Galápagos tortoise

The Galápagos tortoise (Chelonoidis niger), also called the Galápagos giant tortoise, is a very large species of tortoise. It is the largest living species of tortoise and can weigh up to 417 kg (919 lb). These tortoises are also the largest terrestrial cold-blooded animals (ectotherms) and can live for over 100 years in the wild, with some living up to 177 years in captivity.

Native to seven of the Galápagos Islands, Galápagos tortoises have shells and sizes that vary depending on where they live. Those on islands with more moisture and plants have larger, dome-shaped shells, while those on drier islands have smaller shells with longer necks. The differences in shell shapes helped Charles Darwin develop his theory of evolution when he studied the tortoises during his voyage on the Beagle in 1835.

Tortoise numbers dropped sharply from over 250,000 in the 1500s to around 15,000 in the 1970s due to human activities like taking them for food and clearing land for farming. Some island populations were lost completely. Conservation efforts have helped increase their numbers again, with thousands of baby tortoises being raised in safe places and released back to their home islands. Today, all surviving subspecies are still considered Threatened by the International Union for Conservation of Nature.

Taxonomy

Early classification

The Galápagos Islands were discovered in 1535 but first appeared on maps around 1570. They were named "Islands of the Tortoises" because of the giant tortoises living there.

At first, people thought the giant tortoises from the Indian Ocean and the Galápagos were the same. Over time, scientists gave them different names. By 1834, they were recognized as a separate group.

Recognition of subpopulations

In 1875, a scientist named Albert Günther identified different groups of tortoises on the islands. Later, more groups were found. Scientists thought all these tortoises came from one ancestor that spread by land bridges, but we now know the islands formed from volcanoes and the tortoises came from South America.

Current species and genus names

Today, the Galápagos tortoise is known as Chelonoidis niger. It was once part of a different group but was moved to its own in the 1980s.

Subspecies

There are 15 subspecies of Galápagos tortoises, though only 12 still exist. Some live on separate islands, and others on the volcanoes of Isabela Island. One subspecies from Pinta Island went extinct in 2012. Another from Floreana Island may have been hunted to extinction by 1850, but some related tortoises have been found.

Isabela Island

The tortoises on Isabela Island are debated. The group on Volcan Wolf is different from the others. Recent studies show that the groups on Volcan Alcedo and Volcan Darwin are also unique. The group on Sierra Negra may have split from another group more recently.

Floreana Island

Studies suggest that the extinct tortoises from Floreana might be brought back using tortoises from Isabela Island that have some of their genes.

Pinta Island

The tortoises from Pinta Island are most related to those from San Cristóbal and Española islands, not Isabela as once thought.

Santa Cruz Island

Tortoises on Santa Cruz have different genetic lines. In 2015, one group was named a new subspecies.

Pinzón Island

Only a few very old tortoises were left on Pinzón Island because rats were eating the young ones. In 2012, rats were removed, and baby tortoises started to survive again.

Española Island

Only a few tortoises were left on Española Island, so scientists brought them to a breeding center. After many years, the population grew, and now over 1,000 tortoises live there.

Fernandina Island

A single tortoise was found on Fernandina Island in 2019, suggesting the subspecies might still exist there.

Santa Fe Island

The tortoises from Santa Fe Island are gone, but scientists brought tortoises from another island to live there.

Subspecies of doubtful existence

One supposed subspecies from Rábida Island was probably brought there from another island and not a true separate group.

Description

The Galápagos tortoises have a large, bony shell that is usually a dull brown or grey color. Their shells are made of fused plates that protect their bodies. These tortoises can pull their heads, necks, and front legs inside their shells when they feel unsafe. Their legs are big and sturdy, with dry, scaly skin and tough scales. The front legs have five claws, while the back legs have four.

These tortoises are very big, with some weighing over 400 kg and measuring nearly 2 meters long. They come from islands where food can be hard to find, so their large size helps them survive long journeys and tough weather. Their shells can look different depending on where they live—some have rounded, dome-shaped shells, while others have shells that curve upward like a saddle. These differences help them reach food and survive in their specific island homes.

