The Galápagos tortoise (Chelonoidis nigra)

Nov 20, 2016 Features / Columnists, Interesting Creatures in Guyana

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The Galápagos tortoise or Galápagos giant tortoise (Chelonoidis nigra) is the largest living

species of tortoise. Modern Galápagos tortoises can weigh up to 417 kg (919 lb). Today, giant tortoises exist only on two remote archipelagos: the Galápagos 1000 km due west of Ecuador, and Aldabra in the Indian Ocean, 700 km east of Tanzania.
The tortoise is native to seven of the Galápagos Islands, a volcanic archipelago about 1,000 km (620 mi) west of the Ecuadorian mainland. 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. 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.
All subspecies of Galápagos tortoises evolved from common ancestors that arrived from mainland South America by overwater dispersal. Genetic studies have shown that the Chaco tortoise of Argentina and Paraguay is their closest living relative.
The minimal founding population was a pregnant female or a breeding pair. Survival on the 1000-km oceanic journey is accounted for because the tortoises are buoyant, can breathe by extending their necks above the water, and are able to survive months without food or fresh water.
The tortoises have a large bony carapace (shell) of a dull brown colour. The plates of the shell are fused with the ribs in a rigid protective structure that is integral to the skeleton. Lichens can grow on the shells of these slow-moving animals.
Tortoises keep a characteristic scute (shell segment) pattern on their shells throughout life, though the annual growth bands are not useful for determining age because the outer layers are worn off with time. A tortoise can withdraw its head, neck, and fore limbs into its shell for protection. The legs are large and stumpy, with dry, scaly skin and hard scales. The front legs have five claws, the back legs four.
The tortoises are herbivores that consume a diet of cacti, grasses, leaves, lichens, berries, melons, oranges, and milkweed. 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.
Juvenile tortoises eat an average of 16.7 percent of their own body weight in dry matter per day, with a digestive efficiency roughly equal to that of hindgut-fermenting herbivorous mammals such as horses and rhinos.
Tortoises acquire most of their moisture from the dew and sap in vegetation (particularly the Opuntia cactus); therefore, they can survive longer than six months without water. They can endure up to a year when deprived of all food and water, surviving by breaking down their body fat to produce water as a by-product. Tortoises also have very slow metabolisms. 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.
Although they are not deaf, tortoises depend far more on vision and smell as stimuli than hearing.
Females journey up to several kilometres in July to November to reach nesting areas of dry, sandy coast. Nest digging is a tiring and elaborate task which may take the female several hours a day over many days to complete. It is carried out blindly using only the hind legs to dig a 30 cm (12 in)-deep cylindrical hole, in which the tortoise then lays up to 16 spherical, hard-shelled eggs ranging from 82 to 157 grams (2.9 to 5.5 oz) in mass, and the size of a billiard ball. The female makes a muddy plug for the nest hole out of soil mixed with urine, seals the nest by pressing down firmly with her plastron, and leaves them to be incubated by the sun. Females may lay one to four clutches per season.
Temperature plays a role in the sex of the hatchlings, with lower-temperature nests producing more males and higher-temperature nests producing more females. This is related closely to incubation time, since clutches laid early incubate during the cool season and have longer incubation periods (producing more males), while eggs laid later incubate for a shorter period in the hot season (producing more females).
Young animals emerge from the nest after four to eight months and may weigh only 50 g (1.8 oz) and measure 6 cm (2.4 in). When the young tortoises emerge from their shells, they must dig their way to the surface, which can take several weeks, though their yolk sac can sustain them up to seven months.
In particularly dry conditions, the hatchlings may die underground if they are encased by hardened soil, while flooding of the nest area can drown them. Subspecies are initially indistinguishable as they all have domed carapaces.
The young stay in warmer lowland areas for their first 10–15 years, encountering hazards such as falling into cracks, being crushed by falling rocks, or excessive heat stress.
Sexual maturity is reached at around 20–25 years in captivity, possibly 40 years in the wild. Life expectancy in the wild is thought to be over 100 years, making it one of the longest-lived species in the animal kingdom. (Source: Wikipedia)

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