The Island Rule

When animal species become isolated on islands, their body size often changes in surprising ways over many generations. Large mammals, such as elephants and deer, tend to become smaller, a process known as island dwarfism. On the Mediterranean island of Sicily, for example, scientists have found fossils of elephants that stood only about one meter tall. Small animals such as rodents, by contrast, frequently grow larger than their relatives on the mainland.

Biologists have proposed several explanations for this pattern. Islands usually offer limited food, so large animals that need less energy are more likely to survive and reproduce. At the same time, many islands have no large predators. Without the constant threat of being hunted, small animals no longer benefit as much from being tiny and able to hide, and larger individuals may compete more successfully for food and space.

However, the island rule is far from universal. Studies comparing hundreds of species have found that the pattern is strong in some groups, such as mammals, but weak or absent in others. As a result, many researchers now argue that the size of an island, its climate, and the diet of each species must all be considered before general conclusions can be drawn.