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Galapagos Islands-715

A giant tortoise at the Charles Darwin Research Station on Santa Cruz

 

Galapagos Giant Tortoise

The Galápagos tortoise or Galápagos giant tortoise (Geochelone nigra) is the largest living tortoise, native to seven islands of the Galápagos archipelago. The Galápagos tortoise is unique to the Galápagos Islands. Fully grown adults can weigh over 300 kilograms (661 lb) and measure 1.2 meters (4 ft) long. They are long-lived with a life expectancy in the wild estimated to be 100-150 years. Populations fell dramatically because of hunting and the introduction of predators and grazers by humans since the seventeenth century. Now only ten subspecies of the original twelve exist in the wild. However, conservation efforts since the establishment of the Galápagos National Park and the Charles Darwin Foundation have met with success, and hundreds of captive-bred juveniles have been released back onto their home islands. They have become one of the most symbolic animals of the fauna of the Galápagos Islands. The tortoises have very large shells (carapace) made of bone. The bony plates of the shell are integral to the skeleton, fused with the ribs in a rigid protective structure. Naturalist Charles Darwin remarked "These animals grow to an immense size ... several so large that it required six or eight men to lift them from the ground.". This is due to the phenomenon of island gigantism whereby in the absence of natural predation, the largest tortoises had a survival advantage and no disadvantage in fleeing or fending off predators. When threatened, it can withdraw its head, neck and all forelimbs into its shell for protection, presenting a protected shield to a would-be predator. The legs have hard scales that also provide armour when withdrawn. Tortoises keep a characteristic scute pattern on their shell throughout life. These have annual growth bands but are not useful for aging as the outer layers are worn off. There is little variation in the dull-brown colour of the shell or scales. Physical features (including shape of the shell) relate to the habitat of each of the subspecies. These differences were noted by Captain Porter even before Charles Darwin. Larger islands with more wet highlands such as Santa Cruz and the Alcedo Volcano on Isabela have lush vegetation near the ground. Tortoises here tend to have 'dome-back' shells. These animals have restricted upward head movement due to shorter necks, and also have shorter limbs. These are the heaviest and largest of the subspecies.Smaller, drier islands such as Española and Pinta are inhabited by tortoises with 'saddleback' shells comprising a flatter carapace which is elevated above the neck and flared above the hind feet. Along with longer neck and limbs, this allows them to browse taller vegetation. On these drier islands the Galápagos Opuntia cactus (a major source of their fluids) has evolved a taller, tree-like form. This is evidence of an evolutionary arms race between progressively taller tortoises and correspondingly taller cacti. Saddlebacks are smaller in size than domebacks. They tend to have a yellowish color on lower mandible and throat. At one extreme, the Sierra Negra volcano population that inhabits southern Isabela Island has a very flattened "tabletop" shell. However, there is no saddleback/domeback dualism; tortoises can also be of 'intermediate' type with characteristics of both. The tortoises are slow-moving reptiles with an average long-distance walking speed of 0.3 km/h (0.18 mph). Although feeding giant tortoises browse with no apparent direction, when moving to water-holes or nesting grounds, they can move at surprising speeds for their size. Marked individuals have been reported to have traveled 13 km in two days. Being cold-blooded, the tortoises bask for two hours after dawn, absorbing the energy through their shells, then becoming active for 8–9 hours a day. They may sleep for about sixteen hours in a mud wallow partially or submerged in rain-formed pools (sometimes dew ponds formed by garua-moisture dripping off trees). This may be both a thermoregulatory response and a protection from parasites such as mosquitoes and ticks. Some rest in a 'pallet'- a snug depression in soft ground or dense brush- which probably helps to conserve heat and may aid digestion. On the Alcedo Volcano, repeated use of the same sites by the large resident population has resulted in the formation of small sandy pits. Darwin observed that: "The inhabitants believe that these