Giant tortoises spread to western Indian Ocean islands by sea drift in pre-Holocene times, not by later human agency - response to Wilmé et al . (2016a)

Abstract

Evidence from DNA phylogeny, Plio-Pleistocene ocean currents, giant tortoise dispersal, evolution of plant defences, radiocarbon dates and archaeology indicates that the endemic giant tortoises on the Mascarenes and Seychelles colonized naturally and were not translocated there by humans. Giant tortoises have fascinated Indian Ocean travellers since their first discovery on Mauritius by the Dutch in 1598 (Cheke & Bour, 2014), and a Reunion tortoise was among the earliest biological specimens to reach Europe from the Malagasy region (in 1671: Bour, 2004), but Wilme et al. (2016a) are the first to suggest that humans introduced them, their main arguments being that: Sea drift is too rare, uncertain and contrary to currents to account for the presence of tortoises on so many western Indian Ocean islands. The existing phylogeny, with dating considered uncertain, does not rule out rapid evolution in recent millenia as some tortoise mtDNA is known to mutate very rapidly. Hence, they imply that it is more likely that humans populated the Mascarene and Seychelles islands with tortoises. Specifically, they propose that Austronesian transoceanic colonists, with implied identity to settlers on Madagascar c. 4000 yr bp, distributed tortoises from Madagascar or Africa to Aldabra and the Mascarenes (Mauritius, Reunion, Rodrigues) as a food resource; the granitic Seychelles, oddly, are unmentioned. There are numerous reasons to reject this hypothesis. While Austronesians may have taken chickens and various crops from the Sunda Islands to East Africa and Madagascar (e.g. Boivin et al., 2013; but see Anderson et al., in press), there is no archaeological evidence that they discovered or visited the Mascarenes, granitic Seychelles or the Aldabra group atolls (Blench, 2010; Anderson et al., in press). In addition, the phylogeographical, ecological, evolutionary, palaeontological and archaeological evidence all strongly indicates the contrary. (Resume d'auteur)