EXPERT REACTION: Iguanas rafted more than 8,000 km from North America to Fiji

Publicly released:
Pacific; International
Fijian banded iguana via Wikimedia Commons (CC-BY-SA 2.0)
Fijian banded iguana via Wikimedia Commons (CC-BY-SA 2.0)

New genetics support the theory that iguanas voyaged to Fiji on ocean debris, in what scientists claim is the greatest known ocean journey in the history of land creatures. Genetic analysis shows that the Fijian iguana's closest living relative is the North American desert iguana, and that the two probably diverged around the same time as the Fijian archipelago was being created by volcanic activity. They say the desert iguana is well equipped for the gruelling journey around a fifth of the Earth's circumference, as it can withstand heat and go without freshwater for longer than other lizards. The type of raft may have also played a part, with floating vegetation mats providing a life-saving meal for the plant-eating lizards.

Media release

From: PNAS

Iguanas from North America rafted more than 8,000 kilometers to settle in Fiji as early as 31–34 million years ago, according to a study. The long-distance dispersal of organisms across oceans to distant islands can result in the evolution of new species lineages. The origin of critically endangered iguanas on the South Pacific islands of Fiji and Tonga has been debated; how these large, terrestrial herbivores reached the remote islands is unclear. Simon Scarpetta and colleagues explored the biogeographic origins of Fijian iguanas (Brachylophus) using genome-wide sequence data sampled from 14 living iguana species belonging to eight genera. Phylogenomic analysis suggested that the closest living relatives of Fijian iguanas are North American desert iguanas (Dipsosaurus). The estimated divergence time between the two iguana groups was around 31 million to 34 million years ago, near or after the formation of the Fijian archipelago by volcanism. Biogeographic analyses indicated that North America was the most probable ancestral range of the Fijian and North American desert iguanas and that long-distance dispersal was the most likely mechanism by which iguanas arrived on Fiji. Together, the results suggest that the ancestors of Fijian iguanas rafted on floating debris more than 8,000 kilometers across the Pacific Ocean from North America to Fiji. According to the authors, the origin of Fijian iguanas represents the longest known transoceanic dispersal among terrestrial vertebrates.

Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Associate Professor Nic Rawlence, Director - Otago Palaeogenetics Laboratory,
Department of Zoology, University of Otago, comments:

Isolated island ecosystems are natural laboratories for evolution that have fascinated scientists. The tropical island archipelago of Fiji in the western Pacific Ocean is one such example, with the islands forming due to volcanic activity 34-39 million years ago.

Fiji has a rich biodiversity including extinct terrestrial crocodiles and turtles, and strangely, iguanid lizards. Archaeological and/or fossil remains of crocodiles and turtles have been found in other Pacific islands including Vanuatu and New Caledonia, as well as Australia and New Zealand. In contrast,  living and extinct iguanas are only known from Fiji (and nearby Tonga), the Americas, and Galapagos Islands. In Fiji there are four species of living iguana and one giant extinct species, with another giant extinct species on Tonga.

It has always puzzled scientists how iguanids got to Fiji, with various evolutionary scenarios proposed, including trans-oceanic long distance dispersal, or much longer and more complicated dispersal routes that involve movement into Gondwana or southeast Asia, and then onto Fiji.

Now Simon Scarpetta and colleagues have convincingly reconstructed the evolutionary history of these Fijian iguanids using sophisticated genomic and modelling techniques and resolved these competing evolutionary theories.

They showed the closest relative of the living Fijian iguanas is the North American desert iguana from the northwestern USA, with colonisation of Fiji occurring 34-31 million years ago, at or just after the formation of the islands. By reconstructing where the ancestor of the living Fijian iguanas lived before it arrived in Fiji, the most likely location is North America. All this very convincing evidence strongly  supports the idea that iguanas have made the world’s longest trans-oceanic dispersal of more than 8000km, and not the much more complicated and older evolutionary scenarios. Sometimes the simplest explanation is the most likely.

In his fantastic book about the evolution of animals on islands “Song of the Dodo”, David Quammen said that the chance of successful dispersal occurring in any given year is near impossible, but multiplied over geological time, it’s almost evitable for some animal groups. So how did iguanas disperse from North America to Fiji? It’s likely that they floated on vegetation mats coming down rivers in floods and out into the ocean, with the currents taking them to the Pacific. This has been seen in the Caribbean after storms and resulted in successful colonisation of an island. Iguanas are very resistant to heat, dehydration, and starvation, and if they get hungry, as herbivores, can feed on the floating vegetation mat. It’s likely that dispersal was to Fiji and Tonga only, and not other Pacific islands as there is no fossil record that we know of or living iguanas there. Iguanids are very prone to dispersal, with the majority of living species being unique to islands (e.g. Galapagos and Caribbean).

It will be very interesting to see future ancient DNA research on the two giant extinct Fijian and Tongan iguanas as that may show additional dispersal events from North America.

This study goes to show the importance of long distance dispersal to the make-up of island ecosystems. While we tend to think about a lot of Fijian biodiversity as having originated from Australia, New Guinea, and New Zealand, this new research shows the origins are more cosmopolitan…who knows what else will be found when more living and extinct animals are studied.

Last updated:  18 Mar 2025 8:31am
Contact information
Contact details are only visible to registered journalists.
Declared conflicts of interest No conflicts of interest.

Attachments

Note: Not all attachments are visible to the general public. Research URLs will go live after the embargo ends.

Research , Web page EurekAlert registration is free for journalists. For assistance, contact the SMC.
Journal/
conference:
PNAS
Research:Paper
Organisation/s: University of San Francisco, USA; NatureFiji-MareqetiViti, Fiji
Funder: This work was funded by an NSF postdoctoral research fellowship to SGS (DBI # 2109461). Research in Fiji was funded through the International Iguana Foundation.
Media Contact/s
Contact details are only visible to registered journalists.