The giant tortoise is probably the best known of all Galapagos animals and even gave the archipelago its name; ‘Galapago’ means tortoise in Spanish and may derive from the word for saddle, referring to the distinctive saddle-like shell of some of the tortoises. Galapagos giant tortoises can weigh up to 250 kg and live for more than 100 years. They are thought to belong to just one species, Geochelone elephant opus, with 14 different races or sub-species, three of which are believed to be extinct.
In the 1600s, buccaneers started to use Galapagos as a base, restocking on water and repairing their boats before setting off to attack the Spanish colonies on the South American mainland. But the main attraction of the islands were the giant tortoises which were collected and stored live on board ship where they survived for many months, providing invaluable fresh meat.
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In the 1800s, whaling ships and then fur-sealers collected tortoises for food and many more were killed for their fine ‘turtle oil’ from the late 1800s until early this century. Early settlers then hunted them for their meat and cleared large areas of their habitat for agriculture. The settlers also introduced domestic animals, many of which went wild and had a disastrous effect on the tortoises.
No-one knows exactly how many giant tortoises there were originally but it has been estimated that more than 100,000 were hunted in total over the centuries. The result today is that three races of Galapagos giant tortoise are extinct while just one individual survives from a fourth. There are about 15,000 tortoises left altogether. As the hunters found it easier to collect the tortoises living around the coastal zones, the healthiest populations today tend to be those in the highlands. Persecution still continues on a much smaller scale; more than 120 tortoises have been killed by poachers since 1990.
It is likely that all the present races of giant tortoise evolved in Galapagos from a common ancestor that arrived from the mainland, floating on the ocean currents. Although this seems an incredible journey it is known that Galapagos tortoises can float easily in seawater. Only a single pregnant female or one male and one female needed to arrive in this way, and then survive, for Galapagos to be colonised. It is likely that the original colonist first washed up on the shores of San Cristobal Island and from there, its descendants gradually dispersed around the archipelago, carried on the ocean currents. The closest living relative of the Galapagos giant tortoises is Geochelone chilensis, a small tortoise found in Chile.
The original ancestor of the tortoises was probably of normal size and evolved into the present-day giants after its arrival in Galapagos. This is due to a phenomenon seen in many island ecosystems where gigantism evolves because there is no longer any need to hide from predators and because there are no other similar animals to compete with for food. Once the tortoises spread around the archipelago, they evolved on their isolated islands into the different races we see today, some with domed carapaces (shells), and others with saddleback carapaces. The unusual saddle shape is believed to have evolved several times on different islands, showing that it must be a very successful design for life in Galapagos.
The fourteen races
Although the Galapagos giant tortoises are classified as just one species, scientists cannot be sure whether the fourteen races they recognise actually belong to several different species. A current project to analyse their DNA should answer the question of how closely related the different races actually are. Ten races of giant tortoises still exist in the wild, on the islands of Santiago which has about 800 surviving tortoises, Pinzon (300 tortoises), Santa Cruz (3000), San Cristobal (700), Española (120) and on Isabela Island, which has a different race on each of its 5 volcanoes;- Cerro Azul with about 700 tortoises, Sierra Negra (500), Alcedo (5000), Darwin (1000) and lastly Wolf Volcano with 2000 tortoises.
There is only one surviving tortoise from the island of Pinta. Nicknamed ‘Lonesome George’, he lives at the Charles Darwin Research Station on Santa Cruz Island, although hopes of finding a mate for him are fading and the Pinta Island race will therefore become extinct when he dies.
Three races of tortoise are already extinct – those of Fernandina, Santa Fe and Floreana Islands, largely due to hunting by humans. The Santa Fe tortoise is known only from bones found on the island, and only one Fernandina tortoise has ever been found. Ironically, it was immediately collected and skinned by a member of the California Academy of Sciences expedition in 1905. The Floreana tortoise was still common in the early 1800s but became extinct by the beginning of this century.
Domes and saddlebacks
There are a great variety of carapace shapes, from very large domes to smaller saddle-shaped shells, with all gradations in between. The different shapes probably evolved as an adaptation to the particular environment of each island. The large domed tortoises are found mainly in the highlands where there is plenty of food to support their great size. In contrast, the saddleback races live on lower, more arid islands such as Española where food is more scarce and a smaller body is an advantage.
The high opening of their shell allows them to raise their neck higher to reach the sparse vegetation, further helped by their long neck and legs. The tortoises’ behaviour may also have been a factor in the evolution of shell size and shape; when two males meet, especially at mating time, they will rise up on their legs and stretch up their necks to assess who is dominant. The shorter tortoise will back off leaving the taller, larger tortoise to mate with the female. The saddleback shell with its high reach is therefore a good compromise between the need to be small (due to sparse food) and the need to be tall (to win dominance contests).
Giant tortoises are active for much of the day, spending most of it feeding. They are vegetarian, eating a great variety of plants in large quantities but their digestive system is rather inefficient so much of the food passes through their body without being digested. At night they sleep, often in snug depressions in the ground which probably help conserve heat. They can survive for long periods of time without drinking, by breaking down their body fat to produce water. However, they do enjoy drinking and wallowing in water, and on Alcedo Volcano, in the wet season, large numbers of tortoises can be found bathing in muddy pools.
Giant tortoises have an interesting way of ridding themselves of ticks and other parasites. When they see Darwin’s finch or mockingbird they raise themselves up on their legs and stretch up their necks, allowing the little birds to hop around, removing the parasites from their skin in a classic example of symbiosis. When a tortoise feels threatened it withdraws its head, neck and forelimbs into its shell for protection, presenting a daunting shield to a would-be predator.
When saddleback tortoises do this, a large unprotected gap remains at the top of their shell opening. This was originally not a problem because there were no natural predators in Galapagos but it made them very vulnerable when humans introduced dogs and other predators. Significantly, all 3 extinct races of tortoise were saddlebacks.
Giant tortoises probably only reach sexual maturity at the age of about 40. The breeding season is usually at the end of the hot season. Males have a concave base to their shell and mount the females from behind, bringing their tail, which houses the penis, into contact with the female’s genital region. Mating may last for several hours and the males often give hoarse roars. The females then travel up to several miles to their nesting areas which must have dry, sandy ground where they can dig their nests. They lay between 2 and 16 eggs the size of tennis balls. The female makes a muddy plug for the nest hole out of soil mixed with urine and leaves the eggs to incubate in the heat from the sun. The young tortoises hatch after around 130 days.
Many of the Galapagos giant tortoises are still exposed to threats. Domestic animals and plants, brought to the islands by settlers and now living wild in the National Park, cause serious problems in the conservation of tortoise populations already decimated by centuries of hunting. Rats, dogs and pigs eat eggs and young tortoises; goats compete with them for food and devastate the vegetation; and donkeys, cattle and horses trample the ground, squashing their nests. There is also still some tortoise poaching, particularly on the island of Isabela; as many as 120 are thought to have been killed since 1990.
When the Galapagos National Park Service was set up, work began to save the threatened races of a tortoise. Eggs collected from the wild were incubated at the Charles Darwin Research Station. Once they hatched, the young tortoises were kept until they were big enough to resist attacks by dogs and rats, and then repatriated to their home islands. These methods have been remarkably successful and have safeguarded the populations of tortoise on several islands, such as Española. At the same time, eradication campaigns aim to remove the introduced animals which threaten the tortoises in the wild. Several races of tortoise have been safeguarded in this way but much work still remains to be done.
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