The Galápagos tortoise (or Galápagos giant tortoise), is the largest living tortoise, endemic to nine islands of the Galápagos archipelago. Adults of large subspecies can weigh over 300 kilograms (660lb) and measure 1.2 meters (4 ft) long. Although the maximum life expectancy of a wild tortoise is unknown, the average life expectancy is estimated to be 150-200 years.

The Galápagos tortoises have very large shells (carapace) made of bone which is an integral part of the skeleton. The bony plates of the shell are fused with the ribs and other bones to form a rigid protective structure. When a tortoise feels threatened it can withdraw its head, neck and forelimbs into its shell for protection, presenting a thick shield to a would-be predator. The legs have hard scales that provide an effective armour when withdrawn.

The scutes of the carapace are not coincident with the underlying bony plates and grow at their outer edges. Tortoises thus keep their characteristic scute pattern throughout life. These do have annual growth bands but are not useful for telling the age as the outer layers are rubbed off in the normal wear and tear of living.

The shape of the carapace and other physical features correspond to the habitat of each of the 13 subspecies. Larger islands with more wet highlands such as Santa Cruz and Isabela (where the Alcedo Volcano is found) with lush vegetation near the ground have tortoises with 'dome-back' shells; these animals have restricted upward head movement due to shorter necks, and tend to have shorter limbs as well. These are the heaviest and largest of the subspecies.

Smaller, drier islands such as Española and Pinta are inhabited by tortoises with 'saddle-back' shells comprising a carapace elevated above the neck and flared or reverted above the hind feet, and longer, thinner limbs. This appears to allow them to browse taller vegetation. On the drier islands with tortoise populations, the Galápagos Opuntia cactus (a major source of their water) has evolved a more tree-like and taller form, giving evidence of an evolutionary arms race between progressively taller tortoises and correspondingly taller cacti. Saddleback tortoises tend to be smaller (females average 27 kg / 60 lb, males 54 kg / 119 lb) in size than their domeback counterparts.

In all subspecies, males have a concave undershell, which facilitates mating. The male is also always larger than the female. Shells are not either saddleback or domeback; they can also be of intermediate type with some characteristics of both the extreme types. The Sierra Negra volcano population that inhabits southern Isabela Island is notable for the flat, so-called "tabletop" shells, differing from both the domebacks and saddlebacks. There is little variation in the dull-brown colour of the shell or scales between populations.

The tortoises are herbivorous animals with a diet consisting of cactus, grasses, leaves, vines, and fruit. Fresh young grass in particular is the favourite food of the tortoises, and others are the poison apple (Hippomane mancinella), which is highly poisonous to humans, the endemic guava (Psidium galapageium), the water fern (Azolla microphylla), and the bromeliad (Tillandsia insularis). Tortoises studied in the Santa Cruz tortoise reserve ate well over fifty different plant species. (ref Linda Cayot (1981)) In the drier areas, fallen Opuntia cactus pads and fruits are an important element in the diet of tortoises. Galapagos tortoises eat a large quantity of food when it is available at the expense of incomplete digestion.

By acquiring most of their moisture from the dew and sap in vegetation (particularly the Opuntia cactus), they can go for long periods without actually drinking. They can also survive for long periods of time being forcefully deprived of all liquids, by breaking down their body fat to produce water.

The tortoises are slow-moving reptiles with an average long-distance walking speed of 0.3 km/h (0.18 mph). However, although feeding giant tortoises move about slowly, browsing with no apparent direction, if they have a purpose, such as moving to water or nesting grounds, they can move with surprising speed and determination given their size. Marked individuals have been reported to have traveled 8 miles (13 km) in 2 to 3 days.

On the wetter islands, the tortoises migrate down the gentle mountain slopes after the rainy season to feed on the grass-covered plains and they climb back to feed on grasses of the mountain meadows in the dry season (the increased precipitation at that altitude keeps the grasses watered). On these islands, the tortoises appear to be gregarious, often found in large 'herds'.

Being cold-blooded, the tortoises bask for two hours after dawn, absorbing the energy through their shells, then becoming active for 8-9 hours a day. They may sleep for about sixteen hours in a mud wallow or pool or a 'pallet' (snug depressions in soft ground or dense brush) which probably help conserve heat and may aid digestion. On Alcedo Volcano, where there is a large population, repeated use of the same sites has resulted in the formation of small sandy pits.

Although capable of withstanding drought, tortoises seem to enjoy drinking and wallowing in water. When the tortoise arrives at a spring, seemingly ignoring any spectators, it puts its head in the water above its eyes, and swallows many mouthfuls, at the rate of about ten in a minute. On Alcedo Volcano in the wet season, large numbers of tortoises can be found partially submerged in rain-formed pools or even the dew ponds formed by garua-moisture dripping off trees. These pools tend to be churned up into mud baths. This may be both a thermoregulatory response and a protection from parasites such as mosquitoes and ticks

Tortoises have a classic example of a mutualistic symbiotic relationship with some species of Galápagos finch. The finch hops in front of the tortoise to show that it is ready and the tortoise then raises itself up high on its legs and stretches out its neck so that the bird can reach ticks on its skin, thus freeing the tortoise from harmful parasites and providing the finch with an easy meal. Other birds, including Galápagos Hawk and flycatchers, often use tortoises as observation posts from which to sight their prey.

