The Galápagos tortoise (Chelonoidis niger), also called the Galápagos giant tortoise, is a very large tortoise in the genus Chelonoidis, which also includes three smaller species from mainland South America. This species includes 15 subspecies (12 that still exist and 3 that no longer exist). It is the largest living tortoise, and some can weigh up to 417 kg (919 lb). These tortoises are also the largest living cold-blooded animals on land.

In the wild, Galápagos tortoises can live more than 100 years, making them one of the longest-lived vertebrates. Some tortoises in captivity have lived up to 177 years. For example, a tortoise named Harriet lived for at least 175 years. Spanish explorers discovered the Galápagos Islands in the 16th century and named them after the Spanish word galápago, which means "tortoise."

Galápagos tortoises are found on seven of the Galápagos Islands. The size and shape of their shells differ between subspecies and populations. On islands with wet highlands and plenty of low vegetation, tortoises are larger with domed shells and short necks. On drier islands with less ground-level vegetation, they are smaller with "saddleback" shells and long necks. Charles Darwin studied these differences during his second voyage on the Beagle in 1835, which helped him develop his theory of evolution.

The number of Galápagos tortoises dropped from over 250,000 in the 16th century to about 15,000 by the 1970s. This decline was caused by people hunting them for meat and oil, clearing their habitat for farming, and introducing non-native animals like rats, goats, and pigs to the islands. Many tortoise species went extinct because humans hunted them, as tortoises have no natural predators. At least three tortoise populations on islands have gone extinct due to human activities. Museum collections and DNA studies include remains of these extinct species.

Today, 12 of the original 14–15 subspecies survive in the wild. A 13th subspecies (C. n. abingdonii) had only one known individual, named Lonesome George, who lived in captivity until his death in June 2012. Two other subspecies, C. n. niger (the main subspecies of Galápagos tortoise) from Floreana Island and an unnamed subspecies from Santa Fe Island, are believed to have gone extinct in the mid- to late-1800s. Conservation efforts since the 20th century have led to the release of thousands of captive-bred tortoises onto their home islands. By the start of the 21st century, the total number of tortoises was estimated to be over 19,000. Despite this recovery, all surviving subspecies are classified as Threatened by the International Union for Conservation of Nature.

The Galápagos tortoises are one of only two groups of giant tortoises that still exist today. The other group includes Aldabrachelys gigantea, found on Aldabra and the Seychelles islands in the Indian Ocean, 700 km (430 mi) east of Tanzania. While giant tortoises were common in ancient times, humans have caused the extinction of most of them worldwide. Another group, Cylindraspis tortoises from the Mascarenes, was driven to extinction by the 19th century. Other giant tortoise groups, such as Centrochelys tortoises on the Canary Islands and Chelonoidis tortoises in the Caribbean, also went extinct before that time.

Taxonomy

The Galápagos Islands were discovered in 1535, but they first appeared on maps made by Gerardus Mercator and Abraham Ortelius around 1570. The islands were named "Insulae de los Galopegos" (Islands of the Tortoises) because of the giant tortoises found there.

At first, scientists thought the giant tortoises in the Indian Ocean and the Galápagos were the same kind. They believed sailors had brought the tortoises to the Galápagos. In 1676, a scientist named Claude Perrault called both groups "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 Galápagos tortoises as a separate group, naming them Testudo nigrita.

In 1875, Albert Günther, a zoologist at the British Museum, studied giant tortoises for the first time in a detailed way. He found at least five groups of Galápagos tortoises and three from the Indian Ocean. Later, in 1877, he added more groups: six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther thought all giant tortoises came from a single group that spread through sunken land bridges. This idea was later proven wrong because scientists learned the Galápagos, Seychelles, and Mascarene islands were formed by volcanoes and never connected to continents by land bridges. Today, scientists believe Galápagos tortoises came from ancestors in South America, while Indian Ocean tortoises came from ancestors in Madagascar.

At the end of the 1800s, Georg Baur and Walter Rothschild identified five more Galápagos tortoise groups. Between 1905 and 1906, an expedition by the California Academy of Sciences collected tortoises. John Van Denburgh, a scientist at the Academy, studied them and found four more groups, suggesting there were 15 subspecies. His findings still guide how scientists classify Galápagos tortoises, though now only 10 groups are thought to have existed.