animals are absolutely deaf; certainly they do not overhear a person walking near behind them. I was always amused, when overtaking one of these great monsters as it was quietly pacing along, to see how suddenly, the instant I passed, it would draw in its head and legs, and uttering a deep hiss fall to the ground with a heavy sound, as if struck dead." The tortoises can vocalise in aggressive encounters, whilst righting themselves if turned upside down and, in males, during mating. The latter is described as "rhythmic groans". The tortoises are herbivorous animals with a diet comprising cactus, grasses, leaves, vines, and fruit. Fresh young grass is a favorite food of the tortoises, and others are the 'poison apple' (Hippomane mancinella) (toxic to humans), the endemic guava (Psidium galapageium), the water fern (Azolla microphylla), and the bromeliad (Tillandsia insularis). Tortoises eat a large quantity of food when it is available at the expense of incomplete digestion. Its favorite food is grasses. The tortoise normally eat an average of 70 to 80 pounds a day. Tortoises have a classic example of a mutualistic symbiotic relationship with some species of Galápagos finch. The finch hops in front of the tortoise to show that it is ready and the tortoise then raises itself up high on its legs and stretches out its neck so that the bird can pick off ticks that are hidden in the folds of the skin (especially on the rear legs, cloacal opening, neck, and skin between plastron and carapace), thus freeing the tortoise from harmful parasites and providing the finch with an easy meal. Other birds, including Galápagos Hawk and flycatchers, use tortoises as observation posts from which to sight their prey. Mating occurs at any time of the year, although it does have seasonal peaks between January and August. When two mature males meet in the mating season they will face each other, rise up on their legs and stretch up their necks with their mouths open to assess dominance. Occasionally, head-biting occurs, but usually the shorter loser tortoise will back off, leaving the other to mate with the female. In groups of tortoises from mixed island populations, saddleback males have an advantage over domebacks. Frustrated non-dominant males have been observed attempting to mate with other males and boulders. The male sniffs the air when seeking a female, bellows loudly, and bobs his head. The male then rams the female with the front of his shell and bites her exposed legs until she withdraws them, immobilizing her. Copulation can last several hours with roaring vocalisations from the males. Their concave shell base allows males to mount the females from behind. It brings its tail which houses the penis into the female's cloaca. After mating (June-December), the females journey up to several kilometres to reach nesting areas of dry, sandy ground (often near the coast). Nest digging can last from hours to days and is elaborate and exhausting. It is carried out blindly using only the hind legs to dig a 30 cm deep hole, into which she lays up to sixteen hard-shelled eggs the size of tennis balls. The female makes a muddy plug for the nest hole out of soil mixed with urine and leaves the eggs to incubate. In rocky areas, the eggs are deposited randomly into cracks. The young emerge from the nest after 120 to 140 days gestation later (December-April) and may weigh only 80 grams (2.8 oz) and measure 6 centimetres (2.4 in). Temperature plays a role in the sex of the hatchling: if the nest temperature is lower, more males will hatch; if it is high, more females will hatch. When the young tortoises emerge from their shells, they must dig their way to the surface, which can take up to a month. All have domed carapaces, and subspecies are indistinguishable. Galápagos Hawk used to be the only native predator of the tortoise hatchlings, as Darwin remarked: "The young tortoises, as soon as they are hatched, fall prey in great numbers to buzzards". Sex can be determined only when the tortoise is 15 years old, and sexual maturity is reached at 20 to 25 years old. The tortoises grow slowly for about 40 years until they reach their full size. Reproductive prime is considered to be from the ages of 60–90. The shape of the carapace of some subspecies of the tortoises is said to have reminded the early Spanish explorers of a kind of saddle they called a "galápago," and for these saddle-shaped tortoises they named the archipelago. Up to 250,000 tortoises inhabited the islands when they were discovered. Today only about 15,000 are left.

