These iguanas got hooked and ended up in Fiji, 5,000 miles from home

For decades, native iguanas in Fiji and Tonga have presented evolutionary mysteries. From the southwest of the United States to parts of the Caribbean and South America, all other iguana species reside in the Americas. So how did a handful of reptiles end on two islands in the South Pacific, more than 4,970 miles away?

“We've seen a lot of people who have had a lot of fun,” said Simon G. Scalpetta, an evolutionary biologist at the University of San Francisco.

In a study published Monday in Proceedings of the National Academy of Sciences, Dr. Scalpetta and his colleagues allege that the ancestors of the Fijian iguanas crossed on a mat in floating vegetation. Such a voyage across the open ocean, nearly 5,000 miles, is longest known among non-human vertebrates.

Rafting – The term used by scientists to cross tangled oceans of uprooted trees and plants has long been recognized as a way for small land creatures to reach the island. Usually they are invertebrates, and their small size means they can survive a long journey within the trunk of an uprooted tree. Examples of non-fluid vertebrates are relatively rare, he added, but lizards and snakes appear to be able to sniff out further than mammals.

Iguana species have proven to be proficient at creating shorter intersections. In 1995, scientists said they observed at least 15 green iguanas running nearly 200 miles of hurricane fragments from Caribbean islands to another island. And researchers have long agreed that the ancestors of the Galapagos Islands of the Iguanas traveled nearly 600 miles from South America in bobbing the vegetation.

But the intersection to Fiji represents an almost unimaginable challenge. Some researchers who suggested that the Fijian Iguana ancestors also rafted it there said that the reason for their skepticism refers to a vast distance. They rebutted the iguanas as a remnant of an extinct group, possibly crossing regions from America to Asia and Australia, and they countered the relatively easy intersection of Fiji and Tonga.

Dr. Scalpetta's team addressed the question by trying to resolve when the Fijian iguana species (a distinct genus, belonging to the Brachyrophos) split from the closest parent. After the team sampled the genetics of 14 living iguana species from eight genera, the analysis suggested that the closest living relatives of the Fijian species were the genus Dipsosaurus, a group of desert iguanas found in the southwestern and northwestern Mexico.

“Compared to other iguanas, both have relatively thin body shapes,” Dr. Scalpetta said. Team analysis suggests that the two genera were divided approximately 30 million years and 34 million years ago.

The timing is important for several reasons, Dr. Scalpetta said: First, it was around the time the volcano gave birth to the Fijian archipelago. Second, the cold and ice around the poles of that time would have made it impossible for temperature-sensitive iguana strains to arrive from the Americas to Asia and Australia and dive into the Pacific Islands. Also, there is no evidence of iguana fossils anywhere in the eastern hemisphere outside of Fiji or Tonga.

“North America is the most likely home for Fijian iguanas, and overwater rafting is the most supported mechanism,” Dr. Scalpetta said.

The team also claims that their ancestral desert iguanas (whole heat and harsh conditions) were suitable for travel. The 3-4 month intersection would have been almost the length of the winter hibernation of Dipsosaurus. In other words, the lizard may have sailed without starving.

And if herbivorous reptiles rafted the mat of vegetation, Dr. Scalpetta added, “The voyage iguana might even have had food on their journey.”

“In the past, it seemed like this was an unthinkable story that such long-distance dispersal events were not limited solely by imagination.” But while the proposal may seem extraordinary, the team made a very convincing case. After all, other possible origins of Fijan iguanas will require events that may not be as likely as possible, such as the extinction and complete loss of other iguanas along an alternative route.

The study is added to the ever-growing research organization, and Dr. Spencer adds, suggesting that “long-distance dispersion is far more important in the evolutionary history of more animal groups than previously appreciated.”