Galapagos Giant Tortoise (Geochelone nigra)

Volcan-Alcedo-tortoise-in-habitat

  • Name: Galapagos giant tortoise
  • Latin: Geochelone nigra
  • Classification: Reptile
  • Origin: Galapagos Islands
  • Lifespan: 100+ years

Taxonomy

  • Kingdom: Animalia (Animals)
  • Phylum: Chordata (Vertebrates)
  • Class: Reptilia (Reptiles)
  • Order: Testudinidae (Tortoises)
  • Family: Testudinidae (Terrestrial tortoises with high domed carapace and elephantine feet)
  • Genus: Chelonoidis (Found in S.America and the Galapagos Islands)
  • Species: Geochelone nigra (Galapagos giant tortoise)

Appearance

  • Height: 4ft (1.2m)
  • Weight: 475lbs (215kg)

The Galapagos giant tortoise is the largest chelonian in the world and has around 12 subspecies, dotted along the length of the Galapagos Islands. There are variations among size and shape however, two main morphological forms exist – those with a domed carapace and those known as ‘saddle-backed’.

The domed tortoises are usually larger in size and do not have the upward thrust at the front of the carapace. They tend to live on larger and higher islands where forage is abundant and easily available.

The ‘saddle-backed’ tortoises evolved on arid islands and in response to the lack of available food during periods of drought. The front of the carapace angles upward which allows the tortoise to extend the head higher to reach vegetation.

Saddle-back carapace
Saddle-back carapace
Domed carapace
Domed carapace

Relatives

Listed below are the known subspecies of the Galapagos giant tortoise.

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  • Abingdon Island Tortoise (saddle-backed) (C.n.abingdoni) -EXTINCT-
  • Volcan Wolf Tortoise (saddle-backed and domed) (C.n.becki) -VULNERABLE-
  • Chatham Island Tortoise (saddle-backed and domed) (C.n.chathamensis) -VULNERABLE-
  • James Island Tortoise (domed) (C.n.darwini) -ENDANGERED-
  • Duncan Island Tortoise (saddle-backed) (C.n.duncanensis) -CRITICALLY ENDANGERED-
  • Sierra Negra Tortoise (domed) (C.n.guentheri) -ENDANGERED-
  • Hood Island Tortoise (saddle-backed) (C.n.hoodensis) -CRITICALLY ENDANGERED-
  • Volcan Darwin Tortoise (domed) (C.n.microphyes) -VULNERABLE-
  • Florena Island / Charles Island Tortoise (domed) (C.n.nigra) -EXTINCT-
  • Indefatigable Island Tortoise (domed) (C.n.porteri) -ENDANGERED-
  • Volcan Alcedo Tortoise (domed) (C.n.vandenburghi) -VULNERABLE-
  • Iguana Cover Tortoise (domed) (C.n.vicina) -ENDANGERED-

Habitat & Distribution

The Galapagos giant tortoise is endemic to the Galapagos Islands in the Eastern Pacific Ocean and the environment and climate of which varies from island to island. Generally, saddle-backed tortoises are found on hot and dry islands with sparse vegetation, while domed tortoises live on wetter and cooler islands, with plenty of vegetation at ground level.

Galapagos_Giant_Tortoise_Range_MapDiet

Galapagos giant tortoises are herbivorous and graze on grasses, leaves and cactus, as well as poison apple, guava, water fern and bromeliad. They have an extremely slow metabolism and can survive up to a year when deprived of both food and water by breaking down body fat to produce water as a by-product.

A thirsty tortoise can drink large amounts of water in a single sitting and store it in their bladders and ‘root of the neck’ (the pericardium). On arid island, individuals lick morning dew from boulders.

Juvenile tortoises eat on average 16.7% of their own body weight in dry matter a day and have the digestive efficiency equal to that of hind-gut fermenting herbivorous mammals, such as horses and rhinos.

