One of the most unusual and exotic animals in the world is the giant tortoise of the Galapagos Islands. This imposing animal lumbers around grazing on grass and vegetation, taking the role of large herbivore in an ecological niche of the islands.
The islands’ isolation, being almost 1000km west of Ecuador, has made these tortoises unique and several sub-species have evolved differently on separate islands.
The way the Galapagos Islands formed played a part in the tortoises’ development. A hot spot under the Earth’s crust causes volcanic eruptions. This hot spot is near the junction of three tectonic plates that are being pushed apart. The Galapagos are on one of these, the Nazca Plate, which is moving south-east at a rate of about 7cm a year.
The hot spot is stationary, so as the Nazca Plate moves south-east, a volcano is created. Eventually the volcano becomes dormant as it moves away from the hot spot and it cools to become an island. The eastern islands are the oldest in the Galapagos group and were the ones first settled by birds and animals swept ashore by ocean currents.
A complex system of currents, including the South Equatorial Current and the Humboldt Current, brings food and different weather patterns to the islands. The cold-water Humboldt sweeps krill and other nutrients up from the Antarctic. It is also thought to have carried the only species of penguin to live in tropical conditions to the Equator.
Marine iguanas are only found on the islands, where they lie on the hot rock and sand until their bodies warm enough for them to dive into the ocean to graze on algae and seaweed. As they lose heat, they return to shore to take up a mid-crawl pose on a hot rock, looking for all the world as if their batteries have run out.
While mammals dominate most of the world, it is reptiles – and particularly the giant tortoise – that hold sway in the Galapagos. There are 11 surviving sub-species of these animals and although there were once probably 250,000 of these creatures roaming the islands, their numbers have fallen to around 15,000.
Isabella Island has five species of tortoise, there are five other islands with a species each and one species has only one surviving member, known as Lonesome George. These tortoises can weigh more than 250kg and live for 150 years.
Whalers, sealers and pirates discovered the giant tortoise can survive for six months without food or water and the animals were often caught, carried to a ship and put in the hold for when the sailors needed fresh meat. Thousands of tortoises were killed this way and some species are now extinct.
The Charles Darwin Research Station was built in 1964 on the island of Santa Cruz by the Charles Darwin Foundation and it works in partnership with the National Park Service of Ecuador to study and preserve the giant tortoise.
The station’s extensive breeding program has successfully repopulated some of the islands with thousands of baby giant tortoises. It was discovered that eggs incubated at 28-29.5C had a 70 per cent chance of nurturing female hatchlings and this has helped the population grow, as has the culling of feral rats and goats which competed with the tortoises for food.
The success of repopulating 10 of the surviving subspecies has not worked for Lonesome George’s Pinta Island species. There was some excitement recently when eggs were found in an enclosure that he shares with female tortoises of a similar species, but the eggs were infertile.
No doubt the search will continue and maybe, one day, George will father a new generation of Pinta Island hatchlings.
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