Galapagos tortoise

Galápagos tortoise

 The Galápagos tortoise or Galápagos giant tortoise (Chelonoidis nigra) is the largest living species of tortoise and 13th-heaviest living reptile, reaching weights of over 400 kg and lengths of over 1.8 meters (5.9 ft). With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years.
The tortoise is native to seven of the Galápagos Islands, a volcanic archipelago about 1,000 km (620 mi) west of the Ecuadorian mainland. Spanish explorers, who discovered the islands in the 16th century, named them after the Spanish galápago, meaning tortoise.

Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks - on islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. Charles Darwin's observations of these differences on the second voyage of the Beagle in 1835, contributed to the development of his theory of evolution.

Routine

The tortoises are ectothermic (cold-blooded) and therefore bask for 1–2 hours after dawn to absorb the sun's heat through their dark shells before actively foraging for 8–9 hours a day. They travel mostly in the early morning or late afternoon between resting and grazing areas. They have been observed to walk at a speed of 0.3 kilometres per hour (0.2 mph).

On the larger and more humid islands, the tortoises seasonally migrate between low elevations, which become grassy plains in the wet season, and meadowed areas of higher elevation (up to 2,000 ft (610 m) in the dry season. The same routes have been used for many generations, creating well-defined paths through the undergrowth known as "tortoise highways". On these wetter islands, the domed tortoises are gregarious and often found in large herds, in contrast to the more solitary and territorial disposition of the saddleback tortoises.

Tortoises sometimes rest in mud wallows or rain-formed pools, which may be both a thermoregulatory response during cool nights, and a protection from parasites such as mosquitoes and ticks. Parasites are countered by taking dust baths in loose soil. Some tortoises have been noted to shelter at night under overhanging rocks. Others have been observed sleeping in a snug depression in the earth or brush called a "pallet". Local tortoises using the same pallet sites, such as on Volcán Alcedo, results in the formation of small sandy pits.

Diet

The tortoises are herbivores that consume a diet of cacti, grasses, leaves, lichens, and berries. They have been documented feeding on Hippomane mancinella ('poison apple'), the endemic guava Psidium galapageium, the water fern Azolla microphylla, and the bromeliad Tillandsia insularis. A tortoise eats an average of 32–36 kilograms (70–80 lb) per day, though inefficient digestion means that much of this passes through without nutritional extraction.

Tortoises acquire most of their moisture from the dew and sap in vegetation (particularly the Opuntia cactus); therefore, they can survive long periods without water. They can endure 18 months when deprived of all food and water, surviving by breaking down their body fat to produce water as a by-product. When thirsty they may drink large quantities of water very quickly, storing it in their bladders and the "root of the neck" (the pericardium), both of which served to make them useful water sources on ships. On arid islands, tortoises lick morning dew from boulders, and the repeated action over many generations has formed half-sphere depressions in the rock.