Galápagos+Tortoise

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The Galápagos Tortoise, formally known as // Geochelone elephantopus, // is the largest living species of tortoise. A vertebrate, with a life span of o ver 10 0 years in the wild, it can grow to over 1.8 metres long and weigh more than 400kg. The //Chelonoidis nigra// is native to the Gal á pagos Islands off the west coast of Ecuador, and includes numerous subspecies. The following report outlines the adaptations of the Galápagos Tortoise, its habitat and what environmental pressures caused it to evolve.


 * General Information **


 * Scientific Classification **
 * ** Level ** ||||  ** Classification **  ||
 * Kingdom ||||  Animalia  ||
 * Phylum ||||  Chordata  ||
 * Class ||||  Reptilia  ||
 * Order ||||  Testudines  ||
 * Suborder ||||  Cryptodira  ||
 * Family ||||  Testudinidae  ||
 * Genus ||||  Chelonoidis  ||
 * Species |||| C. Nigra ||

** Global Location ** The Galápagos Tortoise is found only on the Galápagos Islands almost 1000km off the west coast of Ecuador (refer to Figure 3). The Galápagos Islands are made of 18 main volcanic islands (refer to Figure 4) and are full of rare flora and fauna, including the Cormorant. The most significant creature on the islands is the Galápagos Tortoise, with a different subspecies found on each separate island, and it is here that Charles Darwin conducted his primary research on the Theory of Evolution.
 * Habitat **

**Habitat Description** The Galápagos Islands house many grasses, vines, cactus, and fruit such as that of the manzanello tree, although the environment can differ slightly between islands. A number of the islands are somewhat drier lowlands, while others are humid highlands. This difference in island climate affects theevolution of the different sub species of the Galápagos Tortoise. But overall, the luscious, vegetation-rich environment presents a perfect habitat to the herbivorous Galápagos Tortoise.

** Environmental Threats to Vegetation and Wildlife ** Originally, the greatest threat to the wildlife, especially tortoises of the Galápagos Islands was humans passing the islands and taking animals on board. But today the islands are threatened by numerous introduced species such as feral cats, dogs, pigs, cattle, goats, black rats and donkeys. The cats, dogs and rats are a direct threat to the population of Galápagos Tortoises, while the cattle, goats and donkeys compete for grazing. This has led to significant destruction of the vegetation of the islands.

** Specific habitat of the Galápagos Tortoise** The Galápagos Tortoise spends most of its time in pools of water, to regulate its temperate and protect itself from insects. It will remove itself from these pools when in need of food, or a fresher source of water. At night, the tortoise will dig itself into soft ground or vegetation.


 * Adaptations **

** Structural Adaptation - Saddleback Shell **

S ome subspecies of the //Chelonoidis nigra// have developed a particular shell, which in shape arches upward at the front edge of the shell resembling a saddle. When the neck and front legs are withdra wn into the saddleback shell, a large gap remains over the neck, unprotected from any attack. This suggests that during the time of evolution into this shell shape, there was a considerable lack of predation. Although now, when humans introduced dogs to the islands, this became a significant vulnerability.

The tortoises with longer necks, due to the saddleback shell, are found in the drier lowland areas (refer Figure 5). This climate has a lesser amount of vegetation, and certain fauna that grows in these areas. For example, the Prickly Pear Cactus grows in this arid environment and grows its fruit higher up on the plant. The extended neck and increased reach of the tortoise, thanks to the saddleback shell, allows the tortoise to reach up and take the resources of the plant. Natural selection would have killed off any shor ter tortoises or those with domed shells; therefore the drier climates are only inhabited by the saddleback-shelled. Several subspecies each on different islands have separately evolved to house the saddleback shell. This indicates that it is a very successful adaptation for the Galápagos tortoise.

Natural behaviour may also be a factor that contributed to the shape of the shell. Tortoises in the drier climates are more territorial, most likely due to the scarcity of resources. During mating season, when looking for a mate and confronted by another male seeking a mate, both tortoises will rise on their hind legs and extend their necks. The tallest tortoise claims dominance and the shorter tortoise backs down.

