Isla Isabela, Galapagos

Isla Isabela, with its large lava fields and volcanic calderas, is famous for its unique endemic species, many of which have a range of only a few miles. I have arrived at the southeastern tip of the island, where the prevailing trade winds have allowed the growth of dense tropical forest, creating an abundant, Eden-like environment that contrasts with the barren volcanic plains and peaks of the island’s north.

As I trek through the lush tropical forest, I spot a giant tortoise, plodding slowly and carefully across my path. Specifically, I see a dome-necked Sierra Negra giant tortoise (Chelonoidis niger guntheri) which makes its home on the verdant Sierra Negra volcano on whose slopes I am now standing. The Galapagos are actually named for these fascinating, lumbering beasts, the name for the islands coming from the Spanish word ‘Galapago’, or tortoise.

These tortoises like to take it easy, and their relaxed attitude and lack of stress must contribute to their unusually long lifespans. They live for an average of 100 years, though some have reached the venerable old age of around 170. One specimen, named Harriet, a tortoise said to have been collected by Darwin on his voyage in 1831, died in Australia in 2006, sometime after her 175th birthday!

There are many species of giant tortoise on the islands; in fact, the slopes of each of Isabela’s volcanoes have their own species. They are generally split into two broad groups: saddlebacked and dome-backed. This difference nudged Darwin towards his theory of natural selection. He observed that in the lush, fertile highlands, the tortoises tended to be larger, with domed shells and short necks. In the sparser desert environments, however, he saw that the tortoises had longer necks that extended from a saddle-shaped opening in their shells, allowing them a good, high reach and access to leaves that stood higher up. By comparing the two species, Darwin observed evolutionary divergence, where a common ancestor species branches off into multiple species to suit new environments.

In the 15th Century, Spanish explorer Tomas de Berlanda observed that the islands had ‘such big tortoises that each could carry a man on top of himself’. People have long noted that some species on far-flung islands become much larger than their mainland cousins. This is called ‘insular gigantism’, and it’s a controversial subject among students of the Galapagos giant tortoise.

Detractors to the claim that the tortoise exhibits insular giganticism point out that the tortoises were giant before they arrived on the islands, showing fossils of extinct species of giant tortoise found in South America, surely the ancestors of the modern-day Galapagos giant tortoise species. They didn’t evolve their giganticism on the islands, ergo, no insular giganticism.

But not so fast! The defence concedes that, yes, giant tortoises evolved on the mainland. But, they would also like to pose the question - why are there no giant tortoises over there, whereas in the Galapagos they’re a common sight? The answer is natural selection. On the mainland, with its plentiful predators, being a giant tortoise was a disadvantage, so they died out or evolved to be smaller. On the Galapagos, with big sizes come big advantages; they’re far too big to be threatened by the islands' few predators, and their larger size means they lose less water to osmosis and can store huge fat deposits, allowing them to live up to a year without food or water. So then, the tortoise’s giganticism allows it unique advantages that suit it to island life, allowing this gigantic species to survive here when it would have died out on the continent. 

I’m unsure where I fall on the great insular giganticism debate, but I do know these are some of the most fascinating creatures in the world, and they need our help. When Tomas de Berlanda landed on the islands, there were an estimated 250,000 specimens. Today, due to overhunting and the introduction of invasive dogs and wild pigs, their number has plummeted to little more than 12,000. Charities dedicated to protecting the giant tortoise are fighting back with a robust collective breeding program. Thanks to hard work and generous donations, they have been successful, releasing over 7,000 tortoises into the wild since 1985. Perhaps the big, lumbering specimen I’m watching take it slow and easy today is an alumnus of this heroic rewilding programme.