(Duméril and Bibron, 1835)
Santa Cruz (Indefatigable) Island tortoise
This is the best known of the Galápagos tortoises. Its black, oval carapace (to 130 cm) is domed, higher in the center than in the front, and broad anteriorly instead of narrow and compressed. A shallow cervical indentation is present; height at this point is less than 40% of the carapacial length. The anterior rim is slightly serrated but not upturned; the posterior marginals are serrated, flared, and may be upturned. Vertebrals are broader than long; the 5th is laterally expanded. Surfaces of the vertebrals and pleurals are rough with growth annuli. There are 11 marginals on each side, and the single, undivided supracaudal is downturned between the posterior marginals. Anterior marginals are expanded but not upturned; their ventral surfaces are horizontal; lateral marginals are vertical or downturned with the 8th not reduced. The black plastron is rather large, but still shorter than the carapace. Its two lobes are narrowed or tapered, and an anal notch is present. The plastral formula is: abd > hum > fem > an >< gul > pect; the paired gular scutes do not extend to the carapacial rim. The bridge is broad (47-49% of carapace length) with the inguinal scute larger than the axillary. The head is small with a nonprojecting snout and a weakly hooked, bicuspid upper jaw. Its divided prefrontal scale is large, as is also the frontal scale which follows. Head, neck, limbs, and tail are dark gray or black. The neck is long with a biconvex 4th cervical vertebra. Anterior surfaces of the forelimbs are covered with rounded, overlapping scales of varying sizes, and the short tail lacks a large terminal scale.
Males are larger and flatter with more concave plastra and slightly thicker tails.
Geochelone nigrita occurs on Santa Cruz (Indefatigable) Island of the Galápagos Archipelago.
The habitat on Santa Cruz is more moist than that of several of the other islands with tortoises. There are many open grass glades, tree stands, and thickets. Mosses and lichens grow on many rocks, and the tortoises often soak in the numerous mud holes.
Most mating occurs between February and April in the inland areas; mating is rare in the arid coastal zone (Carpenter, 1966; Rodhouse et al., 1975). Carpenter (1966) reported that as the male approaches, the female extends her head and he smells it. The female then turns and the male smells her tail and mounts, stretching his neck and tail to the fullest. Intromission follows and males emit bellows or grunts. During the mating season males are quite aggressive toward other males, charging and butting them.
Nesting occurs from July to December (MacFarland et al., 1974a; Cayot, 1987). A female at the Zurich Zoological Gardens, Switzerland excavated a nest cavity and laid 10 eggs on 30 November (Honegger et al., 1996). During the nesting period, wild females migrate to the "campos" areas of silty soil and little vegetation in the lowlands. This has been explained by a lack of suitable soil for nesting in the feeding areas of the southwestern population, but the most important factor for migration to the arid zone may be that the higher air and soil temperatures are necessary to assure successful incubation (Linda J. Cayot, pers. comm.). The flask-shaped nests are dug in areas of full sunlight to a depth of 175-300 mm.
One or two clutches of 3-16 eggs (usually 9-10) are laid each year. The brittle-shelled eggs are almost spherical (56-63 x 56-58 mm). Incubation may take more than 200 days. Studies at the breeding and rearing center indicate that sex is determined by temperature (TSD), and that the temperature of incubation determines the length of incubation (Linda J. Cayot, pers. comm.). This probably means that nests laid early in the season, which incubate during the cool season, will have a longer period of incubation and thus are likely to produce males, while nests that are laid late in the season and incubate mostly during the hot season will produce females. Hatchlings have rounded, domed carapaces about 60 mm long with rough, granulated surfaces. Each vertebral and pleural may have a light-brown or gray ring surrounding a darker area.
Santa Cruz tortoises may graze on grasses or browse on Opuntia cacti or numerous other succulent plants; see Cayot (1987) for a list of food plants. Fruits of the tropical genus Psidium are particularly relished (Marlow, 1989), and G. nigrita may serve as a seed dispersor for these plants. Most plants eaten are below 0.2 m in height (Marlow, 1986). When free water is available, they frequently drink; during the dry season they depend on metabolic water from their food. The tortoises are active throughout the daylight hours. At night they sleep under bushes, beside trees, or under overhanging rocks (Carpenter, 1966).
This species is also being referred to as porteri Rothschild, 1903, but the name nigrita Duméril and Bibron, 1835 has priority. However, Pritchard (1996a) propagated continued use of the name porteri because its type material apparently is more satisfactory than that of nigrita.
See Genus Geochelone for a note on a small, yet undescribed saddleback population from northwestern Santa Cruz.
IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. nigrita (listed as Geochelone nigra porteri) is considered Endangered (C2a).
MacFarland et al. (1974a) estimated the main population of G. nigrita in southwest Santa Cruz to be 2,000-3,000 with another 50-100 in a separate population in the east. However, predators (pigs, black rats, cats) destroy large numbers of nests and young in the main population, and MacFarland et al. (1974a) believed that recruitment was too low to replace naturally lost adults. The smaller population is subject to poaching and is declining. A program of nest protection (surrounding the nest with rock "walls" that prevent destruction by pigs), artificial incubation of eggs, and raising of hatchlings was begun in the 1970s. The young are released into the natural population when large enough to avoid predation. The population has held its own since the improvements were made, and Caporaso (1991) thought the present population to be still be between 2,000-3,000 individuals.