Tortoise evolution: How did they become so big?

Date: October 23, 2018

Source: Martin-Luther-Universität Halle-Wittenberg

Tortoises are a group of terrestrial turtles globally distributed in habitats ranging from deserts to forests and include species such as the Greek and the Galapagos tortoise. Some species evolved large body sizes with a shell length exceeding 1 metre whereas others are no larger than 6-8 centimetres. Despite a particular interest from naturalists ever since the times of Darwin, the evolution of gigantism in tortoises remains enigmatic.

The fact that all living giant tortoises are insular may suggest that their evolution followed the so-called island rule: a trend toward dwarfism of large animals and gigantism of small animals on islands. An example of insular dwarfism is the Florida key deer, a dwarf version of the mainland white-tailed deer; its small size may be an adaptation to the limited resources found on the islands. Insular gigantism is best exemplified by the famous dodo, an extinct flightless pigeon from Mauritius, probably evolving large body size due to release from predatory pressure. Previous studies on extant tortoises were partly inconclusive: giant size has been linked to the absence of predatory mammals in islands but it has been also proposed that tortoises were already giants when they reached the remote archipelagos. Since very few giant tortoise species survive to the present, these hypotheses are impossible to test without analysing extinct species through the help of the fossil record.

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