The African island of Madagascar, situated some 430 kilometers off the coast of Mozambique, is famous for its unique fauna, particularly its charismatic primates, the lemurs. But how the lemurs and other land mammals got there has proved an enduring mystery. To that end, new evidence supports a theory that some experts once considered unlikely: namely, that the forerunners of Madagascar’s modern mammals reached the island millions of years ago by drifting from the African mainland across the Mozambique Channel on giant rafts of vegetation ripped from the shore and launched out to sea by violent storms.
Reconstructing ancient dispersal routes is a complex exercise. On Madagascar this puzzle is complicated by the fact that the fossil record of mammals from the past 65 million years is meager. Based on the paltry available clues, some researchers thought the ancestral mammalian stock arrived via a landbridge that later disappeared with the shifting of landmasses. But geologic evidence of such landbridges is weak at best. Moreover, this theory cannot account for why the island’s many endemic terrestrial mammal species represent only four of Africa’s broader mammal groups called orders. And all of Madagascar’s land mammals are relatively small—no elephants, lions or giraffes there. If landbridges existed, critics argued, why did only small mammals belonging to these four orders make the trip over?
To account for the shortcomings of the landbridge explanation, paleontologist George Gaylord Simpson proposed the rafting theory in 1940. In the decades that followed, his idea gained credence from studies by molecular biologists, who compared DNA sequences of Madagascar’s modern land mammals and concluded that the founding populations of the four orders colonized the island independently at different times between 60 million and 20 million years ago—exactly the sort of sporadic pattern one would expect if the migrants washed ashore in rare rafting events.
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Still, Simpson’s scenario seemed to have a few leaks. Chief among them was the received wisdom that the ancient currents, presumed to be similar to modern conditions, flowed away from Madagascar, not toward it. If so, then castaways would have drifted back to the mainland or to the north or south, never east toward Madagascar.
But the new work, detailed in the February 4 Nature, indicates that, in fact, prehistoric currents were not the same as they are today (Scientific American is part of Nature Publishing Group). Jason R. Ali of the University of Hong Kong and Matthew Huber of Purdue University used paleogeographic reconstructions and paleo-oceanographic modeling to determine that some 60 million years ago, the African continent and Madagascar both sat about 1,650 kilometers south of their modern-day positions, placing them in a different ocean current system. Back then the surface currents would have flowed from Mozambique and Tanzania east toward Madagascar. During peak periods, their eastward flow could have enabled accidental rafters to reach Madagascar from the mainland in as little as 25 to 30 days.
“The success rate of conveyance across the Mozambique Channel was likely extremely low, with many rafts not making it across and, consequently, the animals on them perishing at sea,” observes David W. Krause of Stony Brook University, who wrote a commentary accompanying the report. “But remember that there were millions of years and many more millions of storms involved.” Furthermore, some animals, including lemurs, can enter into hibernation or another energy-conserving state known as torpor, which could have aided survival on the open ocean.
Ali and Huber’s study “is an excellent piece of work demonstrating probable rafting,” comments Elwyn Simons of Duke University. He adds that many groups of mammals that live on the mainland today were not present during the early phases of colonization of Madagascar, limiting potential migrant diversity.
Whether the newly modeled conditions prevailed long enough to account for all four of the migration events molecular biologists have detected is uncertain, although preliminary data suggest that Madagascar did not move into its present location until some 15 million years ago—well after the last proposed dispersal event. More simulations of ocean currents from different time periods would bolster their case—and hang the landbridge theory out to dry.