Santa Cruz Island
Our explorations of the famed Galápagos Islands continued today with a search for one of the archipelago’s most famous and emblematic species, the giant tortoise. In order to find these gentle giants, we sailed north from Floreana Island to reach the central island of Santa Cruz. Second largest in the group, this island is also one of only four inhabited islands in the archipelago, and the roads created to link the humid highlands to the arid coast give us access to the areas where these fascinating creatures roam.
The Galápagos giant tortoise (Chelonoidis nigra) is native to seven of the Galápagos Islands and is both the largest living species of tortoise and tenth-heaviest living reptile, reaching weights of over 880 lbs. and lengths of over 5’9”! With life spans in the wild of over 100 years, it is also one of the longest-living vertebrates on earth. The ancestors of the Galápagos tortoises colonized these remote volcanic islands, located over 600 miles from the nearest mainland, by overwater dispersal from South America about five or six million years ago. These animals are aided in oceanic dispersal by their ability to float with their heads up, and to survive up to a year without food or water. In fact, although it was long thought that tortoises had arrived in South America overland from North America, a recent comparative genetic analysis concludes that the South American genus Chelonoidis (formerly part of the species Geochelone) is actually most closely related to African hingeback tortoises, thus it is now believed that the ancestors of the genus Chelonoidis dispersed from Africa to South America during the Oligocene.
Recent research is now showing that tortoises' gigantism was probably a pre-adapted condition for successful colonization of these remote oceanic islands rather than an example of evolved insular gigantism, as was previously thought (fossil giant tortoises from mainland South America have been described that support this hypothesis of pre-adapted gigantism). Large tortoises would have a greater chance of surviving the journey over water from the mainland as they can hold their heads a greater height above the water level and have a smaller surface area/volume ratio, which reduces osmotic water loss. Their significant water and fat reserves would have allowed them to survive long ocean crossings without food or fresh water, and to endure the drought-prone climate of the islands.
Upon arriving in the Galápagos, the small founding population would have adapted to the new conditions and rapidly given rise to new species, then spread to other islands. This is reflected in recently undertaken mitochondrial DNA analysis, which indicates that the oldest existing islands (Española and San Cristóbal) were colonized first, and that these populations seeded the younger islands via dispersal in a "stepping stone" fashion by means of local currents, thus echoing the volcanic history of the islands. Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern species of which ten still exist to this day.
So as we donned our rubber boots and set out to walk among these relics from a past age, in the pastures of the foggy highlands, we could truly feel that we had stepped 28 million years back in time to the Oligocene, to wander among creatures long disappeared from our planet.
