The Galapagos Tortoise:
Evolution At Work
Everybody loves a giant this or that, so the giant Galapagos tortoise really hits the spot. Many zoos have one or more of these grand creatures so it's possible to get a look at these creatures without having to travel thousands of miles.
Amazingly, DNA studies suggest that the populations of tortoises on the islands actually come from more than one source. There's a little tortoise in Chile in South America that has been shown to be the closest mainland relative of the Galapagos giants. It is thought that one or more ancestral pregnant females were washed out to sea and floated across the ocean to the islands. While this sounds unlikely, it is possible because of the floating ability of tortoises.15 Subspecies In The Galapagos: Shell Shape And Diet
As a result of various colonization events, the Galapagos tortoises were comprised of 15 subspecies at the time of significant interference from humans.These subspecies were spread from island to island, and several even co-existed on the largest island of Isabela. Presumably the large, hot barren lava floes between these populations served to keep them apart. The subspecies evolved in response mainly to their food supply. In areas where one of their favourite foods, the Opuntia cactus, grows tall the tortoises have longer necks and are more prone to standing upright to forage. Also, they tend to have shells that allow the upward movement of the neck with a flared ridge on the top of the carapace. These are called saddleback tortoises.
Similarly, those populations that graze on ground level vegetation have domed shells as there has been no pressure on them to develop the saddleback shape. There are also populations with intermediate shell shapes.Human Abuse Of The Tortoises Sadly, the abuses these great animals have suffered at the hands of humans could fill a book. All islands along trade routes have been plundered of resources, with the giant tortoises chief amongst them. The Galapagos Islands have been port to pirates, whalers, sealers and many others throughout the last three centuries. The turtles have been killed for meat, oil and skin at different times. During this time the tortoise population of the Galapagos has plummeted from the millions to a mere ten thousand across all the islands. This was also true for the islands off the east coast of Africa with the exception of Aldabra Atoll, which was too far from shipping lanes to be regularly visited. Feral animals are adding to the threat for the Galapagos tortoise populations. Goats eat the local plants and carnivores such as cats, dogs and rats eat the eggs and hatchlings. Efforts are being made to reverse these problems by the Galapagos National Park staff. Many young tortoises are raised in the Charles Darwin Research Station on the island of Santa Cruz and then released into the wild when they are large enough to survive on their own.
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