Galapagos Giant Tortoise   1 comment

The Galapagos Giant Tortoise is probably the best known and most emblematic of all Galapagos animals and even gave the archipelago its name.   ‘Galapago’ means tortoise in Spanish and may derive from the word for saddle, referring to the distinctive saddle-like shell of some of the tortoises.

giant_tortoise_1
Eleven species of giant tortoise are found throughout the Galapagos Islands, with the most famous of all is the famous Lonesome George (from Pinta island).

Arrival

giant_tortoise_21All the subspecies of giant tortoise evolved in Galapagos 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 Galapagos to be colonized.   It is likely that the original colonist first washed up on the shores of San Cristobal Island and from there its descendants gradually dispersed around the archipelago, carried on the ocean currents.

The closest living relative of the Galapagos giant tortoises is Geochelone chilensis, a small tortoise found in Chile.   The split between G. chilensis and the Galapagos lineage probably occurred 6-12 million years ago based on mitochondrial DNA analysis, before the origin of the oldest extant Galapagos island.

Evolution

The original ancestor of the tortoises was probably of normal size and evolved into the present-day giants after its arrival in Galapagos.    This is due to a phenomenon seen in many island ecosystems where gigantism evolves because there is no longer any need to hide from predators and because there are no other similar animals to compete with for food.

Once the tortoises spread around the archipelago, they evolved on their isolated islands into the different races we see today, some with domed carapaces (shells), and others with saddleback carapaces.   The unusual saddle shape is believed to have evolved several times on different islands, showing that it must be a very successful design for life in Galapagos.

Espanola and San Cristobal, the oldest islands, were colonized first; this was followed by several migration events to and between other islands via local currents.  The four named southern subspecies on the largest island, Isabela, are possibly not distinct genetic units, whereas a genetically distinct northernmost Isabela subspecies is probably the result of a separate colonization.  Unexpectedly, the lone survivor of the abingdoni subspecies from Pinta Island (“Lonesome George”) is very closely related to tortoises from San Cristobal and Espanola, the islands farthest from the island of Pinta.

It is thought that the saddle-backed type carapace evolved independently several times as a reaction to dry environments, although extinction of crucial populations by human activities confounds whether domed versus saddleback carapaces of different populations are mono- or polyphyletic.

14 Races

Although the Galapagos giant tortoises are classified as just one species, scientists cannot be sure whether the fourteen races they recognize actually belong to several different species.   A current project to analyze their DNA should answer the question of how closely related the different races actually are.

Up to 250,000 tortoises inhabited the islands when they were discovered.  Today only about 15,000 are left.  There were probably twelve subspecies of Geochelone nigra in the Galapagos 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.

alcedo_tortoises
Ten races of giant tortoises still exist in the wild, on the islands of Santiago which has about 800 surviving tortoises, Pinzon (300 tortoises), Santa Cruz (3000), San Cristobal (700), Española (now back to 2,000 thanks to the captive breeding program at the Charles Darwin Research Station) and on Isabela Island, which has a different race on each of its 5 volcanoes; Cerro Azul with about 700 tortoises, Sierra Negra (500), Alcedo (5000), Darwin (1000) and lastly Wolf Volcano with 2000 tortoises.

Sadly, three races of tortoise are already extinct – those of Fernandina, Santa Fe and Floreana Islands, largely due to hunting by humans.   The Santa Fe tortoise is known only from bones found on the island, and only one Fernandina tortoise has ever been found.  Ironically, it was immediately collected and skinned by a member of the California Academy of Sciences expedition in 1905.  The Floreana tortoise was still common in the early 1800s but became extinct by the beginning of this century.

giant_tortoise_5LONESOME GEORGE

Lonesome George [Spanish: Solitario Jorge] is the last known individual of the Pinta Island Tortoise, subspecies Geochelone nigra abingdoni, one of eleven subspecies of Galapagos tortoise native to the Galapagos Islands.  He has been labeled the rarest creature in the world, and is a potent symbol for conservation efforts in the Galapagos and internationally. It is thought that he was named after a character played by American actor George Gobel.

George was first seen on the island of Pinta on 1 December 1971 by American snail biologist Joseph Vagvolgyi.   The island’s vegetation had been decimated by introduced feral goats, and the indigenous G. n. abingdoni population had been reduced to a single individual.

Relocated for his safety to the Charles Darwin Research Station, George was penned with two females of a different subspecies, Geochelone nigra becki from Wolf Island, in the hope that his genotype would be retained in the resulting progeny.   This species looked most similar to the Pinta species.   So far he has failed to breed successfully with these females over the decades, possibly due to the lack of a female of his own subspecies, and prompted researchers at the Darwin Station to offer a $10,000 reward for a suitable mate.  Any offspring successfully hatched from George and his consorts would be mixed, not purebreds of the Pinta subspecies.

giant_tortoise_8Although his exact age is unknown, George is estimated to be 60–90 years of age, and is in good health, weighing approximately 90 kilograms and measure over 100 centimeters across his carapace.  A prolonged effort to exterminate goats introduced to Pinta is now complete and the vegetation of the island is starting to return to its former state.

