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The Galapagos Giant Tortoise is a very large species which may reach a carapace length of 122 cm (4 feet) and weight of 227 kg (500 lb) on the larger islands. Males are much larger than females. The different populations exhibit marked differences in size and shape. The populations may be divided roughly into two groups. Those from the smaller, drier islands tend to be smaller (females average 27 kg, males 54 kg) and have 'saddleback' carapaces (elevated above the neck and flared or reverted above the hind feet) and longer, thinner limbs. Conversely those from the larger, wetter islands are larger with dome-shaped shells. The saddleback would appear to be a modification allowing the tortoises to reach up and browse on the taller vegetation. This is particularly important since on the drier islands with tortoise populations the Opuntia cactus (a major source of water) has evolved an arborescent form. Mating appears to occur at any time of the year although it does have seasonal peaks. Almost any kind of green vegetation is taken as food, including Hippomane mancinella which is highly poisonous to most creatures. When possible the Giant Tortoise spends long periods of time partially submerged in pools; this may be both a thermoregulatory response and a protection from mosquitoes and ticks. At night this species may dig itself into soft ground or vegetation Populations, particularly on the more accessible islands, were severely depleted by passing ships (particularly whalers) taking tortoises on board for supplies. A total of over 15,000 tortoises is recorded in the logs of 105 whaling ships between 1811 and 1844. Increased settlement in the 20th century encouraged commercial hunting of tortoises for oil and extensive collecting for museums. Introduced mammals now pose the greatest threat to the tortoises. Feral pigs, dogs, cats and black rats are extremely effective predators whilst feral goats, donkeys and cattle compete for grazing. Goats have had particularly drastic effects upon the natural vegetation. |