Galápagos tortoise
The Galápagos tortoise, also called the Galápagos giant tortoise, is a very large species of tortoise in the genus Chelonoidis. The species comprises 15 subspecies. It is the largest living species of tortoise, and can weigh up to. They are also the largest extant terrestrial cold-blooded animals.
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. 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".
Galápagos tortoises are native to seven of the Galápagos Islands. Shell size and shape vary between subspecies and populations. On islands with humid highlands and abundant low vegetation, the tortoises are larger, with domed shells and short necks; on islands with dry lowlands and less ground-level vegetation, 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 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 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, 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.
Taxonomy
Early classification
The Galápagos Islands were discovered in 1535, but first appeared on the maps, of Gerardus Mercator and Abraham Ortelius, around 1570. The islands were named "Insulae de los Galopegos" in reference to the giant tortoises found there.Initially, the giant tortoises of the Indian Ocean and those from the Galápagos were thought to be the same subspecies. Naturalists thought that sailors had transported the tortoises there. In 1676, the pre-Linnaean authority Claude Perrault referred to both subspecies as Tortue des Indes. In 1783, Johann Gottlob Schneider classified all giant tortoises as Testudo indica. In 1812, August Friedrich Schweigger named them Testudo gigantea. In 1834, André Marie Constant Duméril and Gabriel Bibron classified the Galápagos tortoises as a separate subspecies, which they named Testudo nigrita.
Recognition of subpopulations
The first systematic survey of giant tortoises was by the zoologist Albert Günther of the British Museum, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther hypothesized that all the giant tortoises descended from a single ancestral population which had spread by sunken land bridges. This hypothesis was later disproven by the understanding that the Galápagos, the atolls of Seychelles, and the Mascarene islands are all of recent volcanic origin and have never been linked to a continent by land bridges. Galápagos tortoises are now thought to have descended from a South American ancestor, while the Indian Ocean tortoises derived from ancestral populations on Madagascar.At the end of the 19th century, Georg Baur and Walter Rothschild recognised five more populations of Galápagos tortoise. In 1905–06, an expedition by the California Academy of Sciences, with Joseph R. Slevin in charge of reptiles, collected specimens which were studied by Academy herpetologist John Van Denburgh. He identified four additional populations, and proposed the existence of 15 subspecies. Van Denburgh's list still guides the taxonomy of the Galápagos tortoise, though now 10 populations are thought to have existed.
Current species and genus names
The current specific designation of niger, was resurrected as a valid species name in 1984 after it was discovered to be the senior synonym for the then commonly used subspecies name of elephantopus. Quoy and Gaimard's Latin description explains the use of nigra: "Testudo toto corpore nigro" means "tortoise with completely black body". Quoy and Gaimard described it from a living specimen, but no evidence indicates they knew of its accurate provenance within the Galápagos – the locality was in fact given as California. Garman proposed synonymizing it with the extinct Floreana subspecies. Later, Pritchard deemed it convenient to accept this designation, despite its tenuousness, for minimal disruption to the already confused nomenclature of the subspecies. The even more senior subspecies synonym of californiana is considered a nomen oblitum.In most of the 1900s, the Galápagos tortoise was considered to belong to the genus Geochelone, known as 'typical tortoises' or 'terrestrial turtles'. In the 1980s, the former subgenus Chelonoidis was elevated to generic status, a conclusion later supported by phylogenetic evidence which grouped the South American members of Geochelone into an independent clade. This nomenclature has been adopted by several authorities. The names of several species names in the genus Chelonoidis, including this one, have often been misspelled, beginning in the 1980s when Chelonoidis was elevated to genus and mistakenly treated as feminine rather than masculine, an error recognized and fixed in 2017.
