|Listed||April 17, 1979|
|Description||The largest North American land tortoise, with a flat-topped, posteriorly flared carapace with laminae that are pale in contrast to dark centers; the plastron is horn yellow.|
|Habitat||Low, grassland slopes with fine soil, where tobosa grows.|
|Reproduction||Three clutches a year, with three to nine eggs per clutch.|
|Threats||Human predation; habitat modification; livestock grazing; overcollection.|
The Bolson tortoise is the largest terrestrial ectothermic vertebrate in North America and one of the four species of the genus Gopherus, a genus confined to North America. This species was only described in 1959 and remains one of the least known reptiles in the northern hemisphere in spite of its size. Adults may measure in excess of 3.3 ft (1 m) shell length, with a flat-topped, posteriorly flared carapace with laminae that are pale in contrast to dark centers. The plastron is horn yellow. In addition, there are various bone features (osteological characters) which distinguish this species from its close relatives. The contrast between the yellow background and the darker concentric rings caused by the growth of the plates is more noticeable in juveniles. The limbs are squat and columnar, with strong claws.
In order to maintain a stable body temperature during the ever-changing seasonal temperatures, the Bolson tortoise basks. Reproduction is not well-known. It is possible that mating does not occur at all in dry years. Breeding possibly occurs as early as May and as late as October. Eggs are laid in the terminal chamber of the burrow. In captivity, females lay up to three clutches a year, with three to nine eggs per clutch. Hatching occurs about three months after incubation; of the 30 eggs laid, only a few hatch. The Bolson tortoise digs burrows up to 34 ft (10.4 m) in length at depths of up to 5 ft (1.5 m). Because of aggressive behavior among males, burrows are located fairly far apart. Only one individual, either male or female, inhabits a burrow. This tortoise prefers to construct burrows on the shores of temporary or dried lakes, a preference that irrigation has exploited. It is presumed that this gopher tortoise spends daylight hours in tunnels and surfaces at twilight for feeding. Because of the changing temperatures, it is most active in spring and summer, with reduced activity in the hot months. In winter, it becomes dormant. The Bolson tortoise eats primarily grasses, but will also eat shrubs and herbs. The principal food is low bushes and thick plants; the preferred food is the tender shoots or flowers.
The Bolson tortoise is found only in the desert of north-central Mexico (southeastern tip of Chihuahua and southeastern Coahuila) in discontinuous populations. It prefers habitat with low, grassland slopes with fine soil, where tobosa (Hilaria mutica ) grows, on which its food supply depends. These areas are generally found at elevations of 1,000-1,300 ft (304.8-396 m), with a changing climate. In winter, temperatures can drop to 38°F (3.3°C) with night frosts. In summer temperatures are near 90°F (32.2°C).
The Bolson tortoise is associated with grasslands and may at one time have been much more wide ranging than at present. It is now confined to southeast Chihuahua, southwest Coahuila, and northeast Durango in northern Mexico. At present there are a few remaining viable populations, although nearly all populations have been impacted directly by man. The population density does not exceed seven individuals per hectare (2.5 acres).
Before the Bolson tortoise was listed as an endangered species, it was in demand for private collections, zoos, and museums in the United States and elsewhere. With the addition of the tortoise to the Convention on International Trade in Endangered Species (CITES), world trade was discouraged; however, Mexico was not a signatory to CITES until 1993, and illegal trade to other countries probably continued. Human predation on the tortoise is a main cause for its extirpation or reduction in numbers over a large area of its range. The tortoise is used extensively for food by the local population, and although much of the area inhabited by tortoises is sparsely settled, the tortoise population is often eliminated as far away as 6.2 mi (10 km) from the nearest habitation. As water supplies have increased, so have settlements, bringing tremendous pressure on tortoise populations. Habitat destruction is accelerating throughout the range of the Bolson tortoise. Plowing and irrigation of fields for cotton, beans, corn, and melons have apparently contributed to the extirpation of the species in certain areas, such as the region west of the Mexican Highway 49 and around Tiahualilo in Durango. As Mexico's resettlement program continues, more and more land has been converted to agricultural and livestock use. Habitat destruction has occurred through overgrazing by cattle and goats. Goat herds have long grazed the foothills of the tortoise country. Water supplies have been increased in the area through underground drilling; as result, cattle have rapidly increased in density in the arid grasslands. Overgrazing has caused the erosion of topsoil, permitting the invasion of mesquite and creosote scrub. Cattle and goats destroy browse needed by the tortoises, as well as destroying burrows and cover sites by trampling.
Conservation and Recovery
With the listing of the Bolson tortoise, the illegal trade to the United States may have been reduced. It is illegal for any person subject to the jurisdiction of the United States to take, import or export, ship in interstate commerce, or sell or offer this species for sale in interstate or foreign commerce.
Instituto Nacional de Ecología
Av. Revolución, 1425
Col. Campestre, C.P. 01040, Mexico, D.F.
Lovich Ph.D., Jeff. 1999. "Synopsis of Conservation Information on the Desert Tortoise." USGS Western Ecological Research Center. http://www.werc.usgs.gov/cc/synopsis.html. [Accessed 4 August 2000].