Madagascar’s fabulously improbable wildlife, from gremlin-like aye-ayes to satanic leaf-tailed geckos, may be thanks to dozens of dramatic oceanic journeys that would put Robinson Crusoe to shame, new research says.

“It seems like a far-fetched idea that animals could survive drifting across the sea, because it’s hard enough for humans to survive that, let alone animals,” says Matthew Borths, a curator of fossils at Lemur Center at Duke University.

A juvenile Labord’s chameleon navigates a dry forest in Madagascar. The reptile lives less than five months, the shortest life span of a four-legged vertebrate. 
PHOTOGRAPH BY CHRISTIAN ZIEGLER, NAT GEO IMAGE COLLECTION

But a comparison of genetic data from modern Malagasy species with the fossil record of their ancestors from the African mainland has revealed that this is likely what happened for most land vertebrates, according to the research, published in May in the journal Biological Reviews.

Reptiles, amphibians, and mammals from mainland Africa would have been stranded on giant rafts of vegetation and floated to Madagascar, where they eventually evolved into the wildlife we know today. Some 95 percent of mammals and 98 percent of reptiles are endemic to the country, which means they live nowhere else in the world.

While it seems improbable for animals to survive the roughly 30 to 35 days it would have taken to get across the Mozambique Channel, the vegetation may have held fruits or other food sources, as well as trapped rain to keep the animals alive.

“With geological time, something which is statistically unlikely or highly unlikely becomes a certainty. If you keep throwing the dice for five million years, eventually you’re going to come up with 10 sixes,” says study co-author Jason Ali, a geophysicist at the University of Hong Kong.

An empty fossil record

Scientists have three major theories to explain how land animals got to Madagascar: The species were there before Madagascar split off and became an island, they swam and/or rafted across from mainland Africa when currents flowed that way, or they crossed land bridges that might have existed at different periods.

The trouble is, the fossil record in Madagascar between the time of the dinosaurs and about 2,000 years ago is nearly nonexistent, leaving a lot of room for debate.

The aye-aye uses its long middle finger to access grubs deep inside trees.
PHOTOGRAPHY BY FRANS LANTING, NAT GEO IMAGE COLLECTION

In their new study, Ali and Blair Hedges, an evolutionary biologist at Temple University in Philadelphia, pulled together a large amount of previously published research to test each of these possible origin stories.

The team compared the genes of 28 clades of land vertebrates, including reptiles, amphibians, and mammals from modern-day Madagascar with the genes of species on mainland Africa that are evolutionarily the closest relatives. Based on this information, they calculated the likely intervals when the ancestor of each species group likely arrived in Madagascar, then evolved into something new. 

Survival stories

The analysis revealed that a handful of the species found today on Madagascar have been there for at least as long as it’s been an island, which at least partially supports the first theory. That means the animals came along when Madagascar split off from the Indian subcontinent, along with Seychelles, more than 81 million years ago.

Species like the critically endangered Madagascar big-headed turtle and several species of worm-like blind snakes probably descended from this time and were among a select few that survived the extinction event that wiped out the dinosaurs 66 million years ago, Ali says.

But most of the rest of the reptiles, mammals, and amphibians that remain today are descended from relatively small animals that rafted from the mainland.

The ancestors of modern lemurs are already thought to have been quite small—something like modern mouse lemurs. Other Malagasy species, like the predatory fossa, also had comparatively small ancestors while rodents and the only other native Malagasy mammal group—the hedgehog-like tenrecs—remain relatively small today. (Read the surprising reason some lemurs grow their own gardens.)

Reptiles such as tortoises, which are very hardy, may have even floated across the Mozambique Channel without a raft, Ali says.

The cat-like fossa is the largest mammal predator on Madagascar.

Rafts more likely than bridges

Borths, who was not involved in the new study, says the idea of animals walking to Madagascar via land bridges has gotten traction in recent years—notably through the work of the late Judith Masters and Fabien Genin.

But the new study’s models ruled out the possibility that animals crossed via land bridges. Even though Ali and Hedges only have estimated windows for when the ancestors of the living species arrived on the island, these are different enough from each other to appear random over time. If land bridges had opened at some period, many species would have crossed in surges, which would show up in the genetic record, Ali says.

Borths agrees the new study likely puts this the land bridge theory to rest with a “powerful set of models.”

He says rafting is not unique to Madagascar, either: A number of animals made the far longer crossing from Africa to South America via raft, he says. And rafts can be quite huge—he points to a video of a massive clump of vegetation floating down the Panama Canal, complete with upright trees. “A monkey could have totally been on that thing,” Borths says.

“The diversity that we see on the island is actually a product of the randomness of Madagascar,” he adds, “and organisms figuring where they sit in this ecosystem.”