Human translocation as an alternative hypothesis to explain the presence of giant tortoises on remote islands in the south‐western Indian Ocean

Abstract

Giant tortoises are known from several remote islands in the Indian Ocean (IO). Our present understanding of ocean circulation patterns, the age of the islands, and the life history traits of giant tortoises makes it difficult to comprehend how these animals arrived on such small, remote and geologically young (8–1.5 Ma) landmasses. For colonization to have occurred by dispersal, giant tortoises must either have originated in Madagascar or Africa and swum for hundreds of km against the ocean currents, or have launched themselves from the eastern IO margin and drifted with the currents over several thousands of km of open ocean. After these navigational feats, the tortoises would have needed to found new, viable populations on potentially inhospitable volcanic or coral outcrops. Geologically recent sea level changes are likely to have eliminated terrestrial life from islands like Aldabra, complicating the scenario. We reviewed information relating to IO geology, the evolution and ecology of giant tortoises, and the spread of humans within the region, and propose an alternative explanation: we posit that giant tortoises were introduced to the IO islands by early Austronesian sailors, possibly to establish provisioning stations for their journeys, just as European sailors did in more recent historical times.

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)

“…Wilmé et al . () did not discuss the granitic Seychelles. The giant tortoises there are conspecific with those on Aldabra, with minor differences (Austin et al ., ; Gerlach, ).…”

“…Given favourable currents, it is unclear why Wilmé et al . () considered sea drift an acceptable process in the mid‐Pleistocene but not viable at its end.…”

“…4000 years as proposed by Wilmé et al . () would require a rate 250 times faster, an unprecedented rate for vertebrates. Their claim of possible ultra‐rapid mtDNA evolution is based on a misinterpretation of Caccone et al .…”

“…Although Wilmé et al . () acknowledged that a living tortoise drifted from Aldabra to Tanzania in 2004 (Gerlach et al ., ), they dismissed sea drift as responsible for tortoises reaching the Mascarenes or leaving Madagascar, arguing that ocean currents are unfavourable. Although true at present for the Mascarenes, prevailing currents in the Miocene would have favoured such movements, and they remain favourable for Aldabra (Gourlan et al ., ; Gerlach & Rioux Paquette, ).…”

“…Giant tortoises have fascinated Indian Ocean travellers since their first discovery on Mauritius by the Dutch in 1598 (Cheke & Bour, ), and a Réunion tortoise was among the earliest biological specimens to reach Europe from the Malagasy region (in 1671: Bour, ), but Wilmé et al . () 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. …”