The unique fauna of Madagascar, including its celebrated lemurs, reached the island by hitching a lift on natural rafts, scientists have confirmed.
The ancestors of present lemurs, civets, mongooses and rodents, found nowhere else, drifted in on logs carried by currents, according to research that resolves a long-standing enigma about their origin.
Madagascar has been cut off from the African mainland for about 120 million years by the Mozambique Channel, about 300 miles wide. Its isolation has produced idiosyncratic biodiversity. More than 90 per cent of its mammals, reptiles and amphibians are unique, and it has more unique species than any island apart from Australia, which is 13 times larger.
As well as 99 species and subspecies of lemur, which are found only on Madagascar and its surrounding islands, it is home to creatures such as the fossa, a cat-like civet related to mongooses, hedgehog-like tenrecs, and rodents such as the bastard big-footed mouse.
Its land mammals are all small, and belong to just four groups — lemurs, tenrecs, carnivores and rodents — which arrived on the island between 60 million and 20 million years ago.
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This limited range of animal life led the palaeontologist George Gaylord Simpson to suggest 70 years ago that small founder populations from each group reached Madagascar on drifting rafts. The rarity of such events would explain why so few groups of animals populate the island, and the absence of large animals, which would have been too heavy to make such a journey. Simpson’s “sweepstakes hypothesis”, however, has been widely disputed, because African coastal currents would carry away from the island the debris on which animals could ride.
Some scientists have instead proposed that a now-submerged land bridge might intermittently have let animals cross, though there is little geological evidence to support it.
The main drawback of Simpson’s idea has now been resolved by a computer model of ancient currents published today in the journal Nature by Professor Jason Ali, of the University of Hong Kong, and Professor Matthew Huber, of Purdue University in Indiana.
During the period when Madagascar was being populated, from about 60 million years ago, both the island and mainland stood well to the south of their present positions.
That produced ocean conditions unlike those that prevail today, including a current flowing to Madagascar from what is now northern Mozambique and southern Tanzania. As the tectonic plates holding Africa and Madagascar moved north, that current was cut off, about 20 million years ago.
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“All signs point to the Simpson sweepstakes model being correct,” the researchers say. “Currents could have occasionally transported rafts of animals to Madagascar from Africa.”
Anne Yoder, director of the Duke University Lemur Centre, who reviewed the paper for Nature, said the rafting hypothesis had always been most plausible, and that its major flaw had now been addressed. “I was very excited to see this paper,” she said. “Dispersal has been a hypothesis about a mechanism without any actual data. This takes it out of the realm of storytelling and makes it science.”