Geochelone becki

(Rothschild, 1901)
Volcan Wolf tortoise

Geochelone becki is one of the five tortoise populations from Isabela (Albemarle) Island. The gray, saddlebacked carapace (to 105 cm) is relatively thick with little or no cervical indentation, the anterior carapacial rim upturned, and the posterior marginals flared and slightly serrated. The carapace is compressed or narrowed anteriorly, but not nearly as much as some other saddlebacked species, and the height at the cervical indentation is 44% or more of the carapace length. Vertebrals are broader than long, and the 5th is somewhat expanded. In adults, the carapacial surface is nearly smooth. There are 11 marginals on each side and the single, undivided supracaudal is bent downward between the flared posterior marginals. Anterior marginals are not greatly expanded, or greatly upturned, and their ventral surfaces are not upturned to become vertical. Lateral marginals are vertical or downturned, and the 8th is not reduced. The gray, well-developed plastron is shorter than the carapace, and is tapered both anteriorly and posteriorly. At best, only a shallow anal notch is present. The plastral formula is: abd > hum > fem > an >< gul > pect; the paired gular scutes do not project beyond the carapacial rim. The bridge is narrow (37-41% of carapace length) with single axillary and inguinal scutes of about the same size. The head is moderate in size with a nonprotruding snout and a slightly hooked, bicuspid upper jaw. Its divided prefrontal scale and the following frontal scale are small. 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 large, nonoverlapping scales. The tail is short and lacks a large terminal scale.
Males are larger, have slightly longer, thicker tails and yellowish lower jaws and throats.

G. becki occurs in the area about Volcan Wolf, Bank's Bay, Cape Berkeley at the northern end of Isabela (Albemarle) Island, Galápagos.

Geographic Variation
Apparently two morphotypes occur on Volcan Wolf, a domed and a saddle-backed form. Pritchard (1996a) postulated this morphological variation may have arisen after a more flattened or dome-shelled population from the south recently crossed the former lava barrier and mixed with a long-isolated population of saddlebacked tortoises. Sequencing of the DNA from the various island populations of Galápagos tortoises by Caccone et al. (1999) has revealed no significant genetic differences among the four southern Isabela Island populations (guntheri, microphyes, vandenburghi, and vicina), and that these are sister taxa to G. nigrita from Santa Cruz. However, the northernmost population on Isabela (becki) is a close match with G. darwini from Santiago, and not with the four southern populations on Isabela, indicating that the population of G. becki originated from a separate colonization of Isabela.

Geochelone becki lives on very rugged, steep slopes with brushy thickets that impede human entry over large areas. Some low tunnels in this vegetation indicate historical use of specific routes of tortoise movements on the slopes of the Volcano.

Natural History
The population is reproducing, but details of their reproductive biology are lacking; Pritchard (1979) presented a photograph of a nesting female.
Wild foods consist of coarse grasses, but apparently not cacti (Van Denburgh, 1914).

IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c); Geochelone becki itself is not listed.
Both MacFarland et al. (1974a) and Caporaso (1991) estimated the population to be between 1,000 and 2,000 individuals (but accurate data is lacking; Pritchard, 1996a), and apparently not endangered. This population has been the most accessible of the tortoise populations in Galápagos in recent decades, with some adults and juveniles present in coastal situations. Hence some illegally collected individuals have been taken from this population (Cayot and Lewis, 1994). Poaching reached its peak in 1994, but on Volcan Wolf it fortunately has declined in the following years. A recent threat is the appearance of feral goats (Linda J. Cayot, pers. comm.).