Galápagos tortoise

With lifespans in the wild of over 100 years, it is one of the longest-lived vertebrates. Captive Galapagos tortoises can live up to 177 years.^[5] For example, a captive individual, Harriet, lived for at least 175 years. Spanish explorers, who discovered the islands in the 16th century, named them after the Spanish galápago, meaning "tortoise".^[6]


Galápagos tortoises are native to seven of the Galápagos Islands. Shell size and shape vary between subspecies and 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. Charles Darwin's observations of these differences on the second voyage of the Beagle in 1835, contributed to the development of his theory of evolution.


Tortoise numbers declined from over 250,000 in the 16th century to a low of around 15,000 in the 1970s. This decline was caused by overexploitation of the subspecies for meat and oil, habitat clearance for agriculture, and introduction of non-native animals to the islands, such as rats, goats, and pigs. The extinction of most giant tortoise lineages is thought to have also been caused by predation by humans or human ancestors, as the tortoises themselves have no natural predators. Tortoise populations on at least three islands have become extinct in historical times due to human activities. Specimens of these extinct taxa exist in several museums and also are being subjected to DNA analysis. 12 subspecies of the original 14–15 survive in the wild; a 13th subspecies (C. n. abingdonii) had only a single known living individual, kept in captivity and nicknamed Lonesome George until his death in June 2012. Two other subspecies, C. n. niger (the type subspecies of Galápagos tortoise) from Floreana Island and an undescribed subspecies from Santa Fe Island are known to have gone extinct in the mid-late 19th century. Conservation efforts, beginning in the 20th century, have resulted in thousands of captive-bred juveniles being released onto their ancestral home islands, and the total number of the subspecies is estimated to have exceeded 19,000 at the start of the 21st century. Despite this rebound, all surviving subspecies are classified as Threatened by the International Union for Conservation of Nature.


The Galápagos tortoises are one of two insular radiations of giant tortoises that still survive to the modern day; the other is Aldabrachelys gigantea of Aldabra and the Seychelles in the Indian Ocean, 700 km (430 mi) east of Tanzania. While giant tortoise radiations were common in prehistoric times, humans have wiped out the majority of them worldwide; the only other radiation of tortoises to survive to historic times, Cylindraspis of the Mascarenes, was driven to extinction by the 19th century, and other giant tortoise radiations such as a Centrochelys radiation on the Canary Islands and another Chelonoidis radiation in the Caribbean were driven to extinction prior to that.

Evolutionary history[edit]

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.^[17] The minimal founding population was a pregnant female or a breeding pair.^[17] 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.^[33] As they are poor swimmers, the journey was probably a passive one facilitated by the Humboldt Current, which diverts westwards towards the Galápagos Islands from the mainland.^[72] The ancestors of the genus Chelonoidis are believed to have similarly dispersed from Africa to South America during the Oligocene.^[33]


The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Chaco tortoise (Chelonoidis chilensis), a much smaller subspecies from South America. The divergence between C. chilensis and C. niger probably occurred 11.95–25 million years ago, an evolutionary event preceding the volcanic formation of the oldest modern Galápagos Islands 5 million years ago.^{[114][115][116]} Mitochondrial DNA analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands via dispersal in a "stepping stone" fashion via local currents.^[117][118] Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern subspecies. The evolutionary relationships between the subspecies thus echo the volcanic history of the islands.^[17]