Galapagos Giant Tortoise by Adnan Khera Names: Galapagos Giant Tortoise Geochelone nigra (Animalia Æ Chordata Æ Sauropsida Æ Testudines Æ Testudinidae) Each individual race of tortoise on the islands has its own name as well. These are included in the figure below along with the region in which it can be found.

Relatives: Although there is only a single species of giant tortoise on the Galapagos Islands, it has evolved into 14 distinct races. Three of these races are now extinct, and Lonesome George is the only remaining abingdoni. Obviously, the closest relative to any individual race is another race. The closest relative outside the islands is the small Chaco tortoise of South America (University 2003). There are giant tortoises of other species in other parts of the world as well, however. The largest population is a 100,000 individual area in the Aldabra Atoll in Seychelles. They also occur or have occurred (as numerous species are now extinct) in Madagascar, Timor, Flores, Mauritius and Java (Wikipedia 2007).

Physical description: Giant tortoises are large and turtle-like. They have a shell, from which protrudes a head and four legs, and can weigh up to 300 kg. Their shell can be up to 1.7 m long (Charles 2006). The shape of their shells can vary as well. There are saddleback and dome shells as well as intermediate variants. Saddlebacks have one side of their shell raised up (the side from which their head protrudes) while dome-shelled tortoises have the same side going down.

Diet: Giant tortoises are herbivorous. They eat a variety of grasses, leaves and cacti depending on where they live (Charles 2006). Saddlebacks are more likely to eat leaves because they can reach higher due to the shape of their shells.

Habitat Description, Geographic Range, Islands: The tortoises habitat and range vary greatly with race. They can occupy a wide variety of environments from dry to moist islands as well as high to lowland areas. The saddleback tortoises tend to live in drier areas because they can reach for food in higher places where grass is more difficult to come across (Charles 2006). Dome-shelled tortoises live in wetter areas where grass is abundant and there is no need to look higher for food. Tortoises will often spend time in small puddles of water in the highlands (Charles 2006). The specific locations of each tortoise and type is given in the figure below.

Abundance and density: There are 15000-20000 tortoises remaining in the Galapagos Islands (Charles 2006). Over 250,000 were once thought to be present before invasion by humans. They are distributed among the islands as shown in the figure below.

Reproduction (University 2003): Mating rituals occur during the wet season, December through August. Males compete for females by stretching their necks as high as possible to see who’s taller. Galapagos Giant Tortoise by Adnan Khera Males also pursue females by ramming their shells and nipping at their legs. Females will retract their hind legs, and males can then mount. Females dig a hole in the sand by pushing in the ground with their legs. They then lay about a dozen eggs in the hole and spread them out into a single layer. Eggs take about 6-7 months to hatch, and hatchlings require rain to soften the dirt so they can dig out of the hole.

Rusello 2005 Interdependence: Giant tortoises feed almost entirely on plants and ends up eating many of their seeds. Accordingly, numerous species of plants are distributed short distances by these lumbering creatures through their feces. Additionally, it is believed that some species of plant seeds may have been dispersed to or between the islands by tortoises (Hnatiuk 1978). Their metabolism and digestive processes are very slow. A tortoise could potentially feed and float to another island before it defecates, and as a result, spread seeds. Studies of the Aldabra giant tortoise have revealed that this is the case with at least 28 various species of plants (Hnatiuk 1978), so it is very probable that Galapagos tortoises do the same thing. Tortoises also have a beneficial relationship with birds such as finches. Birds eat many parasites which make their homes on the necks of tortoises (Charles 2006). Giant tortoises will extend their necks and sit still while the birds feed. This is beneficial for obvious reasons to both the tortoise and the bird. It is presumably not beneficial, however, for the parasites.

Human Impact: Galapagos Giant Tortoise by Adnan Khera Human invasion of giant tortoise homelands around the world have devastated tortoise populations for years. Countless species of precious giant tortoises have become extinct for this reason. The Galapagos giant tortoise is no exception. Since humans landed on the islands, tortoises were hunted or taken aboard ships for fresh meat during voyages (Charles 2006). Introduced species also harmed tortoises. Rats and cats ate tortoise eggs and hatchlings while goats and cattle outcompeted them for vegetation resources (Charles 2006). Overall, humans have caused drastic decline in tortoise populations in the Galapagos and most probably also caused the extinction of several of the unique races of tortoises. Charles Darwin Foundation for the Galapagos Islands (2006) Galapagos giant tortoises. Charles Darwin Research Station Fact Sheet. http://www.darwinfoundation.org/files/species/pdf/tortoises-en.pdf.

Hnatiuk, S H (1978) Plant dispersal by the Aldabran giant tortoise, Geochelone gigantea. Oecologia 36. 345-350.

Rusello, M A et al. (2005) A cryptic taxon of Galápagos tortoise in conservation peril. Biology Letters 1. 287-290.

University of Colorado (2003) Galapagos Tortoise, Geochelone nigra. Galapagos Tortoise Fact Sheet. http://spot.colorado.edu/~humphrey/fact%20sheets/tortoise_galapagos/tortoise .htm.

Wikipedia (2007) Giant tortoise. http://en.wikipedia.org/wiki/Giant_tortoise.