Iguanas: Fiji's first settlers?

Just a year ago, I reported how new biological research into Fiji's bee species solved an international scientific mystery in https://www.bmallsopp.com/post/beelines-a-pacific-mystery-solved. A few weeks ago, Fiji again made headlines in the world scientific media and even attracted attention of the mainstream press like The New York Times. The source of the articles was Proceedings of the National Academy of Sciences March 17, which provided answers to more than a century of speculation about the origin of four species (two 'banded', two 'crested') of the iguana family in the western Pacific islands of Fiji and Tonga.

But, first, let's go back to 1981 ...

Scientists haven't always recognised four species of iguana endemic to Fiji. The banded iguanas have long been named, but only in recent decades have their crested cousins been accorded distinct species status, largely due to the work of Dr John Gibbons, a herpetologist at the University of the South Pacific. Dr Gibbons believed that the crested iguanas he discovered and described in the 1970s, may have been confined to a very few small islands.

However, when the 1980 Hollywood film starring Brooke Shields was filmed on Turtle Island in the Yasawa group, the cinematographer dwelled lovingly on the wildlife walk-ons. While watching the movie, John Gibbons noticed the frequent appearance of a spectacular green lizard, which he thought may be new to science. He travelled to Turtle Island as soon as he could and described a Fijian crested iguana in 1981, subsequently confirmed as a distinct species. If you've read Death on Paradise Island, my first Fiji Islands Mystery, you'll know the Fiji crested iguana, or vokai in Fijian, who's a very important character!

Where did Fiji's iguanas come from?

Scientists were surprised by Fiji's iguanas because all other 45 species of iguana are found in the Americas.

Fiji's crested iguana: brachylophus vitiensis Source: Unsplash

As many species have dispersed over water by rafting on accretions of vegetation, usually debris from storms, it has always been assumed the ancestors of Fiji's iguanas travelled in this way. Even in recent times, iguanas have been observed at sea on such rafts in the Caribbean. As herbivore stowaways, they can browse on their craft, but can also survive without food or water for long periods, and some can semi-hibernate if conditions are extreme.

Itinerary of the iguana ancestors

To discover the route of the iguana's migration and when they reached Fiji, Dr Simon Scarpetta at the University of San Francisco and colleagues investigated extensive biogeographical and phylogenetic relationships and evolutionary timescales. They found the closest living relative of Fiji's iguanas is the North American desert iguana, which lives in the deserts of the US south-west and north Mexico. That means the lizards probably travelled 8,000 kilometres (5,000 nautical miles) from North America before washing up in Fiji. This represents one-fifth of the earth's circumference and could well be the longest overwater species migration of land vertebrates that has ever occurred.

The researchers' analysis determined that the lineages of Fiji's iguanas and the North American desert iguana, diverged about 34 million years ago, which doesn’t fit with earlier theories of the origin of the Fiji iguanas. Excitingly, that's the time when Fiji's volcanic islands erupted from the sea. It seems that the ancestors of today's Fiji iguanas (both banded and crested)may have been one of the earliest settlers on these glorious islands.

How long did the journey take?

"Rayna Bell, an evolutionary biologist at the California Academy of Sciences in San Francisco, points out that it’s difficult to know the precise distance travelled, partially because other islands may have served as stepping-stones but since disappeared. 'The deeper we go [back] in evolutionary time, we’re more limited in terms of what evidence is available to us, because the planet has changed so much,' she says." (Buehler, J, Science News, March 17, 2025)

The diagram below shows scientists' proposed hypothetical migration routes for the ancestors of Fiji's iguanas.

A map of the world 34 million years ago, showing hypothesized scenarios for the colonization of Fiji by Brachylophus. Based on new research, the most likely scenario is represented by the dark blue arrow from western North America to Fiji. The small red stars represent sites in North America where fossil iguanids have been found. The ranges of modern iguanids — in the Americas and Fiji and Tonga — are shown in light blue. Credit: Simon Scarpetta and Jim McGuire. PNAS, March 17 2025

Other co-authors of the paper are Robert Fisher of the U.S. Geological Survey in San Diego, Benjamin Karin and Ammon Corl of UC Berkeley, Jone Niukula of NatureFiji-MareqetiViti in Suva and Todd Jackman of Villanova University in Pennsylvania.

If you have friends interested in Fiji or Pacific islands in general, I encourage you to share this post.

I look forward to hearing from you!

Bernadette

B.M. Allsopp