In May 2007, analysis of genomic microsatellites (DNA sequences) suggested that other individuals of Geochelone nigra abingdoni may still exist.   Researchers have identified one male tortoise from the neighboring Galapagos island of Isabela which has half his genes in common with George’s subspecies.  This animal must be a first generation intergrade between the subspecies of the islands Isabela and Pinta.   It is possible that a pure Pinta tortoise lives among the 2,000 tortoises on Isabela.

On July 21 2008, it was reported that George had unexpectedly mated with one of his female companions.   A total of thirteen eggs were collected and placed in incubators.  However, on November 11 2008, The Charles Darwin Research Center reported that 80% of the eggs showed weight loss characteristic of being infertile.   To the disappointment of the Darwin Center, by December 2008 the remaining eggs had failed to hatch and x-rays had showed them to be infertile.  He probably grew up alone and did not learn proper social and mating behavior.  When “Lonesome George” eventually dies, his species will probably end with him. He will join the other species of giant tortoises that have become extinct
in Galapagos.

giant_tortoise_11Pinta giant tortoise (Geochelone abingdoni)

Only one Pinta tortoise remains to tell the story of the existence of this species. It is a poignant tale illustrating the devastation that followed the arrival of humans to the shores of Galapagos.    Hunting tortoises for meat greatly affected the numbers of tortoises on Pinta Island.   The destruction caused by introduced goats compounded the problem.

Scientists working for the Charles Darwin Foundation (CDF) are attempting to save the Pinta tortoise and are succeeding in restoring the ecosystems on Pinta Island.  The Pinta tortoise is one of the smaller species.   It has a “saddlebacked” shell or carapace.   This has probably evolved as an adaptation to the environment on Pinta Island. Saddle- back types are raised at the front to allow the tortoise’s long neck to reach for higher vegetation on drier islands.

Vulnerability

Hunting by humans was a problem in the past.   Their numbers were heavily depleted by whalers and sealers during the 19th century, some ships taking hundreds of tortoises at a time.   Tortoises were a good food source, living up to a year in the holds of the ships without requiring food or water.   Females were generally taken first as they are smaller than males and were more accessible in lowland areas during the egg-laying season.

The last sighting of tortoises on Pinta was in 1906 when the island was visited by the Californian Academy of Sciences who collected three males.   Today, the biggest problem facing the endemic giant Galapagos tortoise on many islands is that of introduced species.  Fishermen released goats on Pinta Island during the 1950’s as an alternative food source. They destroyed the vegetation and competed with any remaining tortoises for food. The goat population grew rapidly and caused soil erosion. It is unlikely that more tortoises will be found on Pinta.

During a goat monitoring expedition in 2003, a full search performed by the GNPS and visiting scientists failed to find any other tortoises.   A number of carapaces and skeletons were found; these were in poor condition indicating the animals had been dead for a long time.

giant_tortoise_3Domes and saddlebacks

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.   There are a great variety of carapace shapes, from very large domes to smaller saddle-shaped shells, with all gradations in between.   The different shapes probably evolved as an adaptation to the particular environment of each island.  The large domed tortoises are found mainly in the highlands where there is plenty of food to support their great size.

In contrast, the saddleback races live on lower, more arid islands such as Española where food is more scarce and a smaller body is an advantage.   The high opening of their shell allows them to raise their neck higher to reach the sparse vegetation, further helped by their longer neck and legs.   The tortoises’ behaviour may also have been a factor in the evolution of shell size and shape; when two males meet, especially at mating time, they will rise up on their legs and stretch up their necks to assess who is dominant.   The shorter tortoise will back off leaving the taller, larger tortoise to mate with the female.   The saddleback shell with its high reach is therefore a good compromise between the need to be small (due to sparse food) and the need to be tall (to win dominance contests).

Behaviour

Giant tortoises are active for much of the day, spending most of it feeding.   They are vegetarian, eating a great variety of plants in large quantities but their digestive system is rather inefficient so much of the food passes through their body without being digested.   At night they sleep, often in snug depressions in the ground which probably help conserve heat.   They can survive for long periods of time without drinking, by breaking down their body fat to produce water.   However, they do enjoy drinking and wallowing in water, and on Alcedo Volcano in the wet season large numbers of tortoises can be found bathing in muddy pools.

giant_tortoise_910Giant tortoises have an interesting way of ridding themselves of ticks and other parasites.   When they see a Darwin’s finch or mockingbird they raise themselves up on their legs and stretch up their necks, allowing the little birds to hop around, removing the parasites from their skin in a classic example of symbiosis.   When a tortoise feels threatened it withdraws its head, neck and forelimbs into its shell for protection, presenting a daunting shield to a would-be predator.   When saddleback tortoises do this, a large unprotected gap remains at the top of their shell opening.   This was originally not a problem because there were no natural predators in Galapagos but it made them very vulnerable when humans introduced dogs and other predators.

Significantly, all 3 extinct races of tortoise were saddlebacks.   Giant tortoises probably only reach sexual maturity at the age of about 40.   The breeding season is usually at the end of the hot season.   Males have a concave base to their shell and mount the females from behind, bringing their tail, which houses the penis, into contact with the female’s genital region.

Reproduction

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.

giant_tortoise_9After 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 centimeters (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. Galapagos 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.

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