How did iguanas get to Fiji? Chances are, they floated nearly 5,000 miles on vegetation rafts

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Around 34 million years ago, the ancestors of modern iguanas likely embarked on what may be the longest overwater journey undertaken by a nonhuman, land-dwelling vertebrate species.

Starting off the epic trek from the western coast of North America, these iguanas traveled nearly 5,000 miles (8,000 kilometers) — one-fifth of the Earth’s circumference — across the Pacific Ocean, eventually arriving in Fiji, according to a new study.

Using genetic evidence, researchers propose that these iguanas made the extraordinary voyage by rafting on floating vegetation, possibly composed of uprooted trees or plants.

For decades, scientists have debated how Fiji’s iguanas arrived. Previous theories suggested that an extinct species of iguana rafted from the Americas without a clear timeline, while others proposed that the lizards migrated overland from Asia or Australia, said lead study author Dr. Simon Scarpetta, an assistant professor at the University of San Francisco. Scarpetta conducted this research during his National Science Foundation postdoctoral fellowship at the University of California, Berkeley, and in his current role.

The findings, published Monday in the journal Proceedings of the National Academy of Sciences, help clarify the long-standing mystery of how these reptiles reached such remote islands.

Scarpetta and his team aimed to test both the overwater rafting and overland theories, as well as other hypotheses for the biogeographic origin of Fijian iguanas, including dispersal through Antarctica or across the Bering land bridge.

Understanding this type of water dispersal could offer new insights into how other species have colonized isolated areas over time, Scarpetta added.

Tracing genetics

Iguanas have already demonstrated an ability to survive long-distance ocean travel, according to an October 1998 study. At least 15 green iguanas appeared on the beaches of Anguilla in the Caribbean in 1995 on rafts of uprooted trees. Researchers determined the lizards likely floated nearly 200 miles (322 kilometers) from Guadeloupe following a hurricane event.

Scarpetta noted that this type of overwater rafting is often described as “sweepstakes” dispersal, a rare event that allows a species to colonize an otherwise unreachable area. Major weather events, such as hurricanes or floods, can dislodge vegetation and carry animals along with it.

To determine when iguanas arrived in Fiji, researchers analyzed the genes of 14 living iguana species. The team found that the closest living relative of Fijian iguanas is the Dipsosaurus — a type of desert iguana native to the southwestern United States and northwestern Mexico, according to the new study.

Fossil evidence further supports the idea that these iguanas originated in North America, as no fossils of desert iguanas have been found elsewhere in the world, Scarpetta said.

The analysis also suggests that the Fijian iguanas diverged from their American ancestors between 34 million and 30 million years ago, settling around the same time as the volcanic formation of the Fijian archipelago, Scarpetta said.

This timeline challenges previous theories that iguanas may have made a complex overland journey from South America via Antarctica, which would have happened much later in history, said study coauthor Dr. Jimmy McGuire, a professor of integrative biology at University of California, Berkeley.

“In phylogenetic analyses there is always some degree of uncertainty when trying to predict the timing of divergence events between species,” said Dr. Shane Campbell-Staton, an associate professor of ecology and evolutionary biology at Princeton University, who was not involved in the study. “In this case, the authors were very thorough in collecting multiple different kinds of genetic data and use(d) multiple different models … to test their hypothesis and find that most of the results are largely in agreement.”

How was the trek possible?

While a monthslong voyage across the Pacific may seem impossible, iguanas are surprisingly well-equipped for such extreme travel.

“If you had to pick a vertebrate group that could survive a rafting event across thousands of kilometers of open ocean, iguanas are a great choice,” Scarpetta said.

Many iguana species, especially those in desert environments, can endure extreme heat, starvation and dehydration, Scarpetta noted.

“Being ectothermic means that you don’t expend many of your food resources or fat reserves maintaining an elevated body temperature,” McGuire said, referring to the metabolic rate of cold-blooded animals. “Ectotherms can be about 25 times more efficient in this regard than endotherms (warm-blooded animals), so they don’t need to eat nearly as much food nor nearly so frequently.”

Some estimates suggest that an overwater journey from North America to Fiji could have taken anywhere from four to 12 months. However, newer simulations indicate the trip may have been closer to 2 ½ to four months, Scarpetta said.

Despite basic survival challenges, food scarcity likely wouldn’t have been a major one. If necessary, the herbivorous reptiles may have been able to feed on their floating vegetation mats, McGuire said.

Scientists said they hope that by studying dispersal events, they may be able to predict which species can survive long-distance overwater travel, offering new insights into how animals spread across the globe.

“We now know that overwater dispersal is not only possible, but it has likely played a large role in shaping species diversity on islands around the world,” Campbell-Staton said via email.

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