Geochelone guentheri

(Baur, 1890b)
Volcan Sierra Negra tortoise

Geochelone guentheri is one of the five tortoise populations from Isabela (Albemarle) Island. The gray-brown to black carapace (to 120 cm) is intermediate in shape between domed and saddlebacked. There is a slight cervical indentation, and the carapace is rather broad anteriorly. Anterior marginals are not bent upward or only slightly; posterior marginals are downturned; the height at the cervical indentation is not more than 44% of the carapacial length. Vertebrals are all wider than long, and the 5th is expanded. Surfaces of the vertebral and pleural scutes are rather smooth, especially in large specimens, but growth annuli are present. Eleven marginals occur on each side, and the single, individual supracaudal scute is downturned. Lateral marginals are small, but the 8th is not reduced and has a long dorsal border. The well-developed gray-brown plastron is shorter than the carapace. Its forelobe tapers anteriorly, and the hindlobe is notched posteriorly. The plastral formula is: abd > hum > an > fem > gul > pect; the paired gulars do not project beyond the anterior carapacial rim. The bridge is wide with a short axillary and a larger inguinal. The head is moderate in size with a nonprojecting snout and a slightly bicuspid upper jaw. Its short prefrontal scale is longitudinally divided and followed by a larger frontal scale; other head scales are equal in size and rather small. Females have gray heads, and the lower jaw and throat may be yellow in males. The gray neck is long with a biconvex 4th cervical vertebra. Limbs and tail are gray. Anterior surfaces of the forelimbs are covered with large, pointed, slightly overlapping scales. No large terminal scale occurs on the tail.
The larger males have longer, thicker tails and lighter colored throats and chins.

Linda J. Cayot (pers. comm.) reports that the typical, flat-shelled Geochelone guentheri is restricted to Cienco Cerros on Cerro Azul, southern Isabela (Albemarle) Island. This population is restricted to 100-300 tortoises (Cayot and Márquez, 1994). The remaining populations on Sierra Negra (less than 500 individuals; Cayot and Márquez, 1994), generally considered to be G. guentheri, appear much more like G. vicina.

Geographic Variation
Some individuals are more saddlebacked than others. Male G. vicina have more elevated shells than the flatter male G. guentheri, but their morphological features are extremely similar. Until 1925 no known physical barriers separated the two populations; they are now partially separated by an extensive lava flow, but a large area along the southern coast of Isabela Island remains open as a potential pathway for genetic exchange. Linda J. Cayot reports that a field study at Cienco Cerros on Cerro Azul suggests that there is crossbreeding between the domed morphotype (vicina) and the flat-shelled morphotype (guentheri). Fritts (1984) showed a complex distribution with G. vicina and G. guentheri distributed in altitudinal zones differing in humidity, but if this holds up, G. guentheri may have almost disappeared (Linda J. Cayot, pers. comm.). MacFarland et al. (1974a) and Pritchard (1996a) thought that Geochelone vicina and G. guentheri may be combined.

The habitat is relatively dry with cacti, grassy areas, brush, and some trees, but during the fog season the area is quite mesic. Beck (in Van Denburgh, 1914) reported that these tortoises push under shrubs to avoid the heat, and that they frequently bathe in mudholes, but this is true for all Galápagos tortoises (Linda J. Cayot, pers. comm.).

Natural History
Nesting probably occurs from July to November. Pritchard (1979) reported that clutches average 9 (8-17) eggs; clutches examined by Noegel and Moss (1989) contained 8-11 eggs. More than one clutch may be laid a season at intervals of 30-45 days. Eggs are almost spherical; 10 measured by Van Denburgh (1914) ranged from 60 x 58 mm to 57.6 x 57.1 mm.
Foods consist of grasses and cacti.

This species was described by Baur (1890b) as Testudo güntheri; see Pritchard (1996a, p. 47) for a discussion on the use of güntheri, guentheri, or guntheri. Pritchard also proposed synonymization of all Isabela species but G. becki with G. vicina.

IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. guentheri (listed as Geochelone nigra guentheri) is considered Endangered (C2a).
MacFarland et al. (1974a) and Caporaso (1991) estimated the population of G. guentheri to be 300-500 individuals, mostly small to medium-sized animals, and reported natural reproduction. However, few adults remain, as most have been killed by settlers, and continued patrolling will be necessary to protect the remaining tortoises, as Linda Cayot remarks:

"In 1994 there was a major increase in poaching of tortoises in the Galápagos, with the majority occurring on southern Isabela (principally Sierra Negra, but also Cerro Azul) and Volcan Wolf. Since 1995 this has declined but still occurs on southern Isabela. Many of the subpopulations in the breeding and rearing center in Puerto Villamil are there due to the problem with poaching."