Today’s factismal: Galápagos tortoises are an example of island dwarfism.
Quick! What weighs 550 lbs, is big enough to ride on, and is the smallest of its kind? Why, it is the Galápagos Island tortoise. These huge reptiles are amazing animals. They move at a top speed of nearly two miles a day and feed of a wide variety of vegetables (such as cacti and berries) and small animals (such as lizards and fish). Young tortoises will eat more than 1/8th of their body weight each day, turning the excess into fat that they store for food during the lean months. And those lean months can be long, indeed; the Galápagos tortoise can live for a year on the food and water stored in its shell as fat.
But amazing as that is, what is even more amazing is that the Galápagos tortoise is probably the last of its lineage of giant tortoises, all of which were larger than it. Consider the Hesperotestudo crassiscutata, a giant gopher tortoise, which was about twice as big as the Galápagos tortoise and lived in Texas and Central North America until about 12,000 years ago. And then there was Megalochelys atlas; it lived in India and was another tortoise that makes the Galápagos tortoise look like a runt. Let’s not forget the Ninjemys oweni, a spike-covered super tortoise. And if we go back a few million years, there is the great-granddaddy of scary big tortoises, Carbonemys cofrinii. This guy was roughly the size of a Prius and lived on a diet of crocodiles, fish, and molluscs.
Giant tortoises had a survival advantage in a wet world like the Pleistocene. They could jump into the water and survive long ocean voyages until they came to a landing place. Much like a reptilian coconut, they would have used the sea currents to populate islands and new territories. But like the vegetable coconuts, they were too tasty to survive. Other critters would have found them an easy food source. As a result, the mainland tortoises died out as smilodons, cave bears, and hominids munched them into extinction; in the end, only the species that had moved to islands survived.
And those species, including our friend the Galápagos tortoise, suffered the same fate as elephants, hippopotamuses, and even hominids on islands. Each succeeding generation is smaller because the island’s limited resources give a smaller size a survival advantage. In 1964, a biologist came up with the rule that animals that colonize islands tend to either grow larger or smaller to match the resources available on the island. Known as Foster’s Rule, this was a triumph of evolutionary biology. And today we continue to learn about evolution with the help of citizen scientists. One project is the Evolution MegaLab. By counting the number of bands on snail shells, we can see how their predators (thrushes) are forcing snails to change and adapt. To learn more, crawl over to: