Giant tortoises are rare today but once roamed four continents

New Scientist

Image: Charles Luk

Tortoises evolved into giants on at least seven occasions and on four continents. The finding undermines the long-standing idea that tortoises become enormous only if they are stranded on remote islands.

There are more than 40 species of tortoise, the most spectacular being the giant tortoises. On the Galapagos islands in the Pacific and Seychelles islands in the Indian Ocean, these animals can have shells more than 120 centimetres long.

These islands cover just a few thousand square kilometres. In contrast, Earth’s continents cover 150 million square kilometres. Yet they are home to just one truly large tortoise: the African spurred tortoise.

This implies that tortoises are most likely to become huge when they live on islands, in line with a famous but controversial concept, the “island rule“. This states that, on islands, small animals tend to evolve larger bodies while large animals evolve to be smaller.

But tortoise biologists suspect otherwise. Fossils show giant tortoises once roamed Africa, Eurasia and the Americas, suggesting tortoises don’t need islands to evolve to be larger.