(Van Denburgh, 1907)
Santiago (James) Island tortoise
Recognition
The gray to black carapace (to 96.5 cm) is intermediate in shape between the saddlebacked species and those with domed shells. It has only a shallow cervical indentation; the anterior carapacial rim is not appreciably upturned, and the posterior marginals are flared, slightly upturned, and slightly serrated. The carapace is wide and not compressed anteriorly; height at the cervical indentation is 42% or more of the carapacial length. Vertebrals are broader than long, and the 5th is expanded. In adults, the carapacial surface is smooth. There are 11 marginals on each side, and the single, undivided supracaudal is downturned between the posterior marginals. Anterior marginals are not greatly expanded or upturned; their ventral surfaces are nearly horizontal. Lateral marginals are vertical or downturned, and the 8th is not reduced. The well-developed gray plastron is shorter than the carapace, and tapered (narrowed) anteriorly and posteriorly; there is only a very shallow anal notch. The plastral formula is: abd > hum > fem > an >< gul > pect; the paired gular scutes do not protrude beyond the carapacial rim. The bridge is 40-43% of the carapace length, with single axillary and inguinal scutes. The head is of moderate size with a nonprojecting snout and a slightly hooked, bi- or tricuspid upper jaw. Its divided prefrontal scale is small, as is the single frontal scale. Head, neck, limbs, and tail are gray, but the jaws and throat are darker with some yellow markings. The neck is long with a biconvex 4th vertebra. Anterior surfaces of the forelimbs are covered with large, irregularly shaped, nonoverlapping scales. The short tail lacks a large terminal scale.
Males are larger, have thicker tails and concave plastra.
Distribution
Geochelone darwini lives only on Santiago (San Salvador or James) Island of the Galápagos Archipelago, where, according to Pritchard (1979), it is found in four distinct, though interconnected, zones in the west-central area of the island. Each of these zones has a separate nesting area at low altitudes.
Habitat
The tortoises occur at altitudes between 200 and 700 m in rugged, rocky, brushy, wooded areas.
Natural History
Pritchard (1979) reported that nesting occurs from August to October, and that the nests contain 4-10 eggs. Foods consist of grasses, forbs, and cacti.
IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. darwini (listed as Geochelone nigra darwini) is considered Endangered (C2a).
MacFarland et al. (1974a) estimated the remaining population to be between 500 and 700 tortoises, and Caporaso (1991) at 500 individuals, mostly adults. Only few hatchlings are produced due to predation by introduced pigs and competition with goats. An artificial incubation and hatchling raising program has been initiated on Santa Cruz at the Charles Darwin Research Station and the Galápagos National Park Service. Protection of nests has resulted in increased recruitment (Cayot et al., 1994), but a strong male bias in the remaining population impedes a quick recovery of the population.