Behaviour

Galapagos giant tortoises are cold blooded and will spend warm mornings basking in the sun and will spend a large part of the remainder of the day, grazing in small groups. Win the evening, when the temperature cools, they will sleep partially submerged in mud or water to keep warm and during the rainy season, they will wallow in shallow pools. This tortoise is regular in its sleeping, feeding and nesting habits and will sleep for approximately 16 hours at a time. They will travel the same path to feeding sites so regularly that paths have been cut and built into the landscape by them.

This tortoise is slow moving and moves on average 0.3 km/h. When determined to reach somewhere, such as a nesting or feeding site, they can cover around eight miles in 2-3 days.

The Galapagos giant tortoise has also formed symbiotic relationships with some other residents of the islands. For example, Galapagos finches will remove ticks and other parasites from the tortoise’s skin. This keeps the tortoise healthy while providing a meal for the finch. Galapagos hawks have also been seen using the tortoise has a watch post. This allows the hawk a moving point to scout for prey from and give protection to the tortoise.

Galapagos hawk perching on Galapagos giant tortoise
Galapagos hawk perching on Galapagos giant tortoise

Reproduction

Mating can occur at any time of year although it does have seasonal peaks (between January and August). Males become territorial during the mating period and rivals will size each other up by standing tall and reaching out their necks, the tallest male being dominant and thus achieving the right to mate. In mixed populations of domed and saddle-backed tortoises, the latter have an advantage over the former. Non-dominate males have been observed attempting to mate with other males and even boulders.

Males attract females by bellowing and bobbing their heads. The Male will ram the female and nip at her legs until she draws them in. Mating can last for several hours and the male will bellow occasionally throughout. The tail, which houses the penis, is then brought to the cloaca and copulation begins. The male has a concave dip to the base of his shell, which is used to lock into place with the top of the female.

After mating, the female will travel to a nesting site, which can be several kilometers away. This site is dry, sandy and often situated near the coast. The digging of the nest is done blindly with the hind legs and the process is spread out over several days, which results in a 30cm deep hole. The female then lays a clutch of 2-16 hard shelled eggs (about the size of tennis balls) and plugs the nest with a mixture of soil, leaves and urine. The female will then leave the eggs to incubate.

The young emerge from the nest after 120-140 days and each weighs approximately 80g and measures only 6cm. The temperature of the nest can affect the male to female ratio of the clutch. Low temperatures produce more males and high temperatures produce more females. Once hatched, the young must dig themselves out of the nest which can take up to a month. Sex of the young can be determined at around 15 years of age and sexual maturity is reached at around 20-25 years old. Individuals will grow slowly and will reach their adult size at around 40 years old.

Young hood island tortoise
Young hood island tortoise

Adaptations

  1. It is believed that the Galapagos giant tortoise evolved into its large size after its ancestors arrived on the islands during a time of no predators and no competition for resources, in a process known as ‘gigantism’. This large size allows the animal to house large fat and water reserves and tolerate extreme weather.
  2. The saddle-backed shell has evolved in individuals that reside on the more arid islands of the Galapagos, the shape of which allows the tortoise to reach food situated on high branches. This shape however, leaves a large gap of vulnerable skin above the head once drawn in. This suggests that the tortoise suffered little to no predation during its evolution.
  3. The domed shell is seen on individuals living on more humid islands where there is no need to reach as abundant vegetation is found at ground level. Domed tortoises tend to be larger in size and the shell shape allows for more security when the head and limbs are drawn in. This shape suggests that there may have been significant levels of predation during its evolution.
  4. Galapagos giant tortoises generally live for up to and over 100 years, a feat of which a slow metabolism is necessary. The tortoise uses little energy in its daily routine and its metabolism is not required to process energy at a consistently high rate, allowing in the long lifespan.

Threats

Throughout the 1600’s, these tortoises were often captured and stored on whaling ships as a live source of food, as they did not have to eat regularly, so sailors did not have to spend food keeping them alive. This continued throughout the 19th century as a total of over 15,000 individuals were recorded on the logs of 105 whaling ships between the years of 1811 and 1844.

