat.  They are known as the Black Mountain Tortoises, but are currently listed under the Sonoran population  (Blanchard, 1999).

(Distribution map of the Mojave and Sonoran population range)

Illustration by Kerrie Bathel
 

Mojave Desert:
The population of the Mojave Desert tortoise is found west of the Colorado River and north of the Grand Canyon, preferring the flat and open land.  The desert region of southeastern California and portions of Nevada, Arizona and Utah occupies more than 25,000 square miles (Blanchard, 1999).

Sonoran Desert:
The Sonoran Desert tortoise prefers the rocky slopes and inhabits the Sonoran Desert.  The desert covers 120,000 square miles in southwestern Arizona and southeastern California, as well as most of Baja California and the western half of the state of Sonora, Mexico (Blanchard, 1999).
 
 

Natural History

Desert tortoises are well adapted to living in a highly variable and often harsh environment.  This environment can range from temperatures below freezing to temperatures that exceed 140 degrees Fahrenheit (Burge, 2000).  It is understandable then, that 95% of its life is spent in burrows (Burge, 2000).  While in the burrows or caves, they reduce their metabolism and loss of water and consume very little food (National Wildlife Refuge System, 2000).  Adult desert tortoises lose water at such a slow rate, that they can survive for more than a year without access to free water of any kind (National Wildlife Refuge System, 2000).  They eventually emerge to feed and mate during late winter and early spring, remaining active up until early summer and sometimes emerging again after summer storms (National Wildlife Refuge System, 2000).  If a tortoise can surpase an early demise due to human and natural activities, they have a high chance to live past eighty years.
 

Habitat:
The desert tortoise lives in a variety of habitats from sandy flats to rocky foothills, including alluvial fans, washes and canyons.  The various types of plant communities consist of creosote bush, thorn scrub and cacti.  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 (Lawler, 2000).  Prior to the early 1950's, many populations reached densities of several hundred tortoises per square mile (Lawler, 2000).  Today, most populations contain no more than five to fifty tortoises per square mile (Lawler, 2000).

(Desert tortoise in its natural habitat under desert scrub)

Photo credits: Kristin H. Berry, Robert D. Berry, Nathan W. Cohen, Michael J. Connor,
Ralph Crane, Susan Moore, Mary H. Shepherd, Beverly Steveson, Laura Stockton
 

Burrows/Dens:
According to Lawler, the location, extent and type of burrow or den varies geographically.  Tortoises in the Mojave 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 (Lawler, 2000).  They spend most of each day in underground burrows to prevent overheating in the summer and freezing in the winter.  Such burrows stabilize temperature and humidity providing protection from intense winter freezes and extremely summer heat (Lawler, 2000).  It is common for tortoises to use the same den year after year and share often with other 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 (Lawler,2000).  Tortoises also acquire their dens either by excavating burrows themselves, or finding previously occupied dens from other species.  Burrows are typically located under rocks or bushes, preferably along sloping terrain, and along washes, either at the base or elevated from the bottom (Lawler, 2000).

(Photo depicts a desert toroise's burrow/den)

Photo credits: Kristin H. Berry, Robert D. Berry, Nathan W. Cohen, Michael J. Connor,
Ralph Crane, Susan Moore, Mary H. Shepherd, Beverly Steveson, Laura Stockton
 

Diet:
The desert tortoise is an herbivore with a diet of various herbs, grasses, some shrubs and the new growth of cacti, cacti fruit and their flowers.  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 (Lawler, 2000).  In the Mojave Desert, tortoises specialize in spring annuals (Lawler, 2000).  In the Sonoran Desert, grasses dominate, but the diet also includes perennial wildflowers and other herbaceous plants (Lawler, 2000).  However, their food source is declining due to grazing by sheep.  A study was conducted within the desert tortoise range and found that over 65% of the natural area was grazed (Campbell, 1985).  This was bad news, considering sheep grazing is not permitted within the city limits of California City, where the study took place (Campbell, 1985).  Rarely do city officers take action, but if they do, the herders are given three days to move their sheep.
 