Behavior

The Galápagos tortoises are cold-blooded, so they spend about 1–2 hours each morning in the sun to warm up before they start moving around for the day. They usually walk slowly, moving about 0.3 kilometers per hour (0.2 miles per hour). On islands with more rain, these tortoises travel between lower areas during the wet season and higher areas during the dry season, using the same paths for many years.

These tortoises eat many plants such as cacti, grasses, leaves, and berries. They get most of their water from plants and dew, which helps them go many months without drinking. They can even survive a whole year without food or water by using stored fat.

Galápagos tortoises mainly use their eyes and smell to sense their surroundings, rather than their hearing. They have helpful relationships with some birds, who eat small pests off the tortoises’ shells and bodies.

Mating can happen at any time of year, but often occurs between February and June. Males sometimes fight for the chance to mate, but this is usually not too rough. After mating, females travel to sandy areas to dig nests and lay their eggs. The babies hatch after several months and face some dangers as they grow, but adult tortoises live very long lives—often over 100 years.

Evolutionary history

All subspecies of Galápagos tortoises evolved from ancestors that arrived from mainland South America by traveling over water. Genetic studies show that the Chaco tortoise of Argentina and Paraguay is their closest living relative. These tortoises can survive long ocean journeys because they are able to float, breathe while in water, and go many months without food or fresh water. They likely reached the Galápagos Islands with help from the Humboldt Current, which moves westward from the mainland.

The Chaco tortoise and the Galápagos giant tortoise diverged about 12 to 25 million years ago, before the oldest Galápagos Islands even formed. The oldest existing islands, Española and San Cristóbal, were colonised first. Over time, tortoises spread to younger islands, and because each island was isolated, the tortoises evolved separately into the different forms we see today.

Darwin's development of theory of evolution

Charles Darwin visited the Galápagos Islands for five weeks in 1835 and saw the large tortoises there. They appeared in his writings and journals, and they helped him develop his theory of evolution.

Darwin wrote about how the tortoises on different islands looked different. He realized that islands close to each other, with similar conditions, could have different animals. This idea helped him think about how animals might change over time. Even though he took some tortoises back with him, they were not kept for study, but for food for the ship's journey.

Darwin compared the tortoises to other animals he saw, like mockingbirds, and began to think that animals on different islands might change if they were separated for a long time. These observations were important for his ideas about how species might change and differ from each other.

Conservation

When people first found the Galápagos tortoises in the 16th century, there were about 250,000 of them. By 1974, their numbers had dropped to just 3,060 because people took them for food. Today, thanks to efforts to protect them, there are around 19,317 tortoises.

One type of Galápagos tortoise known as C. n. niger disappeared in the 19th century because people took them for food. Another type, C. n. abingdonii, lost its last remaining individual in 2012. The other types are at risk and need protection.

Historical exploitation

Long ago, pirates and sailors took many tortoises for food because they could survive without eating or drinking for a long time while on ships. Later, whalers also took large numbers of tortoises for food and for a special oil they could make. The number of tortoises dropped quickly, especially near the coast where people could easily find them.

When gold was discovered in California in 1849, people took tortoises there to feed the workers. As more people moved to the Galápagos Islands, they hunted tortoises, cut down trees for farms, and brought in animals like pigs and rats that ate the tortoises' eggs. These changes made it even harder for the tortoises to survive.

Modern conservation

Today, the Galápagos tortoises are protected by laws. In 1936, they were declared protected, and in 1959, parts of the islands became a national park. Rules were made to stop people from taking tortoises or their eggs.

Special programs help baby tortoises grow safe and strong before they are released back into the wild. One successful story is about the Española tortoise, which was close to disappearing. Scientists helped them breed in a safe place, and now their numbers are growing again.

Workers also remove harmful animals like goats and rats from the islands. By doing this, they help the tortoises and the whole island ecosystem recover. Scientists also study how tortoises move to better protect them and teach others about their importance.