Galápagos giant tortoise

Name and taxonomy

Spanish sailors reached the islands in 1535 and named them after galápago, their word for a tortoise. For a long time naturalists grouped these animals with the giant tortoises of the Indian Ocean. In 1783 Johann Gottlob Schneider placed every giant tortoise in Testudo indica. Duméril and Bibron split off the Galápagos animals in 1834 under the name Testudo nigrita. The current name niger means black, and it was revived in 1984 because it was the oldest valid name.

During most of the 1900s the species sat in the genus Geochelone. In the 1980s Chelonoidis rose from a subgenus to a full genus, and later DNA work supported the move. Between 14 and 15 subspecies have been named, and 12 survive. Before 2021 most authorities treated each island form as a full species. A 2021 study found the differences were smaller than thought, so the forms became subspecies of one species again. Wikidata still lists Floreana giant tortoise and Charles Island giant tortoise as English names for the type form.

Evolution and relatives

All Galápagos tortoises descend from ancestors that floated across the sea from South America. Their closest living relative is the Chaco tortoise of Argentina and Paraguay, a much smaller animal. The two lines split 11.95 to 25 million years ago, before the oldest current islands formed 5 million years ago. The founders may have been a single pregnant female or one breeding pair. A tortoise floats, breathes with its neck raised above the water, and can go months without food. The Humboldt Current probably carried them on the 1,000 kilometre crossing, because they swim poorly.

DNA shows that tortoises settled Española and San Cristóbal first, because those islands are the oldest. From there tortoises drifted to younger islands on local currents, one stepping stone at a time. Isolation on each island then produced the forms we see today. The Galápagos group is one of only two giant tortoise radiations left, the other being Aldabra in the Indian Ocean. Humans wiped out the rest, including the Mascarene tortoises, by the 19th century.

Appearance

The shell is a large bony case of dull brown or grey. Its plates fuse to the ribs, so the shell is part of the skeleton. The legs are thick and stumpy, with dry scaly skin, five claws in front and four behind. The tortoise pulls its head, neck and front legs inside the shell when it feels threatened. Lichens often grow on the shell and give it a mottled look.

Shell length runs from 75 to 150 centimetres, and the longest known reached 187 cm. Size varies by island: Pinzón tortoises weigh at most 76 kg with a shell about 61 cm long. Two shell shapes occur. Domed tortoises come from large wet islands with highlands above 800 metres and thick ground plants. Saddleback tortoises come from small dry islands below 500 metres, and they have longer necks and legs. The front edge of a saddleback shell arches up, which leaves a gap over the neck when the head withdraws.

Males are larger than females, with longer tails and a concave lower shell that helps them mount. Females have a wider and longer shell outline than males. Growth rings on the shell do not give a reliable age, because the outer layers wear away.

Habitat and range

The species occurs naturally on seven islands of the Galápagos, about 1,000 km west of mainland Ecuador. Five subspecies live on separate islands, and five more live on the volcanoes of Isabela, the largest island. Habitat ranges from dry lowland scrub and grassland to moist highland meadows. The lowlands hold cactus and dry grass, while the highlands hold lush plants and pools.

On the larger islands tortoises migrate with the seasons. They graze the lowlands when rain turns them green, then climb to meadows up to 610 metres in the dry season. Generations of tortoises walk the same routes, which wear into clear paths called tortoise highways. Satellite tags on 83 tortoises since 2009 show that these journeys follow the changing quality of plants.

Young tortoises stay in the warm lowlands for their first 10 to 15 years. GBIF holds records from 9 countries, but only the records from Ecuador show the native range. Fernandina has no people and no feral mammals, yet its tortoise is very rare, probably because lava flows often cover the island.

Diet and feeding

The tortoise is a herbivore. It eats cactus, grasses, forbs, leaves, lichens, berries and fallen fruit. It also takes plants that hurt people, such as stinging nettle and the poison apple Hippomane mancinella. Young tortoises eat about 16.7% of their body weight in dry matter each day. Their gut digests plant matter about as well as a horse or a rhino does.

Most water comes from dew and from the sap of plants, above all the Opuntia cactus. A tortoise can go more than six months without a drink. It can survive up to a year with no food or water at all, because it breaks down fat to make water. When water is available it drinks fast and stores it in the bladder and at the base of the neck. On dry islands tortoises lick dew from boulders, and over generations this has worn hollows in the rock.

Some tortoises also eat birds. A finch that comes to pick parasites may walk under the raised body, and the tortoise then drops and crushes it. It probably does this to add protein to a diet of leaves.

Behaviour and social life

The tortoise is active by day. It cannot make its own heat, so it basks for 1 to 2 hours after dawn to warm its dark shell. It then feeds for 8 to 9 hours. It walks at about 0.3 kilometres per hour. Movement between rest and feeding sites happens mostly in the early morning or the late afternoon.

Domed tortoises on wet islands are sociable and often gather in large herds. Saddleback tortoises on dry islands are more solitary and defend a territory. In zoos, tortoises enter and leave their shelter in a fixed order, and each keeps its own spot inside. Animal Diversity Web reports that they sleep for about sixteen hours.

Tortoises rest in mud wallows and rain pools. The mud keeps them warm on cool nights and probably protects them from mosquitoes and ticks. They also roll in loose soil to shed parasites. At night some shelter under rock overhangs or settle into a hollow called a pallet. On Volcán Alcedo, repeated use of the same pallets has dug small sandy pits.

