Bolson Tortoise

Adults may grow quite large, growing up to 15.75 inches (40 centimeters). Fossil records of carapace lengths measure more than 39 inches (1 meter), as noted by I. Swingland and M. Klemens in 1989. However, the largest known contemporary individuals are much smaller than this and rarely exceed 15.35 inches (390 millimeters) in shell length. Adult males are generally smaller than females in this species and show few, if any, morphological features prominent in other male chelonians. Aspects that could clearly distinguish males from females incluse concave plastrons, prominent chin glands, extended gular projections, distinct eye colors or larger, thicker tails with a more distal cloacal opening.

This species at present is restricted to a relatively small area of the grasslands of north-central Mexico in the states of Chihuahua, Coahuila and Durango where it exists in disjunct sub-populations. The Bolson tortoise once had a much larger range that included much of the southwestern United States. Fossil records indicate that during the Pleistocene Epoch the tortoise was present in the southern Great Plains and northern Mexican Plateau, extending from southwestern Arizona, Oklahoma, Trans-Pecos Texas and the Mexican state of Aguascalientes during the Pleistocene, as noted by W. Auffenberg and R. Franz in 1978. Post-Pleistocene, the Bolson tortoise continued to inhabit the southwestern United States, likely including New Mexico, as noted by D.J. Morafka and others in 1982. As D.J. Morafka and others noted in 1989, its current distribution thus represents a contraction of its range of more than 90%. Population declines are not a thing of the past and tortoise numbers continue to dwindle in Mexico, mainly due to human predation and activities, such that the current wild population is estimated to consist of fewer than 2,500 adults, as noted by A.R. Kiester and others in 2018. 

While Bolson tortoises are herbivores that favor tobosa grass (Hilaria mutica), they are opportunistic grazers that will include other available grasses and herbaceous annuals in their diets.

The tortoise is a land-dwelling reptile that spends more than 95% of its time in a burrow that it constructs with its shovel-like front feet. Burrows have a single opening, are roughly 2.5 meters deep and 10 meters or more in length, and act as refugia from predators and temperature extremes. While the closely-related gopher tortoise (Gopherus polyphemus) uses several burrows in any given active season, Bolson tortoises show strong preference for a single burrow, which they defend against potential intruders by blocking the entrance with their bodies. Plant cover greatly influences burrow occupancy and creates a suitable micro-climate that could help determine appropriate repatriation sites. Considering the very close relationship of the tortoise with its burrow we expect very high site fidelity for this species.  

The tortoises remain out of sight, deep in their burrows while they brumate between November and March. All foraging, nesting and mating activities take place during the tortoise active season from roughly April to October, as J.M. Legler and R.C. Vogt noted in 2013.

The average life span of a Bolson tortoise is not known, but probably lies upward of a century.

Bolson tortoise courting and mating behaviors begin in late spring and continue through the fall. Nesting takes place the following year, starting in late April or early May, and generally commences in mid-July. Most female tortoises will lay two clutches a year, with an average of 5 to 6 eggs per clutch. Nests are dug with the hind legs and are thus typically shallow. How tortoises select nest sites is currently unknown. Approximately half of the nests are in or near a burrow, which may or may not be the burrow occupied by the nest-maker, but nest sites under shrubs or other vegetation far from the nearest burrow are equally as common. Hatchlings emerge from the egg after 75 to 120 days of development. Hatching starts in July or August and ends in October. Depending on the exact date of hatching, neonate tortoises may have the opportunity to forage only for several days or weeks before the onset of cooler temperatures compels them to seek shelter and brumate for the colder months, most often in a rodent burrow. Hatchlings will also occasionally forgo foraging by remaining in the nest and only emerging the following spring or summer. A combination of low metabolic rates and ample yolk provisions by the mother, the only form of parental care enjoyed by hatchling tortoises, allows them to do so.