The name "niger" (originally spelled "nigra" in 1824) was used again in 1984. It means "black" and was first used by Quoy and Gaimard to describe a tortoise with a completely black body. They described it from a living tortoise, but they did not know where it came from. Later, scientists thought it might be the same as the extinct Floreana tortoise. Pritchard used the name "niger" to avoid confusion in naming. Another older name, "californiana," is no longer used.

In most of the 1900s, Galápagos tortoises were grouped under the genus Geochelone. In the 1980s, scientists moved some of them to a new group called Chelonoidis, based on genetic studies. This change was later supported by evidence showing South American tortoises form a separate group. Some names in the Chelonoidis group were misspelled, but this was corrected in 2017.

Today, 14 to 15 subspecies of Galápagos tortoises have been identified, but only 12 remain. Five live on separate islands, and five are found on the volcanoes of Isabela Island. Some are seriously endangered. A 13th subspecies, C. n. abingdonii, from Pinta Island, went extinct in 2012. The last one, Lonesome George, died in captivity. Scientists believe the subspecies from Floreana Island was hunted to extinction by 1850, but DNA tests suggest some tortoises on Isabela may share their genes. A subspecies from Fernandina Island was once doubted, but a live female was found in 2019, proving it exists.

Before scientists understood the differences between tortoise groups, zoos mixed them together. This led to offspring with lower survival rates. Today, scientists debate whether all Galápagos tortoises are part of one species or separate ones. A 2021 study suggested they are all subspecies of a single species, C. niger. This idea is now widely accepted.

C. n. nigra is the main subspecies.

Modern DNA studies show how Galápagos tortoises are related. The five groups on Isabela Island are often debated. The population on Volcan Wolf is genetically different from the others and is considered a separate subspecies. Scientists think it came from Santiago Island, while the four southern groups came from Santa Cruz. Tortoises from Santa Cruz first reached Sierra Negra volcano and then spread north to Volcan Alcedo and Volcan Darwin. Recent studies show these two groups are genetically different from each other and from other populations.

Description

The tortoises have a large, bony shell that is usually a dull brown or grey color. The shell plates are attached to the ribs, forming a strong, protective structure that is part of the skeleton. Lichens can grow on the shells of these slow-moving animals. Tortoises keep a unique pattern of shell segments throughout their lives, but the yearly growth bands on the shell are not helpful for determining their age because the outer layers wear away over time. A tortoise can pull its head, neck, and front legs into its shell for protection. The legs are large and thick, with dry, scaly skin and hard scales. The front legs have five claws, and the back legs have four.

The Galápagos Islands were discovered by Fray Tomás de Berlanga, Bishop of Panama, in 1535. He wrote about "such big tortoises that each could carry a man on top of himself." Three centuries later, in 1835, naturalist Charles Darwin noted that "these animals grow to an immense size … several so large that it required six or eight men to lift them from the ground." The largest recorded tortoises weighed more than 400 kg (880 lb) and measured up to 1.87 meters (6.1 ft) in length. While Galápagos tortoises and Aldabra giant tortoises share similar sizes, Galápagos tortoises tend to be slightly larger on average. Adult males often weigh more than 185 kg (408 lb), while females typically weigh between 136 and 181 kg (300 to 399 lb). However, sizes vary across islands and subspecies. Tortoises from Pinzón Island are smaller, with a maximum weight of 76 kg (168 lb) and a carapace length of about 61 cm (24 in), compared to tortoises from Santa Cruz Island, which can grow up to 1.5 meters (59 in) in length. The large size of Galápagos tortoises likely helped them survive long ocean journeys from the mainland, as their size reduced water loss and allowed them to store more water and fat. Their larger size also helped them tolerate extreme temperatures. Fossil evidence from South America supports the idea that their large size existed before they reached the islands.

Galápagos tortoises have two main shell shapes that relate to their environment. One shape is called "saddleback," which has an upward curve at the front of the shell, resembling a saddle. The other is "domed," which has a rounded, dome-like shape. When a saddleback tortoise pulls its head and front legs into its shell, a gap remains over its neck, showing that these tortoises did not face many predators during their evolution. Tortoises on larger islands with humid highlands, like Santa Cruz, have domed shells and are larger, with shorter necks and legs. Saddleback tortoises live on smaller, drier islands like Española and Pinzón, where food and resources are limited. Two groups of tortoises on Santa Cruz Island have similar growth patterns but can be distinguished by their shell shapes. The shape of the shell changes as tortoises grow, with certain shell segments becoming narrower or wider.

