(DeSola, 1930)
Volcan Alcedo tortoise
Recognition
Geochelone vandenburghi is one of the five tortoise populations from Isabela (Albemarle) Island. The domed, black carapace (to 125 cm) has a shallow cervical indentation, the anterior rim serrated and slightly upturned, and the posterior marginals serrated and flared. It is broad and low anteriorly; height at the cervical indentation is about 36% of the carapacial length. Vertebrals are broader than long with the 5th expanded. Vertebral and pleural scutes are smooth to striated in adults. Each side bears 11 marginals, and the single, undivided supracaudal scute is downturned between the posterior marginals. Anterior marginals are not greatly expanded and their ventral surfaces never vertical. Lateral marginals are vertical or downturned, and the 8th not reduced. The black, well-developed plastron is shorter than the carapace and tapered both anteriorly and posteriorly with no anal notch. The plastral formula is: abd > hum > fem > an >< gul > pect; the paired gulars do not extend beyond the carapacial rim. The bridge is narrow (30-32% of carapace length) with single axillary and inguinal scutes. The head is moderate in size, with a nonprotruding snout and a slightly hooked or bi- or tricuspid upper jaw. Its divided prefrontal and single frontal scales are small. Head and other skin are gray to black. The neck is long with a biconvex 4th cervical vertebra. Anterior surfaces of the forelimbs are covered with large, nonoverlapping scales. The short tail lacks a large terminal scale.
Males have are larger than females, and have concave plastra and slightly thicker tails.
Distribution
Geochelone vandenburghi is restricted to the slopes and crater of Volcan Alcedo on central Isabela (Albemarle) Island, Galápagos.
Habitat
Grassy slopes and thickets; Geochelone vandenburghi seems to enjoy soaking in mud pools formed from geyser runoff and rains, and in drip pools created beneath trees during the garúa or cool season. In the past it was extremely difficult to get through the thick vegetational band near the rim of the outer slope, thus these tortoises have been little disturbed.
Natural History
Copulation probably occurs in almost every month at anytime of the day (DeRoy, 1972), and heavy mating activity has been observed in March (Pritchard, 1979) and April (DeSola, 1930). Males are quite vocal while courting or mounted, emitting deep, basal roars (DeSola, 1930). DeSola (1930: 79-80) described the mating act as follows:
"Making his advances he carefully approaches and observes her and if she shows any signs of response, i.e., as approach toward him, he will quicken his pace and commence the deeply resounding guttural tortoise shout. He collides against her heavily in a manner that appears fierce, bumping her carapace with his own for about five to eight minutes and often for longer periods. During this time he often nips at her legs but in so doing she never retracts her limbs, however brutal his attack may seem. Again he crashes against her while she views his antics unheeding. This constant concussion, appearing painful to the quiet observer, continues and then another hoarse bellow follows. Slowly but persistently he cleaves behind her and awkwardly mounts her from the posterior extremity. Inserting his penis (which before had been concealed and now protrudes from the cloacal vent) into her dilating cloaca, he stretches forth his long thick neck with its heavy head and straddles forward over the hinder neural and costal shields of her carapace. Stretching and tensely holding this equilibrium so difficulty obtained, as he is now fully mounted in a semihorizontal, somewhat slanting, spread-eagle position, the first spasmodic tupping action of their congress begins. Its preliminary jerky motion almost takes the observer unawares. Opening his strangely peculiar and diabolical face he gives vent to another yell, which sounds more shrill and piercing than the others. The female all the while is crouched with forelegs retracted and hinder limbs stretched strongly outward, uplifting and supporting his great weight and bulk."
Clutches average 13.3 (6-26) eggs, of which approximately 26% are undeveloped (Fowler de Neira and Roe, 1984). Incubation lasts 90-150 days, with hatchlings emerging by mid-December.
Natural foods include mostly grasses and sedges (Fowler de Neira and Johnson, 1985), but forbs, cacti, and fruits are also eaten when available. MacFarland and Reeder (1974) observed two species of Darwin's finches, Geospiza fuliginosa and G. fortis, remove ticks from Geochelone vandenburghi, G. ephippium and G. nigrita. They also showed a photograph indicating aggressive behavior between G. vandenburghi males.
Geochelone vandenburghi displays a variety of distinctive behavioral responses during and following an encounter with a potential predator (Hayes et al., 1988). At first the head and neck are fully elevated, presumably to investigate the potential threat. If touched on the anterior portion of the carapace, the tortoise next withdraws its head beneath the shell and folds its forelimbs horizontally in front of the face to protect the head. Finally, the tortoise takes a defensive stance in which it extends the hindlegs, raising the rear of the body while depressing the front, thus reducing access to the forelimbs and head. When the threat is considered past, the tortoise relaxes, drops its plastron to the ground, protrudes the head and usually bobs it, and finally extends the head and neck before the forelegs touch the ground.
Remark
Pritchard (1996a) proposed synonymization of G. microphyes, G. vandenburghi and G. guentheri with G. vicina.
IUCN Red List Status (1996)
The entire Galápagos group (listed as Geochelone nigra) is considered Vulnerable (A2c, B1+2c). G. vandenburghi (listed as Geochelone nigra vandenburghi) is considered Vulnerable (D2). 3,000-5,000 individuals exist (MacFarland et al., 1974a; Caporaso, 1991), the largest population of tortoises on the archipelago. However, Linda J. Cayot reports:
"In the 1990s the tortoise population on Alcedo has come up against a new threat. Feral goats, which crossed the Perry Isthmus from the south, arrived on the lower slopes of Alcedo during the 1970s. Eradication attempts were unsuccessful and the population expanded. By the 1990s, the goat population had reached the point of very rapid expansion, and it was estimated at more than 75,000 by 1996. The goats are destroying the habitat of the tortoises, especially the forested area on the southern rim of the Caldera, where the majority of the drip pools occur and where the tortoises concentrate during the dry season. The goats are now also found on Volcan Darwin and Volcan Wolf. A major project aimed at the restoration of Isabela was begun in 1995 by the Charles Darwin Foundation and the Galápagos National Park Service. This includes the elimination of the threat caused by the goats."