Poaching of the Galapagos giant tortoise also occurs today but at a much lower level. Juvenile tortoises are primarily under threat from introduced species, such as feral dogs, cats and rats. Goats and cattle also compete with adults for resources. Some subspecies of the Galapagos giant tortoise are now extinct, including the Abingdon Island tortoise, of which the last individual ‘Lonesome George’ passed away in 2012.

Conservation

IUCN Status: Vulnerable

The Galapagos giant tortoise is listed under Appendix 1 of CITES, which only allows trade of the animal and / or its products under exceptional circumstances, and has full protection within the Galapagos National Park which was established by Ecuador in 1959. A collaboration between the Charles Darwin Foundation and the Galapagos National Park Service have been running a programme since the 1970’s, in which staff raise hatchlings from eggs until they are able to survive predation from alien species. This project has increase the population of the critically endangered Hood Island tortoise from just 13 in the 70’s to over 1,000 in the wild.

The Galapagos Conservancy are currently carrying out a project called the Giant Tortoise Restoration Initiative and state that within the next 10 years they will:

  • “Restore tortoise populations, including those considered ‘extinct in the wild’, through a combination of in-situ management, breeding and rearing tortoises where appropriate, and repopulation of an island where tortoises are extinct through the use of an analog (closely-related species).
  • Evaluate habitat conditions and restore where necessary.
  • Improve education / outreach in service of giant tortoise conservation.”

Fun Facts

  • Male: Male tortoise
  • Female: Female tortoise
  • Young: Hatchling
  • Group: Bale, Dole, Creep
  • Charles Darwin and Steve Irwin cared for the same tortoise, a female named Harriet.
  • ‘Testidunal’ means ‘pertaining to or resembling a tortoise or tortoise shell’.
  • Roman soldiers formed rows and held shields in front or above them to shelter the unit, known as the testudo formation, which is the Latin word for tortoise.

References

Animal Corner (2003) Galapagos Giant Tortoise [Online] Available at: https://animalcorner.co.uk/animals/galapagos-giant-tortoise/ [Accessed: 3 October 2015]

Arkive (2003) Galapagos Giant Tortoise (Geochelone nigra) [Online] Available at: http://www.arkive.org/galapagos-giant-tortoise/chelonoidis-nigra/ [Accessed: 8 September 2015]

BBC Nature (2014) Galapagos Giant Tortoise [Online] Available at: http://www.bbc.co.uk/nature/life/Gal%C3%A1pagos_tortoise#intro [Accessed: 8 September 2015]

Discover Galapagos (2015) The Endangered Galapagos Giant Tortoise [Online] Available at: https://www.discovergalapagos.com/tortoise.html [Accessed: 21 October 2015]

Fernando, C (2000) Animal Diversity Web [Online] Available at: http://animaldiversity.org/accounts/Chelonoidis_nigra/#behavior [Accessed: 21 October 2015]

Galapagos Conservancy (2015) Giant Tortoises [Online] Available at: http://www.galapagos.org/about_galapagos/tortoises/ [Accessed: 3 October 2015]

Galapagos Conservancy (2015) Ecosystem Restoration: Giant Tortoise Restoration Initiative [Online] Available at: http://www.galapagos.org/conservation/tortoise-restoration/ [Accessed: 21 October 2015]

Galapagos Tortoise (2015) The Galapagos Tortoise [Online] Available at: https://edangalapagostortoise.wikispaces.com/Gal%C3%A1pagos+Tortoise [Accessed: 21 October 2015]

IUCN (2015) Chelonoidis nigra [Online] Available at: http://www.iucnredlist.org/details/9011/0 [Accessed: 21 October 2015]

National Geographic (2015) Galapagos Tortoise (Geochelone nigra) [Online] Available at: http://animals.nationalgeographic.com/animals/reptiles/galapagos-tortoise/ [Accessed: 8 September 2015]

San Diego Zoo (2010) Galapagos Tortoise, Geochelone nigra [Online] Available at: http://library.sandiegozoo.org/factsheets/galapagos_tortoise/tortoise.htm [Accessed: 8 September 2015]

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