Geochelone microphyes

(Günther, 1875)
Volcan Darwin tortoise

Geochelone microphyes is one of the five tortoise populations from Isabela (Albemarle) Island. The brownish gray, oval carapace (to 103 cm) is intermediate between saddlebacked and domed and rather flattened, with a shallow cervical indentation, the anterior marginals not greatly upturned, and the posterior marginals slightly downturned. The carapace is not appreciably narrowed anteriorly, as in the saddlebacked species, and its height at the cervical indentation is not more than 44% of the carapacial length. Vertebrals are broader than long, and the 5th is expanded. Surfaces of the vertebrals and pleurals are rather smooth in adults. Eleven marginals lie on each side, and the single, undivided supracaudal is downturned between the posterior marginals. Anterior marginals are not expanded and are almost horizontal; lateral marginals are vertical or downturned, and the 8th is not reduced. The brownish gray, well-developed plastron is shorter than the carapace, with narrow lobes. Its forelobe tapers toward the front, and the hindlobe is only slightly notched posteriorly. The plastral formula is: abd > hum > fem > gul > an > pect; the paired gulars do not project beyond the carapacial rim. The bridge is wide (45-46% of carapace length) with a small axillary scute and a larger inguinal. The head is moderate in size, with a nonprotruding snout and a weakly hooked, bi- or tricuspid upper jaw. Its divided prefrontal is small, as is the following frontal scale. Head, neck, limbs, and tail are gray. The neck is long with a biconvex 4th cervical vertebra. Anterior surfaces of the forelimbs are covered with rounded, slightly overlapping scales of varying sizes. The tail is short and lacks a large terminal scale.
Van Denburgh (1914) reported that males have low shells with flat backs, the front of the carapace being only a little lower than the middle of the back. The male tail is also somewhat longer and thicker.

Geochelone microphyes occurs in the vicinity of Volcan Darwin in north-central Isabela (Albemarle) Island of the Galápagos Archipelago.

This tortoise lives mostly on the volcanic slopes in dry thickets, cacti, and grass bunches.

Natural History
Nesting occurs in the lowlands and slopes along the western side of Volcan Darwin; about eight eggs are laid in each clutch (MacFarland, in Pritchard, 1979).
Successful mating occurred in October and fertile eggs were laid from November to the end of April at the Gladys Porter Zoo, Brownsville, Texas (Hairston and Burchfield, 1989). Clutch size was 7-21 eggs, and the females laid 1-2 clutches during the nesting season. Oviposition usually was after 16:30, and was completed by dusk.
Natural foods include mostly grasses, cacti, and other succulents.

Pritchard (1996a) thought the characters separating the tortoises from central and southern Isabela Island (overall size, shell texture, degree of bossing of the carapace scutes, details of shell proportions) should be regarded as environmental responses. He proposed synonymization of G. microphyes, G. vandenburghi and G. guentheri with G. vicina, recognizing only becki from the northern and western slopes of Volcan Wolf, and vicina from the rest of Isabela Island.

IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. microphyes (listed as Geochelone nigra microphyes) is considered Vulnerable (D1+2). MacFarland et al. (1974a) and Caporaso (1991) estimated the population to be 500 to 1,000 individuals.