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Age and Dietary Differences of Recently Extinct Indian Ocean Tortoises (Geochelone s. lat.) Revealed by Carbon Isotope Analysis

R. Burleigh, E. N. Arnold

Abstract

All but one of the seven or eight species of giant tortoise (Geochelone) in the southwest Indian Ocean became extinct by the mid-nineteenth century, leaving many aspects of their history and biology unknown. Radiocarbon dating of fossil remains indicates that the population of G. gigantea on the small island of Assumption was a natural one, predating European activity in the area. It also confirms that the two species known from Mauritius, G. inepta and G. triserrata, were indeed synchronous and that this was also true of the pair found on Madagascar, G. grandidieri and G. abrupta. Survival of both the latter species well into the first millennium A.D. is demonstrated, suggesting that they were still present when the first human colonists arrived and were perhaps exterminated by them. Stable carbon isotope ratios indicate that, apparently unlike other Indian Ocean giant tortoises, G. grandidieri ate a high proportion of plants with C$_{4}$ metabolism. This may have reduced potential competition with the sympatric G. abrupta.

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