Watch a video about how iguanas floated 1/5 of the way around the world to colonize Fiji. Thumbnail image via Bjørn Christian Tørrissen/ Wikipedia (CC BY-SA 4.0)

For many years, scientists have wondered where the iguanas that inhabit the remote and isolated islands of Fiji and Tonga came from. Finally, a team of researchers from the University of California, Berkeley, and the University of San Francisco (USF) said on March 17, 2025, they have an answer. These reptiles likely arrived on the islands by rafting from western North America. This means the iguanas traveled 5,000 miles (8,000 km) on natural rafts across the Pacific Ocean.

To solve the mystery, the researchers analyzed the DNA of more than 200 iguana specimens from museums around the world. They also discovered that the iguanas arrived on the islands about 34 million years ago, either immediately after the islands’ formation, or shortly afterward. The scientists published their study in the peer-reviewed journal Proceedings of the National Academy of Sciences on March 17, 2025.

Simon Scarpetta, the study’s lead author, is a herpetologist and paleontologist, former postdoctoral fellow at UC Berkeley, and current assistant professor in the USF Department of Environmental Sciences.

Fiji and Tonga iguanas broke a record

Iguanas are fascinating animals: They can change color, detach their tails, have a third eye on top of their heads, know how to swim and can dive for 30 minutes. But traveling 5,000 miles (8,000 km) from the west coast of North America to these distant islands is a big deal.

The four species that inhabit the islands of Fiji and Tonga have earned the well-deserved record for the longest known transoceanic dispersal of any non-human terrestrial vertebrate. These iguanas belong to the genus Brachylophus.

Although iguanas commonly float on natural rafts made of fallen trees and plants – and transport themselves using this system – making such a long journey seemed impossible. Jimmy McGuire, co-author of the study and professor of integrative biology and herpetology curator at the Museum of Vertebrate Zoology, said:

That they reached Fiji directly from North America seems crazy.

There are 45 species of Iguanidae that live in the Caribbean and the tropical, subtropical and desert regions of North, Central and South America. Therefore, scientists looked for the origin of the Brachylophus genus in nearer locations. Central and South America seemed more likely options than North America.

The mysterious origin of Brachylophus iguanas

Seeing iguanas floating on rafts in the Caribbean is a common sight. In fact, this is what happened centuries ago, when they embarked on a 600-mile (970-km) journey from Central America to colonize the Galapagos Islands.

Scientists hypothesized that, if this had occurred previously, the iguanas could have continued their journey further to reach Fiji and Tonga from the western Pacific. Researchers also proposed the idea that they could have arrived from tropical South America, via Antarctica or Australia. However, there is no genetic or fossil evidence to support these hypotheses.

According to McGuire:

When you don’t really know where Brachylophus fits at the base of the tree, then where they came from can also be almost anywhere. So it was much easier to imagine that Brachylophus originated from South America, since we already have marine and land iguanas in the Galapagos that almost certainly dispersed to the islands from the mainland.

Their origin confirmed!

Previous genetic analyses of some iguanid lizard genes were inconclusive about the relationship of Fiji and Tonga iguanas to the rest. A few years ago, during his postdoctoral studies, lead author Simon Scarpetta began a detailed investigation of all Iguania genera with the goal of clarifying the group’s family tree. McGuire explained that:

Different relationships have been inferred in these various analyses, none with particularly strong support. So there was still this uncertainty about where Brachylophus really fits within the iguanid phylogeny. Simon’s data really nailed this thing.

Scarpetta compiled DNA from genomic sequences of more than 4,000 genes and from tissues of more than 200 iguana specimens found in museum collections around the world. When comparing these data, one result stood out clearly: Fiji and Tonga iguanas are closely related to iguanas of the genus Dipsosaurus.

The most widespread of this genus is the North American desert iguana, Dipsosaurus dorsalis, adapted to the scorching heat of the deserts of the southwestern United States and northern Mexico. Scarpetta stated that:

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.

The origin of the islands and their colonization

In addition to demonstrating that Brachylophus iguanas did indeed arrive from North America, the scientists also established that they reached Fiji and Tonga around 34 million years ago. They rejected alternative models involving colonization from adjacent lands because they didn’t correspond with this period of time.

In fact, biologists had previously proposed that Fijian and Tongan iguanas could have descended from an older, more widespread lineage in the Pacific (now extinct). However, the dates did not match.

This exhaustive analysis also explains when the genetic divergence of Brachylophus iguanas from their closest relatives, the North American desert iguanas, Dipsosaurus, occurred. The study suggests that Brachylophus iguanas may have even colonized the volcanic islands of Fiji and Tonga as soon as land emerged (34 million years ago) or shortly after their formation, thus diverging from Dipsosaurus iguanas. According to Scarpetta:

We found that the Fiji iguanas are most closely related to the North American desert iguanas, something that hadn’t been figured out before, and that the lineage of Fiji iguanas split from their sister lineage relatively recently, much closer to 30 million years ago, either post-dating or at about the same time that there was volcanic activity that could have produced land.

How did they get to the islands?

Despite being very resilient creatures, it’s still surprising how they were able to undertake this adventure. Dispersal over water is the main way newly formed islands are populated with plants and animals.

And this is quite impressive. Let’s imagine the situation … A modern-day sailor using the wind to reach Fiji from California would need about a month to get there. Can you imagine how long it would take the iguanas floating on a raft?

Fortunately, iguanas are accustomed to going long periods of time without food or water. On the other hand, the rafts they traveled on were likely made of fallen trees and other plants. Fortunately, iguanas are herbivores, and the raft itself would have provided them with food.

The dispersal of animals often leads to the evolution of new species and entirely new ecosystems. Other islands besides Fiji and Tonga may have also hosted iguanas, but volcanic islands tend to disappear as easily as they appear. Evidence of other Pacific Island iguanas, if they existed, has likely been lost. So, Fiji’s iguanas are an outlier, lying alone in the middle of the Pacific.

Unfortunately, all four species from Fiji and Tonga are listed as critically endangered. This is primarily due to habitat loss and exploitation by smugglers who fuel the exotic pet trade.

Bottom line: Iguanas are incredible reptiles that can live without food or water for long periods of time. This allowed them to travel 5,000 miles from North America to Fiji and Tonga and conquer the islands.

Via University of California, Berkeley

Source: Proceedings of the National Academy of Sciences

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