North American iguanas rafted 5,000 miles to colonize Fiji: Study
A subset of North American iguanas likely landed on an isolated group of South Pacific islands about 34 million years ago — having rafted some 5,000 miles from the West Coast of the faraway continent, a new study has found.
Their epic journey to what is now Fiji marks the longest known transoceanic expansion of any terrestrial vertebrate species, according to the study, published Monday in the Proceedings of the National Academy of Sciences.
Overwater travel for both animals and plants isn't unique in and of itself, as this is the primary way in which newly formed islands have become populated by plants and animals, the study authors noted.
Such travel has often led to the evolution of new species and ecosystems — a phenomenon that has fascinated scientists since the time of Charles Darwin.
Iguanas themselves are also known for their propensity to float elsewhere and are often found rafting around the Caribbean aboard plant life, the researchers explained.
And while the scientists were already aware that the lizards traveled 600 miles to get from Central American to the Galapagos Islands, the massive 5,000-mile trip to Fiji took them by surprise.
"That they reached Fiji directly from North America seems crazy," senior author Jimmy McGuire, a professor of integrative biology at the University of California, Berkeley said in a statement.
"As soon as land appeared where Fiji now resides, these iguanas may have colonized it," added McGuire, who is also a herpetology curator at the Museum of Vertebrate Zoology.
Previously, biologists had assumed that Fiji iguanas — four species from the genus Brachylophus — descended from an older lineage that was at one point common around the Pacific but ultimately died out. Another possibility was the idea that the lizards "hitchhiked" from South America and then through Antarctica or Australia.
Nonethless, the researchers found there was no genetic or fossil evidence to support those theories.
To draw their conclusions, the researchers acquired genome-wide, sequence DNA from more than 4,000 genes and from the tissues of more than 200 iguana specimens in global museum collections.
As they began comparing the data to that of the Fiji iguanas, the scientists found that these animals were most closely related to iguanas in the genus Dipsosaurus. Within that genus, the most widespread lizard is the North American desert iguana, Dipsosaurus dorsalis, which has adapted to the desert heat of the U.S. Southwest and Northern Mexico, according to the study.
"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," lead author Simon Scarpetta, a former postdoctoral fellow at UC Berkeley.
The findings also indicated a much more recent emergence on Fiji — up to 34 million years ago — than the previously suggested arrival times of either 43 million years ago or up to 60 million years ago.
"The lineage of Fiji iguanas split from their sister lineage relatively recently, much closer to 30 million years ago," added Scarpetta, who is now an assistant professor in environmental science at University of San Francisco.
That appearance in Fiji would have either postdated or occurred "at about the same time that there was volcanic activity that could have produced land," Scarpetta said.
The four species of Fiji iguanas, which inhabit both Fiji and Tonga, are listed as endangered — due to habitat loss, predation by invasive rats and smuggling for exotic pet trades, the authors noted.
McGuire acknowledged it may have been "easier to imagine that Brachylophus originated from South America," since marine and land iguanas dispersed to the Galapagos from the mainland. But this theory can now be ruled out, he and his colleagues confirmed.
Although the scientists could not pinpoint the exact moment and circumstances under which the iguanas arrived in Fiji, they marveled at the prospect of the journey.
"Regardless of the actual timing of dispersal, the event itself was spectacular," McGuire added.