Subspecies
Within the archipelago, 14-15 subspecies of Galápagos tortoises have been identified, although only 12 survive to this day. Five are found on separate islands; five of them on the volcanoes of Isabela Island. Several of the surviving subspecies are seriously endangered. A 13th subspecies, C. n. abingdonii from Pinta Island, is extinct since 2012. The last known specimen, named Lonesome George, died in captivity on 24 June 2012; George had been mated with female tortoises of several other subspecies, but none of the eggs from these pairings hatched. The subspecies inhabiting Floreana Island is thought to have been hunted to extinction by 1850, only 15 years after Charles Darwin's landmark visit of 1835, when he saw shells, but no live tortoises there. However, recent DNA testing shows that an intermixed, non-native population currently existing on the island of Isabela is of genetic resemblance to the subspecies native to Floreana, suggesting that C. niger has not gone entirely extinct. The existence of the C. n. phantastica subspecies of Fernandina Island was disputed, as it was described from a single specimen that may have been an artificial introduction to the island; however, a live female was found in 2019, likely confirming the subspecies' validity.Prior to widespread knowledge of the differences between the populations from different islands and volcanoes, captive collections in zoos were indiscriminately mixed. Fertile offspring resulted from pairings of animals from different races. However, captive crosses between tortoises from different races have lower fertility and higher mortality than those between tortoises of the same race, and captives in mixed herds normally direct courtship only toward members of the same race.
The valid scientific names of each of the individual populations are not universally accepted, and some researchers still consider each subspecies to be distinct species. Prior to 2021, all subspecies were classified as distinct species from one another, but a 2021 study analyzing the level of divergence within the extinct West Indian Chelonoidis radiation and comparing it to the Galápagos radiation found that the level of divergence within both clades may have been significantly overestimated, and supported once again reclassifying all Galápagos tortoises as subspecies of a single species, C. niger. This was followed by the Turtle Taxonomy Working Group and the Reptile Database later that year. The taxonomic status of the various races is not fully resolved.
- Testudo californiana
- Testudo nigra
- Testudo elephantopus
- Testudo nigrita
- Testudo planiceps
- Testudo clivosa
- Testudo typica
- Testudo ''elephantopus
- Geochelone elephantopus
- Chelonoidis elephantopus
C. n. nigra
C. n. abingdoni
C. n. becki
C. n. chathamensis
C. n. darwini
C. n. duncanensis
C. n. hoodensis
C. n. phantastica
C. n. porteri
C. n. vicina
; Chelonoidis nigra nigra
Isabela Island
The five populations living on the largest island, Isabela, are the ones that are the subject of the most debate as to whether they are true subspecies or just distinct populations of a subspecies. It is widely accepted that the population living on the northernmost volcano, Volcan Wolf, is genetically independent from the four populations to the south and is therefore a separate subspecies. It is thought to be derived from a different colonization event than the others. A colonization from the island of Santiago apparently gave rise to the Volcan Wolf subspecies while the four southern populations are believed to be descended from a second colonization from the more southerly island of Santa Cruz. Tortoises from Santa Cruz are thought to have first colonized the Sierra Negra volcano, which was the first of the island's volcanoes to form. The tortoises then spread north to each newly created volcano, resulting in the populations living on Volcan Alcedo and then Volcan Darwin. Recent genetic evidence shows that these two populations are genetically distinct from each other and from the population living on Sierra Negra and therefore form the subspecies C. n. vandenburghi and C. n. microphyes. The fifth population living on the southernmost volcano is thought to have split off from the Sierra Negra population more recently and is therefore not as genetically different as the other two. Isabela is the most recently formed island tortoises inhabit, so its populations have had less time to evolve independently than populations on other islands, but according to some researchers, they are all genetically different and should each be considered as separate subspecies.
Floreana Island
Phylogenetic analysis may help to "resurrect" the extinct subspecies of Floreana —a subspecies known only from subfossil remains. Some tortoises from Isabela were found to be a partial match for the genetic profile of Floreana specimens from museum collections, possibly indicating the presence of hybrids from a population transported by humans from Floreana to Isabela, resulting either from individuals deliberately transported between the islands, or from individuals thrown overboard from ships to lighten the load. Nine Floreana descendants have been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz; the genetic footprint was identified in the genomes of hybrid offspring. This allows the possibility of re-establishing a reconstructed subspecies from selective breeding of the hybrid animals. Furthermore, individuals from the subspecies possibly are still extant. Genetic analysis from a sample of tortoises from Volcan Wolf found 84 first-generation C. n. niger hybrids, some less than 15 years old. The genetic diversity of these individuals is estimated to have required 38 C. n. niger parents, many of which could still be alive on Isabela Island.
Pinta Island
File:Lonesome George in profile.jpg|thumb|C. n. abingdonii Lonesome George at the Charles Darwin Research Station, in 2006
The Pinta Island subspecies has been found to be most closely related to the subspecies on the islands of San Cristóbal and Española which lie over 300 km away, rather than that on the neighbouring island of Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta. This discovery informed further attempts to preserve the C. n. abingdonii lineage and the search for an appropriate mate for Lonesome George, which had been penned with females from Isabela. Hope was bolstered by the discovery of a C. n. abingdonii hybrid male in the Volcán Wolf population on northern Isabela, raising the possibility that more undiscovered living Pinta descendants exist.
