Galapagos Tortoises are the largest tortoises in the world. They are commonly split into two groups depending on the shape of their shell or ‘carapace’, although some believe that there are intermediate varieties as well. The Saddle-back Carapace has a curved shell front which allows the tortoise’s neck to extend and reach food in higher places. There are 14 recognised subspecies of Galapagos Tortoise, three of which are extinct. In the past, nearly 200,000 tortoises were taken from the island by sailors and pirates for food. Today, there are less than 20,000 tortoises living on the Galapagos archipelago. Introduced species are now the greatest threats to these tortoises. Pigs, cats and dogs eat tortoise eggs and hatchlings. Competition from feral goats and cattle has caused a shortage of food. Introduced rats will also eat hatchlings and cause sometimes fatal injury to the adults.
Distribution and Habitat
The Galapagos Tortoise is found on the Galapagos Islands in the Pacific Ocean, straddling the equator due west of Ecuador. They generally live in arid areas, but there are some found in the forests on higher slopes.
Mating occurs throughout the year, but generally peaks between January and August. The male can be identified by the concave underside of his shell. This assists him in balancing on the female. Courting has been said to begin with the male ramming the female with the front of his shell and nipping her exposed legs until she draws them in, immobilizing herself. However our keepers have also observed that copulation can occur by the female elevating the back end of her shell and stretching her cloaca open and positioned towards him. Our female tortoise has even been observed to help the male with his aim, guiding him with her back feet.
The Galapagos Tortoise is a generalized herbivore that feeds on grasses, vines, cactus fruit, and other vegetation. It eats the fruit of the Manzanello tree and fallen fruits and spiny pads of the Prickly Pear. At the zoo the tortoises are fed a diet which includes Lucerne hay and meadow hay, browse such as mulberry, Chinese Elm, acacia, Kurrajong and bamboo amongst others.
They also eat a variety of vegetables including broccoli, lettuce, endive, chicory, carrots, apple, bananas, celery, calcium and mineral supplements. A tortoise eats approximately 35kg per day. They require this much food as they have very inefficient digestion. Tortoises can also go for extended periods without any food at all, up to 12 months! They achieve this by slowing their metabolism right down.
Tortoises don’t need to drink water often, due to the dew/sap from the vegetation that they eat. This allows them to survive with extended periods without drinking (up to 18months!). When they are thirsty, they can drink large amounts at a time and can store it their bladder or at the base of their neck.
Galapagos Tortoises are terrestrial and diurnal, or active in the daytime, and very fond of water. They are slow-moving animals, moving only 0.25 kilometres per hour. They have a dominance hierarchy based on the height to which the tortoise can stretch its head. This dominance tends to be established interaction to interaction, as opposed to having a recognised hierarchy. The Galapagos Tortoise's eyesight is extremely poor, so much so that the males will try to mate with almost anything resembling a female, even a large rock. These tortoises spend their day grazing and relaxing in the sun. They also enjoy drinking and wallowing in puddles of water. Nights are usually spent half-submerged in mud or water or burrowed in dense brush. This keeps them warm during the cool night hours and also protects them from parasites.
Galapagos Tortoise have another interesting way of keeping parasites at bay, and this is through a relationship they share with a number of species of small birds. The birds signal to the tortoise that they would like to ‘clean them’ by hoping around. The tortoise shows that it is ready by rising up and extending its legs and neck. The bird then cleans the neck, legs and skin beneath the shell. This response is also exceptionally important for zoo-based management, health care and monitoring of breeding events.
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 are 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.
In 1959, Ecuador declared all uninhabited areas in the Galapagos to be a National Park, and made it illegal to capture or remove many species from the islands, including tortoises or their eggs; in 1970, it became illegal to export any Galapagos Tortoises from Ecuador, regardless of whether they had been reared in captivity or the wild, or whether from continental Ecuador or the islands.
Taronga Western Plains Zoo has achieved a national breeding success hatching Australasia’s first Galapagos Tortoise in March 2011.