Communication and senses

Tortoises make few sounds. A male bellows and grunts during mating, in a hoarse rhythm. A tortoise hisses as it pulls into its shell, because the movement forces air out of its lungs. Other noises come during fights or when an animal struggles to right itself.

Darwin wrote that local people thought the tortoises deaf, because they did not hear him walk up behind them. They can hear, but they rely far more on sight and smell. A male finds a female in the breeding season by sniffing the air for her scent. Rival males settle disputes by a display: each rises on its legs and stretches its neck as high as it can. The tortoise with the lower head usually gives way.

Tortoises also signal to birds. When finches hop in front of it, a tortoise stands tall and stretches out its neck and legs. This tells the birds that it is ready to be cleaned of ticks.

Reproduction and life cycle

Mating can happen at any time, but it peaks from February to June in the wet highlands. Courtship is rough. The male rams the female with his shell and bites her legs until she pulls them in. He then mounts her, and the hollow underside of his shell helps him keep his balance.

From July to November a female walks several kilometres to a dry sandy nesting site near the coast. She digs a hole about 30 centimetres deep with her back legs, working blind over several days. She lays 2 to 16 hard round eggs, each 82 to 157 grams, about the size of a billiard ball. Domed populations lay bigger clutches, about 9.6 eggs against 4.6 for saddlebacks on Pinzón. She seals the nest with a plug of mud and urine and leaves the sun to warm the eggs. A female may lay one to four clutches in a season.

Incubation takes 4 to 8 months, and cool nests produce more males while warm nests produce more females. Hatchlings weigh about 50 grams and measure about 6 cm. They dig upwards for several weeks, fed by a yolk sac that can last up to seven months. Every hatchling has a domed shell, so the subspecies cannot be told apart at that age. Sexual maturity comes at 20 to 25 years in captivity and perhaps 40 years in the wild. Wild tortoises live 100 years or more, and Harriet at Australia Zoo died in 2006 at an estimated age above 170.

Predators and threats

An adult tortoise has no natural predator. Darwin noted that old tortoises seemed to die of accidents, such as a fall from a cliff. The Galápagos hawk once took many hatchlings, but it is now rare. Introduced pigs, dogs, cats and black rats now eat eggs and young. On Pinzón, black rats killed every hatchling for perhaps more than a century.

Goats, donkeys and cattle eat the same plants and walk over nest sites. Goats also strip the plant cover, which leaves nests easier to find. Hatchlings die in hardened soil during drought, and floods can drown a nest. In the lowlands young tortoises fall into cracks, get crushed by rocks or overheat. Poachers have taken more than 120 tortoises since 1990. Tourism and growing towns now press on the remaining habitat.

Population and conservation

The IUCN lists the species as Vulnerable. About 250,000 tortoises lived on the islands when Spain found them in the 16th century. A 1974 census counted only 3,060. An estimate for 1995 to 2009 put the total at 19,317, and current figures run from about 15,000 to 20,000. Some subspecies are far worse off, and the IUCN rates them from Vulnerable to Extinct in the Wild.

Ecuador protected the tortoise in 1936, made the islands a national park in 1959, and later banned all export. The species sits on CITES Appendix I, which bars commercial international trade. Breeding centres started work in 1965, and they have lifted seven of the eight endangered subspecies. Eggs are collected from threatened nests, and the young are kept for four to five years before release.

The Española subspecies fell to 3 males and 12 females, spread so far apart that they never met. Brought to Santa Cruz in 1971, they produced over 1,200 young in 33 years. One male, Diego, fathered between 350 and 800 of them. Española now holds more than 1,000 tortoises that breed on their own. Rats were cleared from Pinzón in 2012, and wild hatchlings appeared there in 2013 for the first time in a century. On Pinta, more than 41,000 goats were shot in the first campaign from 1971 to 1982.

Relationship with humans

Pirates used the islands from the 17th century as a base and loaded tortoises as food. From the 1790s to the 1860s, whalers and sealers took far more. A tortoise stays alive on deck for a year or more without food, so it gave sailors fresh meat. Whaling logs record over 13,000 tortoises taken between 1831 and 1868. Later, tortoises were boiled down for oil and sold to mainland Ecuador. The California Gold Rush of 1849 opened a new market, and tortoise bones turn up in San Francisco digs.

Scientific expeditions took 661 tortoises between 1888 and 1930. Today the tortoise is the flagship of Galápagos conservation and a major draw for visitors. Like other tortoises, it is harmless to people. iNaturalist holds 6,276 observations of the species.

In culture and myth

Fray Tomás de Berlanga, who found the islands in 1535, wrote of tortoises so big that each could carry a man. Darwin recorded that lifting a large one took six or eight men. He also rode on their backs, rapped the shell to make them walk, and found it hard to stay on.

Vice-Governor Lawson told Darwin that he could tell which island any tortoise came from. Darwin did not take the point at first and mixed his collections from two islands. The Beagle left with more than 30 tortoises on deck, all meant for the pot. Only later, when he compared tortoises with mockingbirds, did he suspect that island forms were varieties of one stock.

Sailors praised the meat. William Dampier wrote that no chicken tasted better, and Captain Porter said all other meat seemed poor afterwards. Lonesome George, the last Pinta tortoise, became a symbol of extinction before his death in June 2012. Diego, the Española male, earned the nickname Playboy Tortoise in 2020 news reports. The tortoise is thought to be the animal most people picture when they think of the Galápagos.