Mating occurs at any time of the year although it does have seasonal peaks, usually between January and August. When two mature males meet in the mating season, they will rise up on their legs and stretch up their necks to assess dominance. The shorter tortoise will back off leaving the taller, larger tortoise to mate with the female. In groups of tortoises from mixed island populations, saddleback males have an advantage over domebacks. Frustrated non-dominant males have been observed attempting to mate with other males and boulders.

The male tortoise bellows loudly and bobs his head to attract a female. The male then rams the female with the front of its shell and nips her exposed legs until she draws them in, immobilizing her. Mating may last for several hours and the males may "roar" hoarsely. Males have a concave base to their shell and mount the females from behind. It brings its tail which houses the penis into the female's cloaca.

After mating (between June and December), the females journey several kilometres to reach nesting areas of dry, sandy ground (often near the coast). Nest digging is an elaborate task and takes several hours sometimes spread out over several days. It is carried out blindly using only the hind legs to dig a one foot (30 cm) deep hole, in which it lays two to sixteen hard-shelled eggs the size of tennis balls (the number varies with population). The female makes a muddy plug for the nest hole out of soil mixed with urine and leaves the eggs to incubate.

The young emerge from the nest 120 to 140 days later (between December and April) and may weigh only 80 grams (2.8 oz) and measure 6 centimetres (2.4 in) (De Vries 1984). Temperature plays a role in the sex of the hatchling: if the nest temperature is low, more males will hatch; if it is high, more females will hatch. When the young tortoises emerge from their shells, they must dig their way to the surface, which can take up to one month. Hawks are probably the only native predator of the tortoise hatchlings.

Sex can be determined when the tortoise is 15 years old, and sexual maturity is reached at 20 to 25 years old. The tortoises grow slowly for about 40 years until they reach their full size.

The Galápagos tortoise is found on the Galápagos Islands west of Ecuador in South America. The shape of the carapace of some subspecies of the tortoises reminded the Spanish explorers of a kind of saddle they called a "galápago", and for these saddle-shaped tortoises they named the archipelago. As many as 250,000 tortoises inhabited the islands when they were discovered. Today only about 15,000 are left (according to estimates by the Charles Darwin Foundation and Galapagos National Park Service), mainly due to harvest by whalers and pirates that killed them for food during the 18th and 19th centuries. Turned on their backs so they could not move, the Galápagos tortoise could survive for months without food or water, making them a good source of fresh meat on a whaling ship (before refrigeration). Their diluted urine could also be used as drinking water. In addition, non-native species such as goats were introduced on some islands resulting in destruction of the vegetation that provides the tortoises' diet; the resulting habitat loss further diminished the tortoise population.

There were probably thirteen subspecies of Geochelone nigra in the Galápagos Islands, although some recognise fifteen subspecies. Now only eleven subspecies remain, five on Isabela Island, and the other six on Santiago, Santa Cruz, San Cristóbal, Pinzón, Española and Pinta. The Pinta subspecies is likely due to become extinct as only one single male, known as Lonesome George, is known to be alive (though the possibility of other individuals remaining cannot be completely discounted).

The previous oldest known living specimen was a giant Galápagos tortoise named Harriet who lived at the Australia Zoo at Beerwah, Queensland, Australia until her death in 2006. Her estimated date of birth is 1830, and was believed to be the oldest living animal in the world after the death of Adwaitya, an Aldabra Giant Tortoise that lived in the Kolkata zoo in India.

* Geochelone nigra abingdoni - Pinta Tortoise (Extinct in the wild)
* Geochelone nigra becki - Volcán Wolf Tortoise
* Geochelone nigra chathamensis - San Cristóbal Tortoise
* Geochelone nigra darwini - Santiago Tortoise
* Geochelone nigra ephippium - Pinzón Tortoise
* Geochelone nigra phantastica - Fernandina Tortoise (Extinct)
* Geochelone nigra elephantopus - Floreana Tortoise (Extinct)
* Geochelone nigra guntheri - Sierra Negra Tortoise
* Geochelone nigra hoodensis - Española Tortoise
* Geochelone nigra microphyes - Volcán Darwin Tortoise
* Geochelone nigra nigrita - Santa Cruz Tortoise
* Geochelone nigra vandenburghi - Volcán Alcedo Tortoise
* Geochelone nigra vicina - Cerro Azul Tortoise

Tortoise remains have been found on Rábida and Santa Fe islands and were considered possible new taxa, but it is now considered doubtful that these were ever natural populations, but more likely that they were introduced by humans.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.