 

The inhabitants...state that they can distinguish the tortoise from different islands; and that they differ not only in size, but in other characters. Captain Porter has described those from Charles and from the nearest island to it, namely Hood Island, as having their shells in front thick and turned up like a Spanish saddle, whilst the tortoises from James Island are rounder, blacker, and have a better taste when cooked.---Charles Darwin 1845

 

There were probably twelve subspecies of Geochelone nigra in the Galápagos Islands, although some recognise up to 15 subspecies. Now only 11 subspecies remain, five on Isabela Island, and the other six on Santiago, Santa Cruz, San Cristóbal, Pinzón, Española and Pinta. Of these, the Pinta Island subspecies is extinct in the wild and is represented by a single individual (Lonesome George). In the past, zoos took animals without knowing their island of origin. Production of fertile offspring from various pairings of tortoises largely confirmed that they are subspecies and not different species. All the subspecies of giant tortoise evolved in Galápagos from a common ancestor that arrived from the mainland, floating on the ocean currents (the tortoises can drift for long periods of time as they are buoyant and can stretch head upwards to breathe). Only a single pregnant female or breeding pair needed to arrive in this way, and then survive, for Galápagos to be colonised. In the seventeenth century, pirates started to use the Galápagos islands as a base for resupply, restocking on food, water and repairing vessels before attacking Spanish colonies on the South American mainland. The tortoises were collected and stored live on board ships where they could survive for at least a year without food or water, providing valuable fresh meat, whilst their diluted urine and water stored in their neck bags could also be used as drinking water. Of the meat, Darwin wrote: "the breast-plate roasted (as the Gauchos do 'carne con cuero'), with the flesh on it, is very good; and the young tortoises make excellent soup; but otherwise the meat to my taste is indifferent." In the nineteenth century, whaling ships and fur-sealers collected tortoises for food and many more were killed for high grade 'turtle oil' from the late 1800s onward. Darwin described this process thus: "beautifully clear oil is prepared from the fat. When a tortoise is caught, the man makes a slit in the skin near its tail, so as to see inside its body, whether the fat under the dorsal plate is thick. If it is not, the animal is liberated and it is said to recover soon from this strange operation." A total of over 15,000 tortoises is recorded in the logs of 105 whaling ships between 1811 and 1844. As hunters found it easiest to collect the tortoises living round the coastal zones, the least decimated populations tended to be those in the highlands. Population decline accelerated with the early settlement of the islands, when they were hunted for meat, their habitat was cleared for agriculture and alien mammal species were introduced. Feral pigs, dogs, cats and black rats are effective predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing. In the twentieth century, increasing human settlement and urbanisation and collection of tortoises for zoo and museum specimens depleted numbers even more. The Galápagos giant tortoise is now strictly protected. Young tortoises are raised in a programme by the Charles Darwin Research Station in order to bolster the numbers of the extant subspecies. Eggs are collected from places on the islands where they are threatened and when the tortoises hatch they are kept in captivity until they have reached a size that ensures a good chance of survival and are returned to their original ranges. The Galápagos National Park Service systematically culls feral predators and competitors where necessary such as the complete eradication of goats from Pinta. The conservation project begun in the 1970s successfully brought 10 of the 11 endangered subspecies up to guarded population levels. The most significant recovery was that of the Española Tortoise, whose breeding stock comprised 2 males and 11 females brought to the Darwin Station. Fortuitously, a third male was discovered at the San Diego Zoo and joined the others in a captive breeding program. These 13 tortoises gave rise to over 1000 tortoises now released into their home island. In all, 2500 individuals of all breeds have been reintroduced to the islands. However, persecution still continues on a much smaller scale; more than 120 tortoises have been killed by poachers since 1990 and they have been taken hostage as political leverage by local fishermen.

 

Santa Cruz

With the largest human population in the Galapagos archipelago, Isla Santa Cruz is the most important of the Galapagos Islands. Meaning Holy Cross in Spanish, this island is also known as Indefatigable, after the HMS Indefatigable landed here long ago. The second largest island terms of land area at 986 sq km, Isla Santa Cruz is home to the key town of Puerto Ayora, the Charles Darwin Research Station and the headquarters of the Galapagos National Park Service. With its own airport on Isla Baltra a few miles away, Isla Santa Cruz is where most visitors who come to the Galapagos Islands usually stay. With a number of bars, hotels, restaurants and shops in Puerto Ayora, most tours of the Archipelago also usually begin from here.

 

Galapagos Islands

The Galápagos Islands (official name: Archipiélago de Colón; other Spanish names: Islas de Colón or Islas Galápagos) are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, some 900 km west of Ecuador. It is a UNESCO World Heritage site: wildlife is its most notable feature. Because of the only very recent arrival of man the majority of the wildlife has no fear of humans and will allow visitors to walk right up them, often having to step over Iguanas or Sea Lions.The Galápagos islands and its surrounding waters are part of a province, a national park, and a biological marine reserve. The principal language on the islands is Spanish. The islands have a population of around 40,000, which is a 40-fold expansion in 50 years. The islands are geologically young and famed for their vast number of endemic species, which were studied by Charles Darwin during the voyage of the Beagle. His observations and collections contributed to the inception of Darwin's theory of evolution by natural selection.

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Taken on April 24, 2009