Iguanas sailed one-fifth of the way around the world on rafts 34 million years ago

Around 34 million years ago, iguanas undertook the longest-known transoceanic trip of any terrestrial species, sailing one-fifth of the way around the world from North America to set up home in Fiji, a new study suggests.

Researchers believe the iguanas made the more than 5,000 mile (8,000 kilometer) journey on rafts made of vegetation, arriving in Fiji shortly after the islands formed. "You could imagine some kind of cyclone knocking over trees where there were a bunch of iguanas and maybe their eggs, and then they caught the ocean currents and rafted over," lead author Simon Scarpetta, lead author and assistant professor of environmental science at the University of San Francisco, said in a statement.

Fiji's bright-green lizards are the only iguanas outside the Western Hemisphere, and how they got there has been a long-standing mystery. In a new genetic analysis published Monday (March 17) in the journal PNAS, researchers found Fiji's iguanas are much more closely related to their Western Hemisphere cousins than previously believed, making the journey directly from the West Coast of the United States to Fiji about 34 million years ago.

"That they reached Fiji directly from North America seems crazy," study co-author Jimmy McGuire, professor of biology at the University of California, Berkeley, said in a statement. "But alternative models involving colonization from adjacent land areas don't really work for the time frame, since we know that they arrived in Fiji within the last 34 million years or so."

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Previously, some biologists posited the Fijian lizards — which comprise the genus Brachylophus — descended from a now-extinct family of iguanas that once populated the Pacific. Others have suggested the lizards could have floated shorter distances from South America and through Antarctica or Australia before finally ending up in the Pacific.

But these ideas were based on past genetic analyses that did not conclusively show how closely Fiji iguanas were related to other iguanids.

The new analysis relies on a genome-wide DNA sequence that Scarpetta collected from over 200 iguana specimens from museums around the world.

The work revealed the Brachylophus genus in Fiji is most closely related to lizards in the Diposaurus genus, which are widespread in the deserts of North America. These desert iguanas are well adapted to searing heat, so potentially had adaptations to survive the long journey.

"Iguanas and desert iguanas, in particular, are resistant to starvation and dehydration, so my thought process is, if there had to be any group of vertebrate or any group of lizard that really could make an 8,000 kilometer journey across the Pacific on a mass of vegetation, a desert iguana-like ancestor would be the one," Scarpetta said.

The researchers estimate these lineages split approximately 34 million years ago — roughly aligning with geological history of the islands' formation. "This suggests that as soon as land appeared where Fiji now resides, these iguanas may have colonized it. Regardless of the actual timing of dispersal, the event itself was spectacular," Scarpetta said.