Fiji's iguanas arrived on the island on floating rafts of vegetation that may have also provided a food source along the way, researchers believe. (iStock)
Millions of years ago, seafaring iguanas may have pulled off one of the greatest long-distance migrations the world has ever seen.
That’s according to a study published Monday in The Proceedings of the National Academy of Sciences (PNAS), which suggests iguanas traveled from North America to Fiji on rafts of vegetation during the late Paleogene period (about 34 million years ago). The epic voyage measures about one-fifth of the world’s circumference and is the longest in the “history of terrestrial vertebrates,” researchers of the peer-reviewed study said.
“We found that the Fiji iguanas are most closely related to the North American desert iguanas, something that hadn’t been figured out before,” lead author Simon Scarpetta, an assistant professor at the University of San Francisco, said in a news release, adding that they split from their “sister” lineage around 30 million years ago.
“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,” he said.
The findings could shed new light on how species are dispersed across oceans in a process that has long captivated scientists and is regarded as critical in the establishment of new species of plants and animals. Other animals such as primates, rodents, geckos, skinks and frogs have all been able to cross oceans, with lizards particularly adept “overwater dispersers,” researchers said. However, nailing down the specifics of how such events occurred and the plausibility of animals traveling between continents on rafts has been “vigorously debated,” researchers said, with other theories, such as land bridges, thought to be more likely.
The study was carried out by biologists at the University of San Francisco, the University of California at Berkeley and Villanova University in Pennsylvania, as well as experts from the U.S. Geological Survey and the Fijian nongovernmental organization NatureFiji-MareqetiViti. The team used a phylogenomics dataset, which helps construct an evolutionary history of a species, to compare lineages of 14 different types of iguana, as well as fossil data and geologic records to determine the time frame of the iguanas’ arrival.
The notion of iguanas traveling on rafts has precedent. They are believed to have traveled about 600 miles from Central America to establish a home in the Galápagos Islands, and have traveled around the Caribbean. In 1995, at least 15 green iguanas on a vegetation mat thought to have come from Guadeloupe were seen making landfall on Anguilla, about 170 miles away — resulting in the successful hybridization of a local species of iguana, researchers said.
However, while there are more than 2,100 species in the Iguania group that also includes chameleons, anoles, bearded dragons and horned lizards, these are mostly concentrated in North, Central and South America. The Fiji iguanas are regarded as an “enigmatic” outlier and their origin had not been tested, the research team said. It has been previously thought the two types of iguana found on Fiji — Brachylophus and Lapitiguana iguanas, the latter of which is now extinct — had come either in a “single rafting event” across the Pacific, or via “stepping stone dispersal” across paleocontinent Gondwanaland, yet strong support for either theory was lacking.
“Our analyses support the hypothesis that iguanas reached Fiji via an extraordinary oceanic dispersal event from western North America,” the researchers said. While the team could not entirely rule out that the Brachylophus iguana arrived in Fiji via island-hopping, it said a route from coastal Mexico to the South Pacific “is plausible” based on floating debris and ocean currents at the time.
“That they reached Fiji directly from North America seems crazy,” said co-author Jimmy McGuire, a UC Berkeley professor of integrative biology. “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. 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.”
Iguanas are the ideal candidate for such a voyage as they are herbivores and can go months without food, according to the authors. They may also have been able to snack on the “floating vegetation mats” along the way. The Fijian iguanas were closely related to those found in the North American desert, which are used to searing heat, researchers said.
“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 in the news release.
Denisse Dalgo, a PhD candidate researching evolutionary biology of marine iguanas at the University of Leipzig, was not involved in the research but said the findings are “truly remarkable” given the distance, and prove just how tough iguanas can be. “Their strong swimming ability allows them to stay afloat despite strong currents, while their high stress tolerance helps them survive extreme conditions. They can withstand prolonged periods of starvation by lowering their metabolism, and some — like the Galápagos marine iguanas — can even shrink their bodies temporarily to conserve energy,” she said.
She said further research into how iguanas change their diet and behavior to adapt to a new environment could help provide even deeper insight into island biogeography and evolution.