“As I was walking along I met two large tortoises, each of which must have weighed at least two hundred pounds: one was eating a piece of cactus, and as I approached, it stared at me and slowly stalked away; the other gave a deep hiss, and drew in its head. These huge reptiles, surrounded by the black lava, the leafless shrubs, and large cacti, seemed to my fancy like some antediluvian animals. The few dull-coloured birds cared no more for me, than they did for the great tortoises.”
On a hot September day in 1835, Charles Darwin met his first giant tortoise on Chatham Island, part of the Galápagos archipelago. After visiting other islands in the archipelago, he came to realize that each island had its own, but slightly different giant tortoise. This was already known by the natives, who could distinguish the tortoises from different islands, but Darwin was struck with wonder (he even brought some tortoises back to Europe as pets). Eventually, his wonder forever changed our understanding of the natural world.
The Galápagos giant tortoise species complex (Chelonoidis nigra) forms an example of an adaptive radiation; a rapid diversification of a lineage when a new food source or ecological niche becomes available. The first giant tortoises are thought to have reached the islands two to three million years ago from South America (Caccone et al., 2002), and subsequently spread through the archipelago as new land emerged from the volcanic sea floor. At one point there were 15 different species (or subspecies, this is a matter of discussion, see Poulakakis et al., 2015). Three of them are now extinct, and scientists are struggling to save the remaining ones.
Together with the Aldabra giant tortoise (Aldabrachelys gigantean) from the Aldabra atoll (part of the Seychelles) in the Indian Ocean, the Galápagos giant tortoises are the only remaining representatives of the giant tortoises that were once widespread across the globe. Although their enormous size has been interpreted as an adaptation to island life, fossil remains of giant tortoises found across the Americas, Eurasia and Africa show that large size was present in continental tortoises as well. The largest of them all was Megalochelys atlas from the Siwaliks Hills in northern India and Pakistan (Falconer & Cautley, 1844), which grew to the size of a small car. Since the Pleistocene era, at least 36 species have gone extinct (Hansen et al., 2010), many of them, especially those on islands, because of human impact.
But how did these gentle giants manage to reach islands across hundreds of kilometers of open sea? Most of the islands they (once) inhabit(ed), such as the Mascarenes and the Canary Islands, are volcanic in origin and have never been connected to the continent by dry land. In contrast to their cousins, the turtles, tortoises are not particularly adept at swimming and it is unlikely that they swam their way across. However, giant tortoises are excellent at bobbing along with their heads held up high by their long neck. In combination with the ability to go without food or water for weeks on end (a feature heavily exploited by early sailors and one that contributed to their demise), the fact that females can store sperm for several years (Pearse & Avise, 2001), and a favorable sea current, the world was their oyster.
In 2004, an Aldabra giant tortoise washed ashore in Tanzania, 740 km from home (Gerlach et al., 2006). Although it was emaciated and covered in goose barnacles, indicating that it had spent at least weeks at sea, it managed to survive the sea crossing. Given this ability for long-distance oceanic dispersal, giant tortoises must have been often among the first large, non-volant vertebrates to colonize islands. In the absence of other herbivores, such as bovids and deer, their presence shaped these isolated ecosystems. For instance, native plants in the Mascarenes developed several adaptations, such as serrated leafs and leafs with red venation, to deter tortoise browsing (Cheke & Hume, 2010).
Despite the evidence from the fossil record, genetics, ecology and observational records for the ability of trans-oceanic dispersal, some researchers have questioned whether the occurrences of giant tortoises on certain islands are natural after all. The giant tortoises of the South West Indian Ocean (Madagascar, the Seychelles and the Mascarenes) are thought to be closely related and share an African origin. Wilmé et al. (2016) argues that the colonization of the Seychelles and the Mascarenes (including Mauritius) in the South West Indian Ocean by giant tortoises from Africa or Madagascar was unlikely to have happened naturally, as present ocean currents in the South West Indian Ocean flow mainly east to west, making it difficult for a non-swimming animal to disperse against the current. They argue it is more likely that early Austronesians that colonized Madagascar from Southeast Asia translocated giant tortoises to islands in the western Indian Ocean ~ 4000 years ago as part of a suite of domesticated animals and crops.
However, many other scientists were quick to point out that there is no evidence that suggests that Austronesians, or any other humans, were involved in the original distribution of giant tortoises to the Mascarene or Seychelles (Hansen et al., 2016; Cheke et al., 2016. While Austronesians may have taken chickens and various crops from the Sunda Islands to East Africa and Madagascar (Boivin et al., 2013), there is no archaeological evidence that they ever visited the Mascarenes or the Seychelles. Moreover, remains of fossil giant tortoises from Mauritius, Reunion and the Seychelles indicate that giant tortoises were present long before humans. Furthermore, the southwestern Indian Ocean is a geologically active region. Particularly during the last million years, sea level, geology and bathymetry have changed dramatically and suggest that past ocean currents differed from the predominant east-west flowing present ones. Transoceanic dispersal from Africa or Madagascar to the smaller islands in the Indian Ocean may therefore have been much more likely in the past then it is today.
References
Boivin, N., et al., 2013. East Africa and Madagascar in the Indian Ocean world. Journal of World Prehistory 26:213–281.
Caccone, A., et al, 2002. Phylogeography and history of giant Galápagos tortoises. Evolution 56:2052–2066.
Cheke, A.S. & Hume J. P., 2008. Lost Land of the Dodo. Yale University Press, New Haven.
Cheke, A.S., et al., 2016, 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). Journal of Biogeography, in press.
Falconer, H. & Cautley, P.T., 1844. Communication on the Colossochelys atlas, a fossil tortoise of enormous size from the Tertiary strata of the Siwalk Hills in the north of India. Proceedings of the Zoological Society of London 1844(12):54–84.
Gerlach, J., et al., 2006. The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40:2403-2408.
Hansen, D.M., et al., 2016. Origins of endemic island tortoises in the western Indian Ocean: a critique of the human-translocation hypothesis. Journal of Biogeography, in press.
Pearse, D. & Avise, J., 2001. Turtle mating systems: behavior, sperm storage, and genetic paternity. Journal of Heredity 92:206–211.
Poulakakis, N., et al., 2015. Description of a New Galapagos Giant Tortoise Species (Chelonoidis; Testudines: Testudinidae) from Cerro Fatal on Santa Cruz Island. PLoS ONE 10(10): e0138779.
Wilmé, L., et al., 2016, Human translocation as an alternative hypothesis to explain the presence of giant tortoises on remote islands in the south-western Indian Ocean. Journal of Biogeography 44: 1–7.
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