The Island Rule describes a biological phenomenon in which animals left alone on islands undergo very different evolutionary paths to their mainland cousins.
When an animal is on an island with a lack of natural predators, evolution typically tends to push an animal towards gigantism. If there are predators about, it can go the other way and the animal may become smaller over time, as it is more effective to shrink in size and find somewhere to hide than it is to grow and fight. There would be little to no benefit for an animal to become larger in size, if it means it would then be competing with already existing larger species for resources such as food or shelter.
The general (but simplified) rule is, in mammals, small animals and omnivores get bigger, while larger animals and carnivores get smaller. Smaller creatures grow in size when there are no predators around to keep them in check. Bigger animals get smaller in response to limited food sources, when living on an island there is often a finite food supply.
Here are six animals that became giants:
1. Saint Helena giant earwig
This is the world’s largest earwig and was endemic to the oceanic island of Saint Helena in the southern Atlantic Ocean. It is now considered extinct by the IUCN as no live individuals have been found since 1967.
2. Chappell Island tiger snake
The largest of the tiger snake species, the Chapel Island resident averages 1.9 m (over 6 ft) in length and is considered as one of Australia’s deadliest snakes. But despite their fearsome and venomous reputation, they are actually rather docile.
3. Madagascar hissing cockroach
These wingless cockroaches are usually found in rotting logs. They are excellent climbers and even smooth glass cannot stop them from scaling a building. The hissing noise is produced when the cockroach forces air out of the spiracles (tiny holes) on their body. They hiss on three main occasions: when they disturbed, in mating, and when they are threatened and ready for a fight.
4. Galapagos tortoise
Now famous as one of the largest tortoises in the world, the Galapagos tortoise is a perfect example of gigantism. Historic accounts report that it was a ‘medium-sized‘ tortoise left on the island, suggesting the abundant food supply and no predators, left them to grow in size.
5. The weta of New Zealand
Endemic to New Zealand, the weta are some of the largest (and heaviest) insects in the world. They are a hardy group, with species living in a variety of habitats from forests to grasslands. The name weta is said to have come from the Maori word wetapunga meaning ‘god of ugly things’.
6. Solomon Island skink
Another record holder, this skink is the world’s largest species of skink. From the tip of their nose to the end of their tail, they can grow up to 81 centimetres (32 inches). Although they have incredibly strong jaws, they are relatively peaceful creatures, surviving on a vegetarian diet of leaves, flowers, and fruit.
Image from www.flickr.com/photos/stuutje