Galapagos tortoises are the largest of the tortoises, weighing up to 900 pounds. When sailors first visited the Galapagos Islands in the 17th century, there were 13 different subspecies of Galapagos tortoises. Unfortunately, several of these are now extinct.
Each island in the Galapagos has its own distinct race of giant tortoise, with the biggest island, Isabella, having five different races. The tortoises have had to adapt to the different environments of the various islands. Tortoises from bigger islands and higher elevations, where there is more rain and lush vegetation, are larger and have dome-shaped shells.
For the most part, Galapagos tortoises are grazers. Smaller islands with no highlands tend to be dry and have sparse vegetation with little variation. Tortoises on these islands are smaller and have saddleback shells that allow then to browse on the leaves of bushes.
Why do giant tortoises get so big? Again, it’s the result of ecological factors. The absence of competing mammalian herbivores, intra-specific competition for food and the ability to store energy in response to fluctuating food supplies all might result in an increase in body size.
During the Pleistocene era, giant tortoises lived on the mainland of North and South America, even in what is now South Carolina. But, they were quickly eliminated after the arrival of humans.