Santa Cruz Island
Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in nonoverlapping population distributions around the regions of Cerro Montura, Cerro Fatal, and La Caseta. Although traditionally grouped into a single subspecies, the lineages are all more closely related to tortoises on other islands than to each other: Cerro Montura tortoises are most closely related to C. n. duncanensis from Pinzón, Cerro Fatal to C. n. chathamensis from San Cristóbal, and La Caseta to the four southern races of Isabela as well as Floreana tortoises.
In 2015, the Cerro Fatal tortoises were described as a distinct taxon, donfaustoi. Prior to the identification of this subspecies through genetic analysis, it was noted that there existed differences in shells between the Cerro Fatal tortoises and other tortoises on Santa Cruz. By classifying the Cerro Fatal tortoises into a new taxon, greater attention can be paid to protecting its habitat, according to Adalgisa Caccone, who is a member of the team making this classification.
Pinzón Island
When it was discovered that the central, small island of Pinzón had only 100–200 very old adults and no young tortoises had survived into adulthood for perhaps more than 70 years, the resident scientists initiated what would eventually become the Giant Tortoise Breeding and Rearing Program. Over the next 50 years, this program resulted in major successes in the recovery of giant tortoise populations throughout the archipelago.
In 1965, the first tortoise eggs collected from natural nests on Pinzón Island were brought to the Charles Darwin Research Station, where they would complete the period of incubation and then hatch, becoming the first young tortoises to be reared in captivity. The introduction of black rats onto Pinzón sometime in the latter half of the 19th century had resulted in the complete eradication of all young tortoises. Black rats had been eating both tortoise eggs and hatchlings, effectively destroying the future of the tortoise population. Only the longevity of giant tortoises allowed them to survive until the Galápagos National Park, Island Conservation, Charles Darwin Foundation, the Raptor Center, and Bell Laboratories removed invasive rats in 2012. In 2013, heralding an important step in Pinzón tortoise recovery, hatchlings emerged from native Pinzón tortoise nests on the island and the Galápagos National Park successfully returned 118 hatchlings to their native island home. Partners returned to Pinzón Island in late 2014 and continued to observe hatchling tortoises, indicating that natural recruitment is occurring on the island unimpeded. They also discovered a snail subspecies new to science. These exciting results highlight the conservation value of this important management action. In early 2015, after extensive monitoring, partners confirmed that Pinzón and Plaza Sur Islands are now both rodent-free.
Española Island
On the southern island of Española, only 14 adult tortoises were found, two males and 12 females. The tortoises apparently were not encountering one another, so no reproduction was occurring. Between 1963 and 1974, all 14 adult tortoises discovered on the island were brought to the tortoise center on Santa Cruz and a tortoise breeding program was initiated. In 1977, a third Española male tortoise was returned to Galapagos from the San Diego Zoo and joined the breeding group. After 40 years' work reintroducing captive animals, a detailed study of the island's ecosystem has confirmed it has a stable, breeding population. Where once 15 were known, now more than 1,000 giant tortoises inhabit the island of Española. One research team has found that more than half the tortoises released since the first reintroductions are still alive, and they are breeding well enough for the population to progress onward, unaided. In January 2020, it was widely reported that Diego, a 100-year-old male tortoise, resurrected 40% of the tortoise population on the island and is known as the "Playboy Tortoise".
Fernandina Island
The C. n. phantasticus'' subspecies from Fernandina was originally known from a single specimen—an old male from the voyage of 1905–06. No other tortoises or remains were found on the island for a long time after its sighting, leading to suggestions that the specimen was an artificial introduction from elsewhere. Fernandina has neither human settlements nor feral mammals, so if this subspecies ever did exist, its extinction would have been by natural means, such as volcanic activity. Nevertheless, there have occasionally been reports from Fernandina. In 2019, an elderly female specimen was finally discovered on Fernandina and transferred to a breeding center, and trace evidence found on the expedition indicates that more individuals likely exist in the wild. It has been theorized that the rarity of the subspecies may be due to the harsh habitat it survives in, such as the lava flows that are known to frequently cover the island.