Tim Radford
Mauritius was the death of more than just the dodo. It may have been home to the world’s only lightweight, racing tortoise – until humans arrived.
Jeremy Austin, of the Natural History Museum, told the British Association festival of science in Cardiff recently that the three Mascarene islands of Reunion, Mauritius and Rodrigues, in the south-west Indian Ocean, may have had six species of giant tortoise. On each island there was a round-shelled tortoise, and sometimes a second species in which the front of the shell was raised.
Most tortoises have thick shells. “The thick, bony shells also make tortoises heavy and slow,” said Austin. “In the Mascarene islands there were no large predators, so the tortoises adapted to the island life by reducing the thickness of their shell to the bare minimum, and having much larger openings for the legs and head. This would have reduced the weight of the shell enormously, leading to the possibility that these animals were the world’s only lightweight racing tortoise.”
In one of the first studies of its kind, Austin has used ancient DNA recovered from bones trapped in caves to answer questions about the evolution of the tortoise that could run but could not hide. Visitors to the islands in the 1600s talked of great herds of tortoises. They were hunted in vast numbers for food and were easy meat for introduced predators. By the early 1800s they were gone.
The tortoises could have got to Mauritius and the other islands only by drifting along on ocean currents, an amazing feat of endurance and long- distance travel in itself.
Biologists were puzzled as to whether the two forms evolved only once, and spread from one island to the other two, or evolved independently on each island. Scientists at the Natural History Museum have enough DNA – which can be read as a kind of clock – to begin to answer some of the questions.
“We now know that the Mascarene islands were colonised only once by tortoises,” said Austin. “The two species on Rodrigues evolved there independently of the other two islands, and at least one of the Reunion species arrived from Mauritius in quite recent times.”