Description: Bolson tortoises are the largest North American tortoises, with a mean carapace length of approximately 11.6 inches. Carapace lengths commonly range from 6.5 to 14.5 inches. The base color of the plastron is yellowish, and that of the carapace ranges from darker shades of straw yellow to brown. The centers of the epidermal laminae are darkest, ranging from brown to black. They have a faint radial pattern on the carapace, most noticeably on the centrals.

Their soft body tissue coloration ranges from dirty cream to yellow on areas where skin is granular. Larger scales (e.g., on their forelegs and heads) are pale yellow with dark centers of varying size. They have a flat-topped, posteriorly-flared carapace with laminae that are pale in contrast to the dark centers. The contrast between the yellow background and the darker concentric rings caused by the growth of the plates is more noticeable in juveniles.

In adults, there are 19 zones of growth, including the actively-growing zone and the zones formed in the season of hatching. Growth zones formed in the same year on different laminae positively correlate to the abdominal lamina, they both grow in equal measures at the same time. These turtles have thick limbs and strong claws. The plastron of males is slightly more concave, in the abdominofemoral region, than that of females. Females' tails are shorter and blunter, while in males, the tail posterior to the anus is more tapered and has a distinct tip.

The iris is dark neutral brown, flecked with gold or greenish yellow on its peripheral half; these flecks are finer, less numerous, and nearer the periphery in males than in females. The zone of the eyeball peripheral to the iris is yellow and is flecked with brown, the flecks more numerous and more heavily developed in males than in females. The nictitating membrane is pinkish gray.

Habitat: Bolson tortoises live at an average elevation of 1500m but are sometimes found in elevations as low as 1000m and as high as 2000m near volcanic slopes. Their habitat is a semi-hot desert climate with winter temperatures around 37℉ and summer temperatures ranging to 97.3℉. The average rainfall in the region is 5.6 inches. The characteristic vegetation is microphyllous desert scrub, rosette desert scrub, grasslands, and other desert scrub. Bolson tortoises commonly inhabit low grade slopes (0.5% to 2%) of fine textured soil (averaging 48% sand, 32% silt, 10% clay, 10% gravel), vegetated by mixed sclerophyll shrub and desert bunch grass. Bolson tortoises dig burrows up to 6.5 feet deep and 26 feet in length as refuge from predators and extremes weather conditions. Burrows are constructed in social aggregations, and clusters show social structuring of individuals. The microhabitat inside their burrows have a mean temperature of less than 86℉ and are commonly found by vegetative cover within 10 feet. If their burrows' temperature exceed 86℉ or there is rainfall, they are commonly found outside their burrows. They spend up to 85% of their lives in their burrows.

Range: Bolson tortoises are located in the Nearctic Region, and are primarily endemic to a portion of the Mapimi Basin in the Chihuahuan Desert of Mexico. They are exclusively found in the northern Mexican states of Chihuahua, Coahulia and Durango. Respectively, they inhabit the south-east region of Chihuahua, the north-east region of Durango and the north-west portion of Coahulia. Bolson tortoises' historic range used to include the southern United States and most of central Mexico.

Found in these States: NM

Diet: Bolson tortoises are herbivores that mainly consume desert scrub, with the halophytic grass Hilaria mutica comprising 60% of their diet. They also consume leaves, bark, stems, fruits, and/or flowers of trees, shrubs, woody vines, succulents, and herbaceous perennials and annuals. They obtain most of their nutritional needs in the spring when the desert plants are in bloom. Their water requirement is minimal, and they can live for weeks without it.

Reproduction: Bolson tortoises are polygynandrous with their mating season beginning in May and ending in August. Species and sex recognition is made by head bobbing. Males tend to exhibit faster head-bobs and longer neck extension, whereas females bob their heads more slowly. Males compete for dominance by having ramming and neck extension bouts. The winning males gain rights to females or territory. Courtship begins when the males or females chase the opposite sex until the individual is cornered or ceases trying to escape. The males head-bob now at every subsequent phase of courtship. Females mainly bob their heads during the initial phase of courtship, when the tortoises are done inspecting each other. The males then begin to ram the females on their sides and rears in this second phase of courtship. Females retract into their shells. Males, with necks fully extended, mount the females. For reasons not explained, males are less aggressive in the presence of other males but turn more aggressive when other males are absent.

Bolson tortoises achieve sexual maturity within the range of 10 to 20 years with the mean being 15 years. They reproduce sexually and are oviparous, with the annual breeding season from May to June. The females nest from July to August. Gestation times average 88 days (range 75 to 100). Their clutches range from 6 to 18 eggs, with an average of 12 offspring. The hatchlings can weigh from 0.35 to 0.7 ounces with the average being 0.5 ounces. Once hatching begins the clutch hatches out of their shells in 15 minutes (range 10 to 20 minutes). Bolson tortoises can lay new clutches throughout their lifetimes once they have hit sexual maturity. After hatching from their eggs, Bolson tortoises lead solitary lives.

The female is the only parental caregiver. Females carry eggs for a gestational period that lasts 75 to 100 days (average 88 days). Females lay eggs by digging a small hole near the entrance of their burrows. This location protects the eggs from the elements and predation. Once the clutch is laid, parental involvement ceases.

Status: Bolson tortoises in the US are federally endangered under the Endangered Species Act (1979). The IUCN Red List categorizes Bolson tortoises as "Vulnerable." These tortoises are protected under Mexican law and are included in CITES Appendix I. An Appendix I listing means that CITES prohibits international trade in specimens except for scientific purposes. The cause for their severe decline is human development and destruction of their habitat.go

Subspecies: None

Taxonomy: