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Raft-less transoceanic dispersal by a giant tortoise

The following image shows an Aldabra Giant Tortoise shortly after it washed ashore in Kimbiji, Tanzania.

The growth of barnacles on the animal is striking. The image was published by Gurlach, Muir, and Richmond (2006) as part of their account and analysis of the occurrence.

In December 2004 an Aldabra giant tortoise Dipsochelys dussumieri was washed ashore on the coast of east Africa, probably having been carried off the shore of Aldabra atoll, 740 km away. Although trans-oceanic dispersal is assumed to be the mechanism by which tortoises and many other animals became established on islands throughout the world, this is the first direct evidence of a tortoise surviving such a sea-crossing.

Although there are several populations of D. dussumieri on less distant islands, the authors state that shell morphology suggests the Kimbiji tortoise originated on Grand Terre (South Island), Aldabra, and not from a closer site. Preliminary data on barnacle settlement and growth rate suggests 6 to 7 weeks since first exposure. The following map (also from Gurlach, Muir, and Richmond 2006) shows relevant locations and oceanic currents.

Note that Dipsochelys are known from the Comoros and Seychelles islands, and Madagascar, but not the mainland. Also note that D. dussumieri has been found in open water previously (see Gurlach, Muir, and Richmond 2006 for details).

The authors draw attention to two interesting conclusions allowed by their brief paper: (1) that giant tortoises are capable of surviving transoceanic crossings without the aid of a raft object, and (2) that this occurrence is the inverse of the mainland-to-island rafting mechanism central to the island biogeography of terrestrial vertebrates.

Gerlach J, Muir C, Richmond MD. 2006. The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41-43): 2403-2408.