** Behavioural Adaptation – Mutualism between Galápagos Mockingbird and Finch** The Galápagos Tortoise has difficulty in reaching all parts of its body to rid itself of ticks, mosquitoes and other parasites. Therefore it has developed a mutualistic relationship with Galápagos finches and mockingbirds. The bird, or sometimes a group of them, initiates the process, by hopping around in front of the tortoise. When the tortoise is ready, it extends its neck and legs to give the bird(s) full access to otherwise inaccessible places of its body such as the joints between shell and skin, between the toes and around the head and neck (refer Figure 6). This cleans the tortoise and provides a food source for the birds.

What may have caused this behavioural evolution between these birds and the Galápagos tortoise is that firstly, the tortoises were being attacked by parasites and dying of disease passed on by them, and second the finches and mockingbirds finding a secondary source of food. The cooperation seen by both parties is a classic example of symbiosis and ensures the survival of both.

** Structural/Physiological Adaptation – Gigantism ** It is said that the original ancestor that moved to the GalápagosIslands was not a giant, but instead an average sized tortoise. This means that the //Chelonoidis nigra// evolved into a giant species (refer Figure 7) after arriving on the islands.

Many benefits come with gigantism. Due to the large size of the tortoise, it becomes more successful in colonising remote oceanic islands – a larger tortoise would have a greater chance of surviving long journeys across water to another islands (e.g. between islands in the Galápagos archipelago) as they can hold their heads higher and have a smaller surface area to volume ratio, which reduces osmotic water loss. Larger tortoises also have larger fat and water reserves giving them the ability to last out in the ocean for a long time without food or fresh water (they can last up to 18 months without either resource). Finally, the giant size of Galápagos tortoises allows them to tolerate extremes of weather better, due to gigantothermy, by regulating temperature through their enormous bodies and keeping a relatively high body temperature.

It is likely that the Galápagos Tortoise evolved into a giant due to the fact that there were no predators to hide from and because there are no competition for food from similar animals.

** Structural Adaptation – Domed Shell ** As indicated earlier, there are numerous subspecies spread through the islands of the Galápagos archipelago. Those who live in humid, wet highlands are larger in size and have a large domed shell (refer Figure 7). The luscious climate of the higher islands is abundant with the required dietary vegetation of the Galápagos tortoise. The flora of the wetter areas is closer to the ground, resulting in the tortoise have a shorter neck and legs, as there is no need to reach up higher for food. With the abundance of resources, the dome-shelled Galápagos tortoise can support a larger body size.

The domed shell acts as a much larger and secured shell than that of the saddleback. When the limbs and neck/head are withdrawn into the shell, there is a much smaller opening, increasing the defences of the tortoise. This increased defence suggests that the dome-shelled Galápagos tortoise was, or is, threatened by more or a larger amount of predators than the saddleback- shelled Galápagos tortoise, and ensures survival in the wetter climates that these predators may inhabit. ** Physiological Adaptation – Slow Metabolism ** The Galápagos tortoise generally lives for over 100 years (refer Figure 8). This is due to its slow metabolism. Considering the animal uses little energy in its daily routine, its metabolism is not required to process energy for the body at a consistently high rate. It is also strongly suggested and partially evident that giant tortoises in fact don’t die of old age, but instead only of disease or injury, as their organs and other important bodily functions and parts do not cease to work after years of use, as humans do. Instead, the Galápagos tortoise reaches adulthood and plateaus in age development from there, although it continues to grow very, very slowly its entire life.

The main reason for the Galápagos tortoise to develop a slow metabolism is its necessity of a long life span. Mating routine is infrequent andirregular and the climate and environment of the Galápagos islands can change regularly and is often unpredictable (being such a young set of islands), therefore not always being perfect for mating and laying eggs. This adaptation has helped to ensure the survival and continuation of the species by giving the tortoise a longer life span in which to find the most suitable time(s) to mate.


 * ** Bibliography **
 * Figure one: Swarthmore College. //Boy riding Galapagos Tortoise.// Retrieved 23rd June 2011, from []
 * Format from: Hugo Chan’s Horseshoe Crab wiki. http://designeranimals.wikispaces.com/Horseshoe+Crab
 * Sangay. //Galapagos Giant Tortoise//. Retrieved 23rd June, 2011 from []
 * Slate. //Why do tortoises live so long?// Retrieved 26th June, 2011, from []
 * Wikipedia. //Galápagos Tortoise.// Retrieved 26th June, 2011 from []