(Van Denburgh, 1907)
San Cristóbal (Chatham) Island tortoise
Recognition
Geochelone chathamensis has a wide, black shell, its shape intermediate between the saddlebacked and domed species: adult males are rather saddlebacked, but females and young males are wider in the middle and more domed. The carapace (to 89 cm) has only a slight cervical indentation; the anterior rim is little upturned, if at all, and the posterior marginals are downturned. Height of the shell at the cervical indentation is less than 40% of the carapace length. Vertebrals are broader than long, and the 5th is expanded. Surface of the scutes varies from smooth to rough with growth annuli in adults. There are 11 marginals on each side, and a single, undivided, downturned supracaudal scute. Anterior marginals are little serrated, and usually only slightly upturned, at best; their ventral surfaces are never vertical. Lateral marginals are vertical or downturned, and the 8th is not reduced. The black plastron is well-developed, shorter than the carapace, and tapered (narrowed) on both lobes; an anal notch is usually absent. The plastron formula is: abd > hum > fem > gul >< an > pect. This is the only species of the Galápagos complex in which the pectoral scutes do not always meet at the plastral midline (in 9 of 18 examined by Pritchard, 1979, and 3 of 4 examined by Ernst). The paired gulars extend almost to the carapacial rim in adult males. The bridge is broad (about 42-45% of carapace length) with the inguinal scute slightly larger than the axillary. The head is small with a nonprojecting snout and a bicuspid upper jaw. Its divided prefrontal scale and the frontal scale are small. Skin of the head and soft parts is dark gray or black; white or cream-colored pigment may occur around the mouth, chin, and nostrils. The neck is long with a biconvex 4th cervical vertebra. Anterior forelimb surfaces are covered with large nonoverlapping scales. The short tail has no large terminal scale.
Like in the other Galápagos species, males are larger than females, and have a thicker tail.
Distribution
Geochelone chathamensis is restricted to the northeastern part of San Cristóbal (Chatham) Island of the Galápagos Archipelago. A now extinct, more flat-shelled form occurred throughout the wetter and higher regions of the island most altered by man when the island was colonized.
Habitat
Geochelone chathamensis lives in brushy thickets, woods, and grassy areas, and often soaks in mud puddles.
Natural History
Nesting begins in September and four to six eggs are deposited in each nest (Pritchard, 1979).
Natural foods include cacti, grasses, forbs, and the leaves of bushes, most of which is foraged below a height of 0.2 m (Marlow, 1986).
Remark
Van Denburgh's description of this species was based on a flat-shelled individual from a now extinct population that lived in a relatively humid area of central San Cristóbal (Pritchard, 1996a). Pritchard thought that the extant, more saddlebacked population from the northeastern part of the island is not true chathamensis, but may actually represent an undescribed form.
IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. chathamensis (listed as Geochelone nigra chathamensis) is considered Vulnerable (D1+2).
Originally the tortoises were found throughout San Cristóbal Island, but, due to easy accessibility, mariners and early colonists collected more tortoises there than on any other islands during the 1800s, extirpating them from all but the rarely visited northeastern part of the island. MacFarland et al. (1974a) estimated the population to be between 500 and 700 individuals; however, while mating and nesting occurred naturally, all hatchlings were eaten by dogs, or the nests destroyed by feral donkeys. Fencing of nests until dogs were eradicated in the mid-1970s reduced these threats and the population is in good condition at present (Cayot et al., 1994). About 800-1,000 individuals are now present on San Cristóbal (Caporaso, 1991).