The largest of the known species of Gopherus tortoises, the bolson tortoise, G. flavomarginatus, is also the largest terrestrial reptile in North America. In its native habitat, the bolson tortoise is called la tortuga grande, which translates into English as the large (or great) turtle. In addition, other Spanish common names for the species include la tortuga del monte (turtle of the mountain) and la tortuga del llanera (turtle of the plains).
The bolson tortoise is endemic to central northern Mexico, where it currently inhabits isolated pockets within the region of the Chihuahuan Desert known as the Bolsón de Mapimí. This phrase translates into English as the Mapimí Basin. While there is fossil evidence that the species once roamed the entire Chihuahuan Desert, and possibly beyond, G flavomarginatus currently inhabits a drastically reduced area within the Mapimí Basin.
The species was scientifically described in 1959 by American herpetologist John M. Legler in a paper titled “A new tortoise, genus Gopherus, from north-central Mexico” published by the University of Kansas Publications, Museum of Natural History. At first the newly described species was met with considerable skepticism. Subsequently, the species became widely accepted as valid. However, the bolson tortoise is likely the least studied of the Gopherus species.
Even though it inhabits a comparatively harsh and remote ecosystem, the bolson tortoise faces significant threats to its continued existence. The origin of these threats is largely due to the human presence in its habitat of the tortoise and will be discussed later in this article.
Gopherus flavomarginatus is a large land-dwelling tortoise, typically reaching a carapace length of 14.5 inches (37 centimeters) at maturity. According to research by herpetologist Dr. David Morafka, fossil evidence suggests that the carapace length could have been 39 inches (1 meter). The mass of modern mature individuals is generally 33 to 40 pounds (15 to 18 kilograms) or more.
The carapace of the species is oblong in shape and varies in coloration from straw-colored through greenish-yellow to brown. The central area of each scute, known in biology as the areola, is darker brown or black.
The species name, flavomarginatus, derives from two Latin word roots: flav-, meaning yellow, and -marginato, meaning edge or border. This specific name describes the lateral marginal scutes, which tend to be lighter in color than the rest of the carapace. The plastron of the bolson tortoise is typically yellow with a dark-brown to black blotch on each scute in younger animals. The dark blotches tend to fade as the animal matures. As is typical of other Gopherus species, the skin on the head, limbs and tail of the bolson tortoise is yellow to brown.
While there is minimal sexual dimorphism (differences in appearance between male and female bolson tortoises), males tend to be somewhat smaller at maturity than females. The male usually has a more concave plastron than the female, but there are exceptions to this generalization.
The tail of the male is somewhat larger than that of the female, and his gular scutes are longer. The male’s cloaca is located outside the rear marginal scutes, whereas the female’s cloaca is underneath the rear marginals. The male displays enlarged chin glands, particularly during mating season.
An unusual characteristic of the male bolson tortoise is the two dark-tipped spurs on the thigh of each of its hind legs; these spurs are absent on the female of the species (Gopherus flavomarginatus). Of the recognized Gopherus species, the bolson tortoise is physiologically most closely related to the gopher tortoise, G. polyphemus, a native of the southeastern United States. Biologists consider the species of Gopherus in the desert southwest to be “more distant cousins” of the bolson tortoise (Species: Bolson Tortoise (Gopherus flavomarginatus). ).
Historically the bolson tortoise occupied a significantly larger range than it does today. During the Pleistocene epoch (spanning the time from 2.6 million to 11,700 years ago), there is fossil evidence that the species ranged as far north as the modern state of Oklahoma in the United States and as far south as the Transverse Volcanic Axis. Also known as the Trans-Mexico Volcanic Belt, this region of active volcanoes runs from the state of Jalisco eastward to Veracruz in central southern Mexico.
The modern range of G. flavomarginatus is considerably more limited than its estimated historical range. In the decade between 1970 and 1980, Dr. David J. Morafka conducted extensive field studies on the species. In a paper published in 1982, he estimated that the wild population at that time was confined to less than 20,000 square miles (50,000 square kilometers) in central northern Mexico in the Mapimí Basin.
Subsequent research indicated that this range estimate was probably too optimistic. A paper published in 1997 estimated that the Mexican range of G. flavomarginatus could be as small as 2.3 square miles (6,000 square kilometers), and “the actual area occupied by tortoises may not be larger than 0.38 square miles (1,000 square kilometers), supporting a maximum population of 10,000 adults” (Aguirre, et.al, 1997).
