A yellow and black spotted frog (Eleutherodactylus counouspeus) from caves on the Caribbean island of Hispaniola.

Credit: S.Blair Hedges

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SYDNEY: New DNA evidence suggests South American frogs travelled over the sea, trapped on mats of floating vegetation, to populate Central America and the Caribbean Islands.

"They just happened to end up there during a storm," said S. Blair Hedges, an evolutionary biologist at Pennsylvania State University in Philadelphia, USA.

This challenges the notion that major frog faunas of the region migrated across land bridges that may have existed around 70 million years ago.

The find, reported this week in the U.S. journal Proceedings of the National Academy of Sciences, could help researchers untangle the confusing evolutionary relationships between frogs in the New World tropics – which are home to half of all amphibian species.

Confusing family tree

Scientists have long debated the evolutionary history of frogs found in the region. To discover more, evolutionary biologists led by Hedges sequenced the DNA of 277 species of eleutherodactyline frogs.

Eleutherodactyline frogs – which make up almost a third of tropical frogs in the Americas – are tiny, reaching a maximum length of around five centimetres. Unlike many other frogs, they reproduce on land with leathery-shelled eggs, sidestepping the tadpole stage. These eggs hatch to produce miniature, fully-formed frogs.

The team compared the DNA of the frogs to discover that they formed three major groups, found in South America, Central America and the Caribbean. They further analysed the data using a method called the 'molecular clock', which allows researchers to predict when several species might have diverged from one another by estimating how much their DNA has changed over time.

Using the molecular clock, Blair and his team estimated that frogs from South America migrated to populate the Caribbean Islands and Central America between 47 and 29 million years ago – a period when these regions were not connected by land bridges, as they were 70 million years ago.

Froginson Crusoe?

Even though experts have been resistant to the idea of ocean dispersal – mainly because frogs were thought too fragile to survive the journey – Blair's team now argue that this is the most likely possibility.

Many species are thought to have populated islands by getting trapped on mats of floating vegetation or "flotsam", said Hedges.

"During storms and hurricanes, dead trees and vegetation get washed into rivers, say in South America, and get carried out into the ocean currents where most disappear," he said. "Rarely, one of these flotsam 'islands' will make it hundreds of miles to another place and land on a beach with its passengers, which could include frogs."

Other animals known to migrate in this way include those of the Hawaiian and Galapagos Islands. Hedges says that "any island that arose in the ocean from a volcano and has animals is evidence" of over-water dispersal.

"The new work is significant," commented Michael Tyler, an evolutionary biologist and frog expert at the University of Adelaide in Australia. "We have argued about the origins of the South American fauna for many years."