FOR the past century and a half, the Seychelles giant tortoise was thought to be extinct. Then came a dramatic announcement in 1997. Ten specimens had been found alive, a discovery that sparked a frenetic breeding programme.
Now there is another twist to the tale of this ancient behemoth. A genetic analysis of the giant tortoises shows they may not be what they seem. Rather than being the few survivors of a long-lost population, the Seychelles tortoises may simply be members of another extant species. They just sport different-shaped shells.
In the 18th and 19th centuries, giant tortoises provided a supply of fresh meat for European sailors arriving at the Seychelles, a group of 115 islands in the Indian Ocean.
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From the shapes of their shells, zoologists believe that six species of the tortoise genus Dipsochelys originally lived in the region. Until recently, it was thought pillaging sailors had destroyed all but one species by the 1840s. After appeals by Charles Darwin among others, D. dussumieri was preserved on Aldabra, a geologically distinct atoll within the archipelago (see Map). Some were transported to the Seychelles as pets, where they and their descendants live today.
But in 1998, Justin Gerlach of the University Museum of Zoology Cambridge stunned zoologists by announcing that 10 captive tortoises on the Seychelles actually belonged to two native species that were supposedly extinct, D. arnoldi and D. hololissa. Their shell shapes 鈥� which are slightly different to the Aldabran giant tortoise鈥檚 鈥� and an initial DNA study, backed this conclusion (Chelonian Conservation and Biology, vol 3, p 133).
Gerlach asked Eric Palkovacs and his team at Yale University to carry out a more complete genetic analysis. They examined 55 tortoises, representing all three supposed species. All share an almost identical mitochondrial DNA sequence that should change under natural selection as species diverge. That suggests all descend from the Aldabra giant tortoise (Molecular Ecology, vol 12, p 1403).
Palkovacs says the most likely explanation is that the Seychelles species are extinct. External factors such as diet could cause the different shell shapes, he says.
But although Gerlach is a co-author on the latest paper, he still believes the Seychelles giant tortoises have survived. He argues the species might have diverged so recently 鈥� perhaps 7000 years ago 鈥� that the mitochondrial sequences remain the same while other genetic regions changed.
Late last year, eggs from the two purported Seychelles species produced hatchlings that had characteristic shell shapes which became more different over time, despite both species sharing the same environment. That strongly suggests the distinction between the presumed species is real, says Gerlach, who will soon publish the details in a peer-reviewed journal. 鈥淪ometimes I look at these data and think, 鈥楬ave I got that right?鈥� Then I look at the tortoises again. You can鈥檛 possibly think they鈥檙e not different species,鈥� he says.
Work at Yale may lay the matter to rest. Palkovacs is testing DNA from the remains of Seychelles tortoises that died before the European sailors arrived. He will compare it to DNA from the living tortoises, which could establish their origin once and for all.