The saddleback shell shape, along with longer necks and legs, helps tortoises reach tall plants, such as the Opuntia cactus, in dry areas. Saddleback tortoises are smaller and more territorial than domed tortoises, which may be an adaptation to limited resources. Larger tortoises may be better suited to cooler, high-elevation areas because their size helps them stay warm.

Another idea is that the saddleback shape and longer limbs and necks might be a feature used by male tortoises to attract mates. Male tortoises compete for mates by showing off their height, not their size. This explains why saddleback males are more aggressive than domed males. The shell shape and longer limbs of saddleback tortoises may be a balance between needing a small body in dry areas and needing height for dominance displays.

The saddleback shell shape likely developed independently in different dry areas because similar shell shapes do not always mean the tortoises are closely related genetically. Instead, similar shell shapes may result from similar environmental conditions.

Male tortoises are generally larger than females. In saddleback populations, males have more angled and higher front shell openings, making their saddle shape more pronounced. All male tortoises have longer tails and shorter, curved underparts with thick bumps at the back to help during mating. Adult males weigh between 272 and 317 kg (600 to 699 lb), while adult females weigh between 136 and 181 kg (300 to 399 lb).

Behavior

The tortoises are cold-blooded, so they sit in the sun for 1–2 hours after sunrise to absorb heat through their dark shells before searching for food for 8–9 hours each day. They move mostly in the early morning or late afternoon between places where they rest and where they eat. Scientists have observed that they walk at a speed of 0.3 km/h (about 0.2 mph).

On larger, more humid islands, the tortoises move seasonally between low areas, which become grassy plains during the wet season, and higher areas covered with meadows during the dry season. These same paths have been used for many generations, creating clear trails through the plants called "tortoise highways." On these wetter islands, domed tortoises often travel in large groups, unlike the more solitary and territorial saddleback tortoises.

Tortoises sometimes rest in mud puddles or pools formed by rain. This helps them stay warm during cool nights and protects them from parasites like mosquitoes and ticks. To fight parasites, they take dust baths in loose soil. Some tortoises sleep under rocks at night, while others rest in small, cozy depressions in the ground or among plants called "pallets." On some islands, like Volcán Alcedo, tortoises using the same pallet sites create small, sandy pits over time.

Tortoises are herbivores that eat plants such as cacti, grasses, leaves, lichens, berries, melons, oranges, and milkweed. They have been seen eating specific plants like the poison apple (Hippomane mancinella), the endemic Galápagos guava (Psidium galapageium), the water fern (Azolla microphylla), the bromeliad (Tillandsia insularis), and the Galápagos tomato (Solanum cheesmaniae). Young tortoises eat about 16.7% of their body weight in dry food each day, and their digestion works similarly to that of animals like horses and rhinos.

Tortoises get most of their water from dew and sap in plants, especially the Opuntia cactus. This allows them to survive more than six months without drinking. They can live up to a year without food or water by breaking down their body fat to create water. Tortoises have very slow metabolisms. When they are thirsty, they may drink large amounts of water quickly, storing it in their bladders and a part of their neck called the pericardium. This ability made them useful as water sources for ships. On dry islands, tortoises drink morning dew from rocks, and over time, this activity has created half-sphere depressions in the rock.

Regarding their senses, Charles Darwin wrote, "The people here believe these animals are completely deaf. I was always amused when I passed one of these large tortoises, and it would suddenly pull its head and legs in, hiss loudly, and fall to the ground as if struck." Although they are not deaf, tortoises rely more on sight and smell than hearing.

Tortoises have a mutualistic relationship with some Galápagos finches and mockingbirds. The birds eat parasites on the tortoises, and the tortoises benefit from this. The process begins when small groups of finches hop near the tortoise. The tortoise signals it is ready by standing up and stretching its neck and legs, allowing the birds to reach hard-to-reach areas like the neck, legs, and skin between the shell and underbelly.

Some tortoises use this relationship to eat birds. After rising and extending its limbs, the bird may go under the tortoise to investigate. Then, the tortoise suddenly pulls its limbs in, flattens itself, and kills the bird. It then steps back to eat the bird, likely to gain extra protein.

