Giant tortoise

thumb|An [[Aldabra giant tortoise, an example of a giant tortoise.]]

Giant tortoises are any of several species of various large tortoises, which include a number of extinct species, as well as two extant species with multiple subspecies formerly common on the islands of the western Indian Ocean and on the Galápagos Islands.

right|thumb|A [[Galápagos tortoise|Galápagos giant tortoise on Santa Cruz Island]]

History

As of February 2024, two different species of giant tortoise are found on two remote groups of tropical islands: Aldabra Atoll and Fregate Island in the Seychelles and the Galápagos Islands in Ecuador. These tortoises can weigh as much as and can grow to be long. Giant tortoises originally made their way to islands from the mainland via oceanic dispersal. Tortoises are aided in such dispersal by their ability to float with their heads up and to survive for up to six months without food or fresh water.

Giant tortoises were once all placed in a single genus (often referred to as Testudo or Geochelone), but more recent studies have shown that giant tortoises represent several distinct lineages that are not closely related to one another. However, giant tortoises are no longer considered to be classic examples of island gigantism, as similarly massive tortoises are now known to have once been widespread. Giant tortoises were formerly common (prior to the Quaternary extinctions) across the Cenozoic faunas of Eurasia, Africa and the Americas. Older (Early Miocene) meiolaniids are also known from the St. Bathans fauna in New Zealand.

Although often considered examples of island gigantism, prior to the arrival of Homo sapiens giant tortoises also occurred in non-island locales, as well as on a number of other, more accessible islands. During the Pleistocene, and mostly during the last 50,000 years, tortoises of the mainland of southern Asia (†Megalochelys atlas), Indonesia, The giant tortoises formerly of Africa died out somewhat earlier, during the Late Pliocene. While the timing of the disappearances of various extinct giant tortoise species seems to correlate with the arrival of humans, direct evidence for human involvement in these extinctions is usually lacking; however, such evidence has been obtained in the case of the distantly-related giant meiolaniid turtle Meiolania damelipi in Vanuatu. One interesting relic is the shell of an extinct giant tortoise found in a submerged sinkhole in Florida with a wooden spear piercing through it, carbon dated to 12,000 years ago.

Today, only one of the subspecies of the Indian Ocean survives in the wild; the Aldabra giant tortoise The Madagascar radiated tortoise Tu'i Malila was 188 at her death in Tonga in 1965. Harriet (initially thought to be one of the three Galápagos tortoises brought back to England from Charles Darwin's Beagle voyage, but later shown to be from an island not even visited by Darwin) was reported by the Australia Zoo to be 176 years old when she died in 2006.

On 23 March 2006, an Aldabra giant tortoise named Adwaita died at the Alipore Zoological Gardens in Kolkata. He was brought to the zoo in the 1870s from the estate of Lord Clive and is thought to have been around 255 years old when he died. Around the time of its discovery, they were caught for food in such large numbers that they became virtually extinct by 1900. Giant tortoises are now protected by strict conservation laws and are categorized as threatened species.

List of insular species

Taxonomy of extant and extinct insular giant tortoise species follows Rhodin et al. (2021), unless otherwise noted.

Aldabrachelys

{| class="wikitable"

!Archipelago

!Island

!Species

|rowspan="6"|Seychelles

|Granitic Seychelles

|Aldabra Atoll

|Aldabra giant tortoise (A. gigantea gigantea)

|Cosmoledo

|rowspan="4"|†Aldabrachelys sp.

|Denis Island

|Assumption Island

|Astove Atoll

|Glorioso Islands

|†Aldabrachelys sp.|†Grandidier's giant tortoise (A. grandidieri)

Chelonoidis

{| class="wikitable"

!Archipelago

!Island

!Species

|rowspan="10"|Galápagos Islands

|San Cristóbal

|San Cristobal giant tortoise (C. niger chathamensis)

|Isabela

|Santiago

|Santiago Island giant tortoise (C. niger darwini)

|Santa Cruz

|Ferdandina

|Fernandina Island Galápagos tortoise (C. niger phantastica)

|Pinta

|†Pinta Island tortoise (C. niger abingdonii)

|Floreana

|†Floreana Island tortoise (C. niger niger)

|Pinzón

|Pinzón Island giant tortoise (C. niger duncanensis)

|Española

|Hood Island giant tortoise (C. niger hoodensis)

|Santa Fe

|†Santa Fe Island tortoise (C. niger ssp.)

|rowspan="7"|Lucayan Archipelago

|Andros

|rowspan="5"|†Abaco tortoise (C. alburyorum alburyorum)

|Nassau

|Mayaguana

|Crooked Island

|Gran Abaco

|Grand Turk

|†Turks tortoise (C. alburyorum keegani)

|Middle Caicos

|†Caicos tortoise (C. alburyorum sementis)

|rowspan="4"|Greater Antilles

|Cuba

|†Cuban giant tortoise (C. cubensis)

|Hispaniola

|Mona

|†Mona tortoise (C. monensis)

|Navassa

|†Chelonoidis sp.

|rowspan="2"|Lesser Antilles

|Sombrero

|†Sombrero tortoise (C. sombrerensis)

|Curaçao

|†Chelonoidis sp.

Other genera

{| class="wikitable"

!Archipelago

!Island

!Species

|rowspan="3"|Mascarene Islands

|Réunion

|†Réunion giant tortoise (Cylindraspis indica)

|Rodrigues

|Mauritius

|Malta

|†Centrochelys robusta