Potential new species of giant tortoise discovered in the Galápagos Islands

Some of the giant tortoises living in Ecuador's Galápagos Island archipelago may not be the species they're thought to be.

Genetic analysis of living and dead giant tortoises found on the island of San Cristóbal suggest that the dead animals may be a distinct lineage from those surviving on the island today. As the current species used for both living and dead tortoises, Chelonoidis chathamensis, was described using one of the dead specimens, confirmation they are separate species would mean the animals alive today would have to be given a new name. 

To confirm this, samples of tortoise DNA itself, rather than DNA contained in cell components called mitochondria, would be needed.

Ecuador's environment minister Gustavo Manrique Miranda, however, has already welcomed this potential new species of giant tortoise. He says, 'Good news! Genetic studies carried out by the University of Newcastle, Yale, the Galápagos Conservancy and other institutions revealed that San Cristóbal Island in the Galápagos is home to a species of giant tortoise not yet described by science.'

His ministry added, 'The species of giant tortoise that inhabits San Cristóbal Island, until now scientifically known as Chelonoidis chathamensis, genetically corresponds to a different species, which was believed to be extinct since the beginning of the 20th century.'

The findings of the study, conducted by an international team of scientists, were published in the journal Heredity. 

How did the Galápagos giant tortoises evolve?

The Galápagos Islands are an archipelago in the Pacific Ocean, comprised of hundreds of islands formed by volcanic activity. They are relatively new in geological terms, with the oldest current island, San Cristóbal, only emerging between two and six million years ago.

As with many archipelagos across the world, the different conditions found on each island provide the opportunities for individual species to radiate into a variety of different forms to match each environment. In the case of the Galápagos, one notable example are its finches, whose common ancestor has given rise to 18 species across the various islands.

These helped inspire Charles Darwin to formulate his theory of evolution by natural selection alongside Alfred Russel Wallace. 

Another example of adaptive radiation on these islands are the distinctive giant tortoises. Arriving while the islands were still forming around two to three million years ago, the tortoises gradually spread to the new islands as they rose out of the ocean.

While they are not good swimmers, giant tortoises are able to float with their heads above water and can survive without food and water for up to six months. This means they could have been taken to new islands by ocean currents, and began evolving into new species based on the conditions found there.

The reason for their size is not known, with some studies suggesting they may have been big before arriving, while others point to a process known as island gigantism.

Overall, there are 14 recognised species of Galápagos giant tortoise, with one other yet to be described. Of these, 13 species are currently alive, while the Pinta Island tortoise went extinct in 2012 after the last of its species, Lonesome George, died.

The new study suggests that the number of species may need to be updated after the tortoises of San Cristóbal were reassessed. 

Is there a new Galápagos giant tortoise species?

The scientists took blood samples from hundreds of giant tortoises living on the island today, as well as genetic samples from the bones of species collected over 100 years ago. The bones included five specimens found dead in a cave, as well as one specimen found alive elsewhere on San Cristóbal.

Genetic material from the mitochondria, the part of the cell involved in energy production, was obtained from the bones, but DNA from the cell itself couldn't be extracted. Mitochondria have their own set of genes distinct from the rest of the organism they are a part of.

Comparisons of the mitochondrial DNA revealed that the living tortoises all shared the same set of genes, while the living specimen collected in 1906 had only a few small differences from them.

However, the dead specimens collected in 1906 had genes which are distinct, and connected more closely to species on other islands such as the Española and Pinta Island tortoises rather than those found on San Cristóbal.

The scientists believe the evidence suggests that one group of tortoises on the island split into two groups, possibly highland and lowland varieties. This may be a result of sea level rises in the past splitting the island in two.

The lowland group, which may be the ancestors of the dead specimens, could then have colonised some of the other Galápagos Islands with their descendants becoming the species now found there. Those related to the ancestors of living specimens in the highland group spread to other islands.

In any case, it is likely that both groups of tortoises were living alongside each other on San Cristóbal over the past few hundred years, when sea levels weren't as high. These giant tortoises may then have come back together to form one population, as is currently being observed in Volcán Wolf giant tortoises on another of the Galápagos Islands.

Depending on how distinct these groups were, they may have been different species or just distinct populations sharing an island. Having been unable to extract nuclear DNA, this study is unable to offer a conclusive answer either way.

If they are separate, however, then the extinct species would take the name of the San Cristóbal giant tortoise as the type specimen. Under scientific naming rules, this means the living species would need a new common and scientific name.

Only time will tell if these gentle giants will need a new moniker after all.