Science Puzzle
Why Did the Tortoises Grow So Big?
Tortoises on the mainland are the size of a dinner plate. On remote islands like the Galapagos, their relatives grew to the size of an armchair and can live over a century.
Islands are small and short of resources. So why did the tortoises get bigger, not smaller?
The Answer
Because on an island the costs of being big vanish and the benefits remain. On the mainland, growing large is punished: you need enormous amounts of food, you cannot hide, and you are a slow, obvious meal for predators. Selection keeps tortoises small.
Now strand a few tortoises on a volcanic island where no mammal predator has ever set foot. Suddenly nothing hunts you, so size costs you nothing in danger. And size starts paying: a bigger body stores more water and fat, which lets a tortoise survive months of drought, and a bigger tortoise loses heat more slowly through the cold nights. Generation after generation, the largest animals survive the hard years and leave the most offspring, and the lineage swells.
The same isolation runs the other way too. Large mammals stranded on islands often shrink, because food is scarce and there is nothing left to defend against. Islands do not make animals big or small; they remove the mainland's rules and let new ones take over.
The principle: Island evolution. Isolation removes predators and changes selection pressures, so traits penalised on the mainland, such as great size, can become advantageous.