Tortoises can mate any time of the year, but they often mate between February and June during the rainy season in humid areas. When male tortoises meet during the mating season, they face each other in a display of dominance, standing on their legs and stretching their necks with their mouths open. Sometimes, they bite each other, but the smaller tortoise usually backs away, allowing the larger one to mate. This behavior is most common in saddleback tortoises, which are more aggressive and have longer necks.

Before mating, males may aggressively ram the female’s shell and bite her legs. Mounting is difficult, and the male must stretch to balance on top of the female. The curved under part of the male’s shell helps him stay balanced and brings his cloacal opening (where the penis is located) closer to the female’s cloaca. During mating, the male makes deep, rhythmic sounds called "groans." These are among the few sounds tortoises make; other noises occur during fights or when they hiss while pulling into their shells.

Female tortoises travel several kilometers in July to November to reach sandy nesting areas. Digging a nest is a hard and time-consuming task, often taking several hours each day over many days. Using only their hind legs, the female digs a 30 cm (12 in)-deep hole and lays up to 16 eggs, each about the size of a billiard ball and weighing between 82 and 157 grams. Some observations suggest that domed tortoises (like C. porteri on Santa Cruz) lay more eggs per clutch (about 9.6) than saddleback tortoises (like C. duncanensis on Pinzón, which lay about 4.6 eggs per clutch). The female covers the nest with a plug made of soil and urine, pressing it down with her underbelly. She may lay one to four clutches each season. The temperature of the nest determines the sex of the

Evolutionary history

All Galápagos tortoise subspecies evolved from ancestors that traveled from mainland South America across the ocean. Genetic research shows the Chaco tortoise of Argentina and Paraguay is their closest living relative. The smallest group of ancestors that reached the islands was likely one pregnant female or a pair of breeding tortoises. These tortoises could survive the 1,000-km ocean journey because they float well, can breathe by lifting their heads above water, and can live for months without food or fresh water. Since they are not strong swimmers, the journey was probably helped by the Humboldt Current, which carries water westward toward the Galápagos Islands from the mainland. The ancestors of the genus Chelonoidis are believed to have traveled from Africa to South America during the Oligocene era.

The closest living relative (but not a direct ancestor) of the Galápagos giant tortoise is the Chaco tortoise (Chelonoidis chilensis), a smaller tortoise from South America. The split between C. chilensis and C. niger likely happened 11.95–25 million years ago, long before the oldest modern Galápagos Islands formed 5 million years ago. Studies of special DNA show that the oldest islands, Española and San Cristóbal, were first colonized. Populations on these islands then spread to younger islands through a "stepping stone" pattern, guided by local ocean currents. Limited movement of genes between isolated islands led to the separate evolution of populations into the distinct subspecies seen today. The relationships among the subspecies match the volcanic history of the islands.

Darwin's development of theory of evolution

Charles Darwin visited the Galápagos Islands for five weeks during the second voyage of HMS Beagle in 1835. He saw Galápagos tortoises on San Cristobal (Chatham) and Santiago (James) Islands. These tortoises appeared in his writings and journals and helped shape his ideas about evolution.

Darwin wrote about the voyage:

He did not understand the importance of the differences between tortoises on different islands until after leaving the Galápagos. He continued,

When the Beagle left the Galápagos, over 30 adult tortoises were on the ship. These were not for scientific study but to provide fresh meat for the journey. Their shells and bones were thrown overboard, leaving no remains to study. Some people think Darwin missed this chance because he only reported seeing tortoises on San Cristobal (C. chathamensis) and Santiago (C. darwini). These tortoises have similar shell shapes and are not very different from each other. Although he visited Floreana, the C. niger subspecies there was nearly extinct, and he likely did not see any adult tortoises there.

Darwin had four live juvenile tortoises from different islands. He collected one from San Salvador, his captain FitzRoy collected two from Española, and his servant Syms Covington collected one from Floreana. These young tortoises could not show differences in shape because they were not yet mature. The British Museum had some tortoises, but their exact origin in the Galápagos was unknown. However, Darwin talked to a naturalist named Gabriel Bibron, who had seen adult Galápagos tortoises in Paris. Bibron confirmed that different varieties of tortoises existed.

Darwin later compared the tortoises with mockingbirds, making his first connection between his Galápagos observations and the possibility that subspecies could change over time. He wrote in his notebooks: "Animals on separate islands ought to become different if kept long enough apart with slightly differing circumstances. —Now Galápagos Tortoises, Mocking birds, Falkland Fox, Chiloe fox, —English and Irish Hare." These observations challenged the idea that subspecies were created separately.

Darwin also found the tortoises to be interesting and fun to observe. He wrote, "I frequently got on their backs, and then giving a few raps on the hinder part of their shells, they would rise up and walk away;—but I found it very difficult to keep my balance."

Conservation

Over time, humans hunted tortoises for food, which caused their numbers to drop from about 250,000 in the 16th century to just 3,060 in 1974. Conservation efforts later helped increase their population to 19,317 between 1995 and 2009.

The subspecies C. n. niger disappeared by the 19th century due to human hunting. Another subspecies, C. n. abingdonii, became extinct on June 24, 2012, when the last individual, a male named Lonesome George, died in captivity. All other surviving subspecies are classified by the IUCN as "vulnerable" or worse in conservation status.

About 200,000 tortoises were taken before the 20th century. These slow-moving, defenseless animals were stored alive on ships, where they could survive for at least a year without food or water. Their urine and stored water in their bodies were used as drinking water. In the 17th century, English explorer William Dampier wrote that Galápagos tortoise meat was "very sweet" and enjoyable. Captain James Colnett of the Royal Navy said the tortoise meat was "the most delicious food we had ever tasted." US Navy captain David Porter described the meat as "easily digested" and "excellent" for eating in large amounts. Charles Darwin noted that the meat was "indifferent" to his taste, though he found the soup made from young tortoises and roasted meat with skin to be good.

In the 17th century, pirates used the Galápagos Islands as a base to restock supplies before attacking Spanish colonies. At that time, the tortoises did not face survival challenges because the islands were far from shipping routes and had few valuable resources. In contrast, tortoises in the Indian Ocean were already near extinction by the late 17th century. Between the 1790s and 1860s, whaling ships and fur sealers collected tortoises in much larger numbers than earlier pirates. Some were used for food, while many were killed for "turtle oil" sold to Ecuador. Logs from whaling ships recorded over 13,000 tortoises taken between 1831 and 1868, and about 100,000 were taken before 1830. Tortoises near coasts were most vulnerable during nesting seasons. The whalers stopped collecting tortoises due to their scarcity and the rise of cheaper energy sources like crude oil.

The California Gold Rush in 1849 increased tortoise exploitation. Tortoises and sea turtles were brought to towns in Alta California to feed miners. Bones from these animals were found at an archaeological site in San Francisco.

Population decline worsened with early 19th-century settlements, which led to unregulated hunting, habitat destruction for farming, and the introduction of non-native animals. Feral pigs, dogs, cats, and black rats prey on tortoise eggs and young, while goats, donkeys, and cattle compete for food and damage nests. The Floreana subspecies became extinct in the mid-19th century due to hunting, farming, and invasive species.

Between 1888 and 1930, scientists collected 661 tortoises. Over 120 tortoises were taken by poachers since 1990. Today, threats include tourism growth and expanding human settlements. The number of tortoise subspecies dropped from 15 to 11 since Darwin’s time.

Major threats to tortoises include:
– Introduced mammals
– Poachers
– Habitat destruction
– Traits that make them vulnerable: slow growth, late maturity, limited to Galápagos Islands, large size, and slow movement
– Collection for oil, museum displays, and private collections

The remaining tortoise subspecies range from "extinct in the wild" to "vulnerable" on the IUCN list. Their slow growth, late maturity, and island-specific habitats make them highly vulnerable to extinction without conservation help. The Galápagos giant tortoise is a key species for conservation efforts.

The Galápagos giant tortoise is now protected under Appendix I of CITES, which bans most international trade. Ecuador protected the tortoise in 1936, established the Galápagos National Park and Charles Darwin Foundation in 1959, and banned tortoise removal in 1970. Exporting tortoises from Ecuador became illegal, and the U.S. banned their import in 1969. A 1971 law made it illegal to harm or disturb anything in the national park.

After the park and foundation were created in 1959, only 11 of the original 14 tortoise populations remained, most of which were endangered. A breeding program began in 1965 to save the Pinzón subspecies, which had fewer than 200 adults due to black rat predation. Breeding centers now raise young tortoises to improve their survival. Eggs are collected for these programs.