Why are Island Tortoises so Large?

New research has shown that giant tortoises may not be large because of their island lifestyle, as previously thought. A team of researchers has gathered extensive genetic data from both living species and fossils of extinct species to discover the truth of how tortoises have evolved, in an attempt to answer the question of how they have become such giants.

Tortoises are an extremely diverse group of animals, ranging in size from 8cm long to one metre long. The largest species is the Galápagos tortoise, which can weigh nearly 500kg. So why has gigantism evolved in tortoises? Despite studies on these fascinating animals since Darwinian times, the answer is still unclear. It was thought that, like many species, tortoises followed the “island rule”: a tendency of dwarfism among large animals and gigantism of small animals living on islands. For example, in mainland Florida the white-tailed deer is abundant, but in the Florida Keys – a tropical archipelago of islands off the south coast of the state – a dwarf version of the mainland deer is found, known as the Florida key deer.

It is thought that island dwarfism is caused by limited resources, while a reduced pressure from predators triggers island gigantism. However, in the case of tortoises, it was suggested that these animals were already large in size before colonising remote island habitats. With so many giant tortoise species now extinct, it is impossible to uncover the reason behind gigantism in these animals without using the fossil record.

Galapagos_giant_tortoise Matthew Field
A dome-shelled Galápagos giant tortoise (Credit: Matthew Field) http://www.photography.mattfield.com

Now, Dr Evangelos Vlachos from the Paleontological Museum of Trelew in Argentina and Dr Márton Rabi from the Martin Luther University Halle-Wittenberg put together the most complete family tree of both extinct and surviving tortoise species so far, in an attempt to piece together the reasons for their variation in size. Interestingly, it was discovered that gigantism occurred on the mainland too, in Africa, Europe, Asia, North and South America. Interestingly, all of these mainland species became extinct.

“The fossils highlight a great number of extinct mainland giant species and suggest that the evolution of giant size was not linked to islands,” says Dr Evangelos Vlachos, “We expect that warmer climate and predator pressure plays a role in the evolution of giant size but the picture is complex and our sampling of the fossil record is still limited.”

So what led to the extinction of these giant mainland tortoises, and why have their island variants survived? It is thought that predation and climate change contributed to these extinctions, but it is intriguing to think that the island rule may not be the overriding factor for giant evolution, calling for more research into what causes such variation in size among these animals.

Tortoises have roamed the planet for over 55 million years – they survived the mass extinction that wiped out the dinosaurs, but now face an even graver threat: people. Human-induced habitat loss is the most significant threat for tortoises today, and they face an uncertain future. Thanks to research such as Vlachos’ and Rabi’s, we are able to better understand how these successful animals have evolved, and will hopefully continue to evolve for many more years to come.

To read more on this research, visit Science Daily.

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