EVOLUTION
Only four ancestral mammals survived the perilous journey to Madagascar

THE mammals of modern Madagascar appear to be descended from just four ancestral species that survived the long journey from the mainland by clinging to rafts of plant material.

Madagascar split from Africa 165 million years ago, and has been isolated from all other land by deep water for some 88 million years. The 100 or so terrestrial mammal species that live there are unique to the island, but belong to four main groups - carnivores, lemurs, rodents and tiny hedgehog-like insectivores called tenrecs. How they and other terrestrial animals reached the isolated island is "one of the greatest unsolved mysteries of natural history", says palaeontologist David Krause of the State University of New York at Stony Brook.

Now researchers have compared genetic variation of the seven living carnivorous mammal species on Madagascar with carnivores elsewhere. The analysis shows that the Malagasy carnivore species had a single common ancestor whose closest living relative is the African mongoose, says John Flynn of the Reid Museum of Natural History in Chicago, whose team carried out the research.

An earlier genetic study by Anne Yoder of Yale University in New Haven, Connecticut, had shown that lemurs -primates unique to Madagascar - also share a common ancestor. By combining their data, Yoder and Flynn have worked out that the ancestral carnivore must have arrived 18 to 24 million years ago, while the first lemurs turned up between 62 and 66 million years ago, well after the island split from the mainland (Nature, vol 412, p 734). The researchers have yet to date the arrival of the rodents or tenrecs, but Flynn believes that they too are descended from single species.

None of the mammal groups could have swum the 400 kilometres from Africa, so Flynn thinks they must have ridden on large clumps of plant material, as other animals have over shorter distances. Three of the four groups go into hibernation or torpor when food and water are scarce, which would have helped them survive the crossing.

Jeff Hecht