by Greg Mayer
Dennis Hansen, our Aldabra correspondent, sent Jerry a very nice video of a swimming Aldabra giant tortoise (Aldabrachelys gigantea). This immediately brought to mind what is, in my view, one of the most important recent papers in biogeography, “The first substantiated case of trans-oceanic tortoise dispersal” by Justin Gerlach, Catharine Muir, and Matthew Richmond.
In the paper, they report an Aldabra tortoise that came ashore on a beach in Kimbiji, Tanzania in 2004. After considering several possibilities, they conclude that the tortoise had floated in from Aldabra, over 700 km away across the Indian Ocean. The copious growth of large barnacles on the limbs and lower parts of the carapace certainly suggested that the tortoise had spent a considerable amount of time in the sea.
Dennis’s video shows that the tortoises enter water, and how they move about in the shallows. The Kimbiji tortoise, despite the ability to swim, could not have swum to the main, but was rather mostly carried by the currents, and presumably spent its time at sea keeping its head, and most of the dome of its carapace, above water. It was, however, walking ashore, apparently intentionally, when found.
So why is this important? It has long been supposed that animals and plants get to oceanic islands by what Darwin called “occasional means of transport“: carried along on logs, masses of vegetation, ice floes, attached to birds, or floating by themselves in the water. (Darwin carried out a series of experiments on the ability of various plant seeds to float in sea water, and their ability to germinate after varying periods of immersion.) Tortoises have usually been thought to float by themselves because of the difficulty they would have in clinging to vegetation, and also because the practice of mariners of earlier centuries of putting giant tortoises in the holds of their ships as a living food supply had shown that tortoises could live for many months without food or water.
Although Darwin and many subsequent zoogeographers (e.g., P.J. Darlington) invoked such occasional means of transport, there has always been a school of thought arguing that such crossings of the ocean by land animals were nigh impossible, and that the presence of non-flying land animals on an island implied a past land connection. In the first half of the 20th century, this school constructed speculative land bridges crisscrossing the oceans, in order for every island animal to have had a dry-shod passage to the island from its home of origin. In the later 20th century, with the development of plate tectonics, the land bridge builders were succeeded by drift enthusiasts, who thought drifting crustal plates could serve to bring oceanic islands into juxtaposition with continents, so that, again, animals might get to islands without having to cross water (or at least not much). (The drift enthusiasts had the advantage that continental drift actually does occur, even if not in the exact plate configurations they hoped for, whereas the land bridge builders’ long, thin isthmuses crossing abyssal oceans have not been borne out by geology.) So, the more or less direct observation of transoceanic crossing by an occasional means of transport provides a crucial link—a vera causa—in the argument for the occurrence and importance of such means in the colonization of islands.
Some younger biologists, raised (and properly so!) on plate tectonics, and perhaps lacking acquaintance with older literature and island organisms in the field, had taken the drift enthusiasts’ claims too much to heart, and seemed to be unaware of the importance of transoceanic dispersal. Now that molecular phylogenies often bolster the argument for the importance of occasional means of transport, they seem a bit surprised to find out that there indeed has been a lot of ocean crossing, not just to oceanic islands, but between continents and continental islands as well. We discussed one such case here on WEIT, the ratite birds, where what had seemed to actually be a good case for continental drift seems to actually involve a fair amount of oceanic crossing. The zoologist Alan de Queiroz has written a popular book on the biology of oceanic dispersal, and the sociology of its rediscovery by some biologists.
de Queiroz, A. 2014. The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life. Basic Books, New York.
Gerlach, J, C. Muir and M.D. Richmond. 2006 The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41–43): 2403–2408. pdf