Island Gigantism And
The Galapagos Tortoise

What does that term island gigantism mean? Simply put it means "things get bigger on islands". It does not always happen. Sometimes the opposite can happen, where species shrink on islands. This is called Island Dwarfing.

Why Does Gigantism Occur?

It's to do with the kind of pressures animals face. While it seems to go against common sense, large size for a plant eater on an island is usually a big benefit.

Tortoises are almost always plant eaters, and for plant eaters size is always an advantage. It allows them to have a longer digestive system, essential for extracting all the goodness out of their tough diet. It also gives better protection from predators. Past a certain size, Galapagos tortoises have no natural predators on the islands.

Large size also means that nesting females can lay larger eggs which in turn allow for larger hatchlings. This reduces the number of predators that can prey on the young. In the case of the Galapagos tortoises, this strategy was so successful that the landscape was virtually covered with tortoises when the islands were discovered in the 1500s.

How Does Island Gigantism Happen?

You can see variation between members of the same species all around. This is not only true for familiar creatures such as people, dogs and cats, but is true of almost all species. It is certainly true of reptiles, even though they may all look the same to our eyes.

In any one clutch of hatchlings, there will be a range of characteristics among the young tortoises. Since even the smallest advantage can mean the difference between life and death in the wild, bigger baby tortoises will stand a much better chance of surviving than smaller ones. Once all the little ones have been weeded out, the bigger ones mate with the bigger ones and the process starts again.

In this way the tortoises can become progressively larger from one generation to the next, as a direct response to the threats posed to them by predators eating the little guys.

Not Just In The Galapagos

Giant tortoises have also been found on several other widely spaced islands. These tortoises cannot all be descended from the same root species, as DNA studies have shown. They have been discovered on the Seychelles, Mauritius, Réunion and Rodrigues Islands, and Aldabra Atoll. These tortoises are thought to have evolved from at least three different ancestors, yet they all share common traits of size, behavior and reproduction. The locations of these islands is shown on the map to the left.

The resemblance between the Indian Ocean giant tortoises and the distant Galapagos tortoises is astounding. Have a look at the comparison of two species shown below.

Island Gigantism Is Not Just For Tortoises

The feared Komodo Dragon is another example of a giant island dwelling reptile. Interestingly it is a carnivore, so its path to gigantism was not due to the improved digestion of plant matter.

What About Mammals?

Being warm blooded is a very energy expensive way of life. Large mammals eat many more times the amount of food that a similar size reptile would eat. Mammals are therefore unlikely to become very large on islands due to the limited food supply. A classic example of this are the miniature island dwelling elephants whose remains have been found on the islands of Sicily and Malta.

Predatory mammals tend to follow patterns set by their prey, so dwarfing of prey would eventually also cause the predators to become smaller and therefore more energy efficient.

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