 

Breeding: (statistics are from Burge, 2000)

During the late summer and early fall when tortoises are above ground, the males testosterone levels begin to peak and courting occurs (National Wildlife Refuge System, 2000).  Once the male and female unite (copulation), the female stores the sperm and lays her eggs around the months of May, June or July (National Wildlife Refuge System, 2000).  The number of eggs varies depending on the size and maturity of the female.  The range is from 2-14 eggs in a clutch, with usually 2-3 clutches a season (Behler/King, 1979).  The eggs are hard, chalky and elliptical or spherical and buried in a funnel-shaped nest (Behler/King, 1979).  Hatchlings from only a few eggs out of every hundred actually make it to adulthood (Burge, 2000).
 

Conservation

Currently, it is only the desert tortoise of the Mojave Desert that is federally listed as a threatened species. State and federal wildlife and land management agencies and local jurisdictions are actively involved in conservation programs to help the recovery of the desert tortoise throughout the Mojave Desert (Burge, 2000).  The populations have declined by 90 percent since the 1980s primarily due to predators such as gila monsters, kit foxes, badgers, roadrunners coyotes and most of all ravens, which account for more than 50 percent of the juvenile desert tortoise deaths (Burge, 2000).
 

Other major threats remaining to the desert tortoises include:


**NOTE….
It is unlawful to touch, harm, harass or collect a wild desert tortoise. There are programs run by tortoise clubs in Arizona, California and Nevada through which legally acquired captives may be adopted.  For more information in California, contact the the California Turtle and Tortoise Club at http://www.tortoise.org/cttc/adoption.html
 
 

To learn more about the desert tortoise, please visit these interesting sites:

King, F. Wayne and Burke, Russel L. (1997). Crocodilian, Tuatara, and Turtle Speies of the World, An Online Taxonomic and Geographic Reference. [Online]. Available: http://www.flmnh.ufl.edu/natsci/herpetology/turtcroclist/default.htm[2000]

Connor, Michael J. (2000). California Turtle and Tortoise Club's World Wide Website [Online]. Available: http://www.tortoise.org/[2000]
 
 
 
 

Bibliography
Behler, John L. and King, F. Wayne. National Audubon Society, Field Guide to North American Reptiles and Amphibians. New York: Alfred A. Knopf, New York, 1979

Blanchard, Donald L. (May, 1999). Desert Tortoises (Gopherus agassizii) from Arizona's Black Mountains.  [Online]. Available: Http://www.coloherp.org/cb-news/cbn-9905/DesertTorts.html [October 12, 2000],

Biological Resources Research Center, University of Nevada, Reno. Gopherus agassizii, Desert Tortoise. [Online]. Available: http://www.brrc.unr.edu/data/reptiles/gophagas.html [October 12, 2000]

Burge, Betty. (2000). The Desert Tortoise.  [Online]. Available: http://www.desertusa.com/june96/du_tort.html [2000]

Berry, Kristin H., Rober D. Berry, Nathan W. Cohen, Michael J. Connor, Ralph Crane, Susan Moore, Mary H. Shepherd, Beverly Steveson, Laura Stockton. (2000) Desert Tortoise Preserve Committee. (2000). Desert Tortoise Natural Area Virtual Field Trip. [Online]. Available:
http://www.tortoise-tracks.org/dtna/tour.html

Lawler, Howard. (2000). A Natural History of The Desert Tortoise, Gopherus [Xerobates] agassizii.  [Online]. Available: http://www.biopark.org/destort1.html[2000]

National Wildlife Refuge System. (2000). Desert Tortoise, Mojave Population.  [Online].  Available: http://bluegoose.arw.r9.fws.gov/NWRSFiles/WildlifeMgmt/SpeciesAccounts/Reptiles/DesertTortoise.html[2000]

Campbell, Tom. (1982). Hunting and Other Activities on and Near the Desert Tortoise Natural Area, Eastern Kern Couty, California: The Desert Tortoises Council (p.90-98). Desert Tortoise Council, Inc: 1985

Redrobe, Sharon. (2000). Chelonians - Veterinary Practice and Husbandry.  [Online]. Available: http://home.earthlink.net/~rednine/chelonia.html [2000]
 

Rieppel, Olivier. (1999). A Commentary Piece from the Journal of Science, 1999 Feb 12;283(5404): 998-1001 on the Prion Gene found in Turtles, by Hedges SB, Poling LL. [Online]. A webmaster publication. Available: http://www.mad-cow.org/00/mar00_turtle.html
 

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