Only in the movies could a lion, a zebra, a giraffe, and a hippo wash ashore onMadagascar to start a new life.
But a new computer model suggests there may be a grain of truth in the animated fiction: The ancestors of ring-tailed lemurs, flying foxes, and other mammals that live on the Indian Ocean island got there aboard natural rafts.
The model supports a 70-year-old theory that mainland mammals from southeastern Africa "rafted" to the island on large logs or floating carpets of vegetation after being swept out to sea during storms.
The ancient refugees were carried to Madagascar by ocean currents, drifting on the open seas for several weeks before finally coming ashore, the model says.
Based on genetic and ecosystem evidence, this theory makes more sense than the alternative, which holds that Madagascar's mammals arrived via a land bridge that was later destroyed by shifting continents.
One of the problems with the rafting theory was that ocean currents and prevailing winds around Madagascar today move east to west—away from, not toward, the island.
Now, using computer simulations normally employed to study global warming, scientists think the currents might have been more favorable for drifitng to Madagascar 50 million years ago.
"The biologists were right all along," said study team member Matthew Huber, a paleoclimatologist at Indiana's Purdue University.
Lemurs on a Log
One weakness of the land-bridge theory is that today only four major groups of mammals live in Madagascar. These animals are distantly related to mammals in mainland Africa, and they are all relatively small.
"If there was a land bridge between Africa and Madagascar, why didn't large animals like elephants or lions cross?" Huber said.
In addition, genetic evidence suggests Madagascar's mammals arrived in discrete waves spaced several million years apart.
Lemurs started the migration about 50 million years ago, followed by hedgehog-like tenrecs, then mongoose-like carnivores such as the fossa, and finally rodents 24 million years ago.
Since rafting fits the biological evidence, Huber and co-author Jason Ali of the University of the Hong Kong wanted to find out if currents around the island might have changed over time.
In fact, 50 million years ago Africa and Madagascar sat about 1,000 miles (1,600 kilometers) south of their current positions due to continental drift, the pair note in the paper, appearing today in the online edition of the journal Nature.
By plugging data about the ocean and atmosphere of ancient Earth into modern climate models, Huber and Ali found that ocean currents around the two land masses once flowed eastward, toward Madagascar, after all.
Rafting Debate Settled?
Huber predicts the 270-mile (430-kilometer) ocean voyage may have taken about three weeks when the currents were flowing at their swiftest.
"The simulation suggests that these very fast currents occurred very rarely, maybe one month every hundred years," Huber said. And small mammals with naturally low metabolisms could have survived for weeks without much food or fresh water, Huber and Ali suggest.
Anne Yoder, director of the Duke University Lemur Center, said she was "very excited" about the new findings.
"Although I am not surprised by the results, I am gratified to see them," said Yoder, who was not involved in the research but who reviewed the study for publication in Nature.
"For me the debate is settled: Madagascar's mammals arrived in Madagascar via ... overwater rafting from Africa."
The new computer simulation might also help solve other biological mysteries, study co-author Huber added.
"We're going to see if we can explain how monkeys made it to South America, because as far as paleontologists can tell, they arrived there sometime during the Eocene"—55.8 to 33.9 million years ago—"when South America wasn't connected to anything," he said.
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