At the 14. December 2004 an Aldabra giant tortoise landed on the coast of Tanzania – a species that actually lives in the Seychelles in the middle of the Indian Ocean, almost 2000 Kilometers away. “The emaciated but apparently healthy animal has to be driven in the Indian Ocean for a few months,” says Uwe Fritz from the Museum of Animal Science of the Senckenberg Natural History Collections in Dresden: barnacles settled on the legs and on the shell during the long journey across the sea .
This extraordinary ability of turtles to survive for a long time without eating and drinking is probably the reason why turtles but hardly any mammals live on some remote islands. The giant tortoises that lived on the Mascarene Islands of Reunion, Mauritius and Rodrigues until about 200 years ago originally came from far away.
About 40 millions of years ago, a female must have taken the reverse path of the Aldabra giant tortoise and driven across Africa. This has now been shown by the analyzes of genetic residues from 19 animals, carried out by Senckenberg researchers Christian Kehlmaier and Uwe Fritz.
Enigmatic trip across the ocean
“It was a huge surprise for all of us,” says Walter Joyce from the University of Freiburg in Switzerland, who participated in the study, published in the journal “Scientific Reports ”was involved. Because the Maskarenes were created only ten million years ago. So how can this be?
So far, researchers had assumed that the “Cylindraspis” giant tortoises of the Mascarene Islands had only reached the islands from Madagascar, Africa or Asia a few million years ago. There were even speculations that the animals had only been brought there by humans. However, the genetic analysis by Uwe Fritz and his colleagues clearly contradicts such an assumption: The genetic makeup of the mascarene species differs greatly from all other tortoises and has been around for about 40 Have lived separately from them for millions of years. But where was the original home of these animals, when the oldest Mascarene island of Mauritius only emerged from the sea less than ten million years ago?
A volcano that produces islands like clockwork
Fritz, Kohlmeier and Joyce see the solution to the riddle in the volcanic hotspot that created the Mascarenes. This is what geophysicists call a certain form of volcanism, in which powerful columns of more than a hundred kilometers of very viscous rock rise slowly from a few thousand kilometers at speeds of a few centimeters a year. “Such a hotspot obviously broke out around 66 millions of years ago in what is now western India,” explains Walter Joyce. While the Indian continental plate continued to move towards Asia and 40 millions of years ago began to unfold the Himalayas, the hotspot remained in place. The Indian Ocean sloshed over him, but the lava from the hotspot soon towered so high that an island emerged from the sea. “A tortoise might have washed up on such a hotspot volcanic island,” says Uwe Fritz.
According to Kehlmeier's genetic analysis, it came from the African continent. And it must have been a female – because that would be enough to start a new population. “Tortoise females often store the seeds of several males and only fertilize their eggs long after mating,” says Fritz.
Survived millions of years – until humans came
The island on which this female ran aground no longer exists. Over the course of millions of years, the ocean plate has moved hundreds of kilometers. A new volcano, a new island, is created there, while waves and tropical storms slowly erode the old island until it disappears into the sea.
But before that, there was a female turtle from old to new Volcanic island has been washed over. After some “island hopping” and splitting into different subspecies, the animals finally reached the three still existing Mascarene Islands and survived – until humans came and wiped them out about 200 years ago ,