Scientists Have Noticed a Peculiar Transformation in Animals That Live in Isolated Islands

On the many islands across the world’s surface, animals sometimes seem strangely different from those on the continents. Some become alarmingly small, while others grow alarmingly large. Biologists have long referred to this phenomenon as the “island rule,”which is a broad pattern of evolution by which large species become smaller after colonizing an island, while small species become larger.

The idea first gained attention when paleontologists began comparing fossil animals from islands with their continental ancestors. Dwarf elephants that once lived on Mediterranean islands such as Cyprus, Malta, and Sicily seem to be one of the clearest examples, and fossil evidence discussed in evolutionary research and summarized in studies cited by Palkopoulou and colleagues in Proceedings of the Royal Society B shows that these elephants evolved smaller bodies dramatically after becoming isolated on islands. It was believed that Limited food resources and restricted space favored individuals who required less energy to survive.


This concept is known as insular dwarfism, and it repeatedly appears in the fossil record. Large mammals that arrive on islands quite often face environments where there is limited vegetation and predators are scarce. Under those conditions, smaller body sizes may be advantageous because they require fewer resources, and natural selection can gradually produce populations far smaller than their mainland relatives over generations.

The opposite trend can also occur. Some animals evolve island gigantism, growing much larger than comparable mainland species. One well-known example is the giant tortoises found in the Galápagos Islands and Seychelles. The absence of large predators allowed these reptiles to grow slowly over long lifespans, as summarised by zoological studies in sources such as the Encyclopaedia Britannica and by evolutionary analyses in journals such as Evolution. Their large bodies help store energy and water, which can be useful in environments where resources fluctuate seasonally.

The island rule is not universal, despite its name. A large comparative study published by Benítez-López and colleagues in Nature Ecology & Evolution analyzed hundreds of island populations and found that body size changes depend strongly on local ecological conditions. What they found was groundbreaking: some species shrink dramatically, some grow larger, and others remain relatively unchanged. Researchers have identified several factors that influence how body size evolves on islands. Island size and isolation appear to play major roles, and studies summarized in evolutionary biology journals such as Evolution indicate that animals on smaller or more remote islands often show stronger size changes. Limited resources and fewer predators create new environmental pressures that shape evolutionary outcomes.

Another example of these dynamics is provided by reptiles. Research on insular lizards published in Evolution has documented widespread Holocene dwarfism, where many island populations gradually evolved smaller bodies over thousands of years. Scientists believe these changes help reduce energy demands in ecosystems where food supplies are limited.

The impact of predators can also be a determining factor in the size of large mammals. Research carried out on the size of various species of mammals, including those published in Evolution and other scientific journals such as Ecology, shows that when food becomes scarce while predators are absent, the size of the species tends to reduce. However, when a new ecological niche presents itself, the species tends to increase in size to exploit the available resources to the full potential.

Islands provide a good platform to observe the evolution of species. The isolated ecosystem provides a unique mix of ecological conditions that often result in the evolution of species that are either larger or smaller than their counterparts. For instance, the existence of dwarf elephants and large tortoises only goes to prove that the island rule provides one of the most obvious instances of how geography influences the evolution of species.