Encyclopedia Britannica Online describes a bolsón as “a semiarid, flat-floored desert valley or depression, usually centered on a playa or salt pan and entirely surrounded by hills or mountains. It is a type of basin characteristic of basin-and-range terrain. The term is usually applied only to certain basins of the southwestern United States and northern Mexico.” Rivers and streams in the Mapimí Basin flow toward the center of the valley floor rather than to the Gulf of Mexico. These rivers and streams generally end in swamps or ephemeral lakes.
The Mapimí Basin is some 50,000 square miles (129,000 square kilometers) in total area and overlays portions of the Mexican states of Durango, Chihuahua, Coahuila and Zacatecas. The area is named after the town of Mapimí in Durango.
G. flavomarginatus inhabits the plains and low hills in the Mapimí Basin at elevations of 3,280 feet (1,000 meters) to 4,265 feet (1,300 meters), possibly up to 4,593 feet (1,400 meters) of elevation. Soils in this region are alkaline, coarse, and ordinarily composed of sand.
The bolson tortoise prefers areas in the Mapimí Basin with a slope of 1 to 2.5%. Poorly drained areas with a slope of less than 0.5% are less than desirable for burrowing, due to the possibility of summer flooding. In areas where the slope exceeds 3%, no burrows are found. This is likely due to the fact that the sandy soil of these lesser in–clines is replaced by the coarser desert pavement that overlies rocky substrate, making burrow excavation nearly impossible as well and as limiting plant growth (Morafka, 1982).
This region, according to Dr. Morafka, is “characterized by strong seasonality.” Winter is cold and dry, with evening frost from November through March, and January is typically the coldest month of the year. In spring, April and May are warm and dry, while summer, June through October, are hot and wet; June is ordinarily the warmest month of the year. Summer is the season during which most of the significant rainfall occurs.
The vegetation of the Mapimí Basin is collectively described as a tobosa grassland. The dominant plant material in much of the area is tobosa grass (Pleuraphis [=Hilaria] mutica), on which the bolson tortoise feeds. Other plant life in the tobosa grassland ecosystem includes creosote (Larrea divaricata), prickly pear (Opuntia species), guayule (Parthenium argentatum), honey mesquite (Prosopis glandulosa), tarbush (Flourensia cernua), etc.
The bolson tortoise is usually dormant in its burrow from the onset of the cold, dry winter season through early spring. During this time temperatures on the soil surface are dangerously low and food is in short supply. The tortoise may bask at the mouth of its burrow on sunny days, and may even occasionally forage for food. This winter dormancy occurs from November through April.
G. flavomarginatus emerges from winter dormancy with periodic foraging sessions in May and early June, particularly when spring brings enough rainfall to promote the germination of seedlings. Observers in Mexico report that courtship and mating occur at the same time.
When the warmer, wetter weather arrives in late July and continues through September, tortoises are active and out of their burrows grazing and hydrating. Generally speaking females nest in June and July, following which neonates emerge from July through early October.
During periods of intense heat, the bolson tortoise will retreat into its burrow to escape the searing surface temperatures. The burrow provides a thermal refuge from the temperature extremes of the surface environment. As temperatures begin to fall and rainfall diminishes in late October, bolson tortoises again take to their burrows for their annual winter dormancy.
Incidentally, G. flavomarginatus evolved during the Pleistocene Epoch, when Planet Earth was generally warmer and wetter, plant life in the grasslands of Mexico was more lush and plentiful than it is today.
Because of the increasingly hotter and drier conditions in its range, scientists surmise that climate change contributes significantly to the decline of the bolson tortoise due to reduced supplies of quality foods coupled with the skewing of the sex ratios in the tortoise populations. Like many other chelonians, the bolson tortoise is subject to temperature-dependent sex determination, meaning the sex of hatchlings depends on the incubation temperature of its eggs.
Mexican wildlife laws issued by the Ministry of Social Development (Secretaría de Desarrollo Social [SEDESOL]) legally protect the bolson tortoise (Zylstra, 2007).
The U. S. Fish & Wildlife Service listed G. flavomarginatus as “endangered” in the Endangered Species Act in 1979. At the second meeting of the CITES Conference of the Parties in San José, Costa Rica in the same year, uplisting of the bolson tortoise to CITES Appendix I occurred. The species was formerly listed in CITES Appendix II.
As of its last assessment in 2017, the International Union for the Conservation of Nature (IUCN) listed the bolson tortoise as “critically endangered” with a “decreasing” population trend. The IUCN estimated the number of mature tortoises in the Bolsón de Mapimí to be 2,500 individuals (Kiester, et al., 2018).