The oval carapace (to 43.3 cm, but usually less than 25 cm) is flattened dorsally with descending sides, a cervical indentation, and a serrated marginal rim. The posterior marginals are slightly upturned, and no cervical scute is present. Vertebral 1 is usually broader than long, but may be only as broad as long. Other vertebrals are broader than long, and the 5th is expanded. There are usually 11 marginals on each side, and a single, undivided supracaudal scute which is downturned between the posterior marginals. The carapace may be either totally yellowish brown or have dark-brown or black growth annuli surrounding tan areolae, and a dark wedge of pigment at the posterior seam of each marginal. Many Patagonian individuals have dark or dark-ringed areolae that constrast with the remainder of the carapacial scutes; aged individuals may be completely dark gray. The well-developed plastron has a deep anal notch. Its forelobe tapers toward the front, and the paired gulars may be slightly divided by a notch. The forelobe is longer but slightly narrower than the hindlobe. The plastral formula is: abd > hum >< fem > gul > pect >< an; the paired gulars are thickened, but do not project much past the carapacial rim. The bridge is broad with an axillary about half as large as the single inguinal. The plastron varies from uniformly yellow brown to having a dark triangular wedge along the seams of each scute. The head is moderate in size with a nonprojecting snout and a hooked, bi- or tricuspid upper jaw. Its large prefrontal scale is divided longitudinally, and followed by a large frontal scale which may be either entire or subdivided; other head scales are small. Head, limbs, and tail are yellowish brown. The anterior surface of each foreleg is covered with large, angular, non- or slightly overlapping scales, and several enlarged tubercles occur on each thigh. The tail ends in an enlarged scale.
Like other Geochelone, this species has 52 chromosomes (Bickham and Carr, 1983).
The sexes are difficult to distinguish, but males are smaller and have slightly longer tails. Old males, at least in Patagonia, are slate gray with large heads and very enlarged, even columnar antebrachial scales; females and juveniles have apposed, pointed antebrachial scales (Buskirk, 1993c).
Geochelone chilensis ranges from southwestern Bolivia, western Paraguay, and northwestern Argentina southward to about 40°S in northern Patagonia (Auffenberg, 1974; Buskirk, 1993c).
Freiberg (1973) described two other tortoises from Argentina, the large Geochelone donosobarrosi and the small G. petersi. Buskirk (1993c) could not differentiate donosobarrosi from G. chilensis based on a detailed morphometric and meristic study, and relegated it to the synonymy of G. chilensis. The small, elongated, thick-shelled petersi may represent males.
Geochelone chilensis inhabits plains and sub-montane habitats of deserts and semi-deserts with scrubs and trees, corresponding broadly to the Chaco Region, but excluding the wettest eastern part. This species occurs from below sea level to elevations over 1000 m (Waller and Gruss, 1986). In the southernmost part of the range, tortoises are reported to hibernate as long as five months in burrows or dens of their own construction (Buskirk, 1993c).
Sexual maturity is thought to be reached in 12 years (Waller et al., 1989). While competing for a mate, males may bite each other in the forelimbs, sometimes inflicting bleeding wounds (Buskirk, 1993c); almost 90% of the population studied by Waller and Micucci (1997) showed scarring of the front scales of the forelimbs. Freiberg (1981) reported courting males gently push the females about. During intercourse, the male may emit nasal cries (Buskirk, 1993c). Mating occurs in November and December; nesting from January to March (Walker, 1989). A captive female was observed digging a nest using her hindlimbs; the boot-shaped nest was 15-20 cm deep and 15 cm wide (Calvo, in Buskirk, 1993c).
One to seven oval to spherical (42-49 x 32-38 mm), brittle-shelled eggs are laid at a time, and up to three clutches are laid each season. Hatching occurs after 12-16 months, and hatchlings are rounded (45-65 mm), flat topped, and yellowish brown.
In northern Patagonia, G. chilensis digs short (50-60 cm) burrows in sandy soil at the beginning of each spring. Burrows are used as a refuge during the night or during mid-day heat. Dens are deeper (usually over 2 m), dug in hard soil, and used over several seasons.
Most activity is in spring, particularly before noon (Waller and Micucci, 1997). In spring, G. chilensis chiefly feeds on ephemeral Plantago; in summer, grasses, succulents, and fruits of perennial shrubs are taken (Waller and Micucci, 1997). Local inhabitants from Patagonia told Buskirk (1993b) that this species is able to smell water, and that it drinks whenever sporadic rainfall enables it to do so.
IUCN Red List Status (1996)
Vulnerable (A1cd). The Chaco tortoise is exploited as a food source and for the pet trade (Waller, 1997). In Argentina, 20,000-50,000 tortoises are collected annually for the domestic pet trade, mainly in the provinces of Córdoba and Santiago del Estero (Waller, 1997). Also free-ranging livestock poses a serious threat to this species, as domestic animals compete for food and trample both vegetation and tortoise burrows. Local burning practices diminish the overall diversity of perennials and directly injure or kill tortoises, in particular juveniles (Waller and Micucci, 1997).