Geochelone ephippium

(Günther, 1875)
Pinzón (Duncan) Island tortoise

This saddlebacked species is one of the smallest of the Galápagos tortoises. Its brownish gray, oblong carapace (to 84 cm) has only a very shallow cervical indentation, the anterior marginals little to much upturned, and the slightly serrated posterior marginals flared and upturned. The carapace is usually compressed or narrowed anteriorly, and its height at the cervical indentation is usually more then 40% of the carapacial length. Vertebrals are broader than long, and the 5th is expanded. Although growth annuli may be present on old tortoises, the surfaces of the vertebral and pleural scutes are generally smooth. There are 11 marginal scutes on each side, and the single, undivided supracaudal is downturned between the posterior marginals. Anterior marginals are not greatly expanded, and, while often upturned, their ventral surfaces are not totally vertical. Lateral marginals are vertical or downturned, and the 8th is not reduced. The well-developed brownish gray plastron is tapered (narrowed) toward both front and back, and shorter than the carapace. Its hindlobe bears only a slight posterior notch, at best. The plastral formula is: abd > hum > fem > gul > an > pect; the paired gulars do not project beyond the carapacial rim. The bridge is narrow (38-42% of carapace length) with single axillary and inguinal scutes of about the same size. The head is small with a nonprotruding snout and a weakly hooked, bicuspid upper jaw. Its divided prefrontal scale and the following single frontal scale are small. Head, neck, limbs, and tail are dark gray. The neck is long with a biconvex 4th cervical vertebra. Anterior forelimb surfaces are covered with large, rounded, nonoverlapping scales. The tail is short and lacks a terminal scale.
Males have yellowish lower jaws and throats, and are larger and have slightly longer, thicker tails.

Geochelone ephippium is restricted to Pinzón (Duncan) Island in the Galápagos Archipelago.

The surviving wild population of about 100 old adult tortoises (Caporaso, 1991; Linda J. Cayot, pers. comm.) lives on the western and southern slopes of the volcanic crater island in thickets and grassy areas. Repatriated tortoises occur in the same areas as the wild population, and in the central crater and on the northwestern slopes (Linda J. Cayot, pers. comm.).

Natural History
Males mature at about 75 cm carapace length in about 13.4 (12-15) years; females mature at 72 cm in about 14.1 (12-15) years (Siaca-Colon, 1994; Siaca and Fritts, 1995). Nesting occurs between August and December; two to eight spherical (58-60 mm) (MacFarland et al., 1974a), brittle-shelled eggs are laid at one time, and several clutches are laid each year. Natural incubation may take from 85 to 120 days.
Natural foods include grass, cacti, moss, lichens, forbs, and leaf litter. Food taxa recorded by Rodhouse et al. (1975) included Alternanthera filifolia, Chiococca alba, Lantana peduncularis, and Maytenus octogona. Most foraging takes place below a height of 0.2 m (Marlow, 1986). Feeding usually occurs in the morning and afternoon, and is more frequent after rains (Rodhouse et al., 1975).

There is a strong indication that the holotype of Geochelone ephippium in fact is a misidentified G. abingdoni, meaning that G. abingdoni is a junior synonym of G. ephippium (Pritchard, 1984, 1996a). Pritchard (1996a) therefore proposed the name duncanensis Garman, 1917 for the Pinzón Island tortoise.

IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. ephippium (listed as Geochelone nigra ephippium) is considered Extinct in the wild (incorrect according to Linda J. Cayot, pers. comm.).
MacFarland et al. (1974a) estimated that approximately 7,000-19,000 tortoises hatched on Pinzon Island in the 10 years prior to 1974, but no juveniles could be found, as all were apparently eaten by introduced black rats. A program of egg incubation and hatchling rearing has been established on Santa Cruz by the Charles Darwin Research Station and the Galápagos National Park Service, and 4-5-year old tortoises have been returned to Pinzon Island (Metzer and Marlow, 1986). The population of black rats, the main threat to the tortoises' survival, was brought down to a very low level in the late 1980s. At that time, the population of native tortoises was estimated at less than 100, but over the years it has been supplemented with 291 repatriated animals (Pritchard, 1996a). Censuses revealed that survival of the 268 released tortoises from 1970 to 1990 exceeds 75% (Cayot et al., 1994).