Iguanas traveled 5,000 miles from North America to Fiji

Iguanas have often been observed drifting on vegetation in the Caribbean, including a past 600-mile journey from Central America to the Galapagos. 

However, when it comes to extremely long-distance travel, Fiji’s iguanas surpass all others, according to new work by biologists at the University of California, Berkeley, and the University of San Francisco (USF).

The study shows that, at some point after approximately 34 million years ago, the ancestors of the Fiji iguanas arrived on the South Pacific islands following a 5,000-mile voyage from the western coast of North America. This feat stands as the longest recorded oceanic journey by any land vertebrate.

Overwater dispersal and its significance

Overwater dispersal is the primary means by which newly formed islands become populated by plants and animals, paving the way for the emergence of new species and distinct ecosystems. 

This puzzle of how life colonizes distant islands has intrigued scientists since the time of Charles Darwin, who put forth the theory of evolution by natural selection.

According to the experts, the arrival of these iguana ancestors likely coincided with the volcanic origins of the Fiji archipelago. 

The researchers estimate that the iguanas’ landing time – 34 million years ago or slightly later – matches the date of the genetic split between Fiji’s Brachylophus iguanas and their nearest relatives, Dipsosaurus desert iguanas in North America.

Mystery of Fiji’s iguanas 

In earlier work, scientists speculated that Fiji iguanas might hail from an older lineage once spanning the Pacific, leaving Brachylophus as the single iguana genus in the western Pacific after the rest died out. 

Some theorized they could have traveled by land and sea from South America or via Australia, but genetic and fossil data offered no support for that. The new research dismisses those ideas. 

Study lead author Simon Scarpetta is a herpetologist and paleontologist who is a former postdoctoral fellow at UC Berkeley.

“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,” said Scarpetta. 

“That they reached Fiji directly from North America seems crazy,” said co-author Jimmy McGuire, a UC Berkeley professor of integrative biology and herpetology curator at the Museum of Vertebrate Zoology. “But alternative models involving colonization from adjacent land areas don’t really work for the time frame, since we know that they arrived in Fiji within the last 34 million years or so.”

“This suggests that as soon as land appeared where Fiji now resides, these iguanas may have colonized it. Regardless of the actual timing of dispersal, the event itself was spectacular.”

The iguanas’ voyage to Fiji

Whereas modern sailors can harness favorable winds to travel from California to Fiji in about a month, an iguana (or a group of them) would probably have drifted for far longer on uprooted plant life, crossing the doldrums and equator to arrive in Fiji and neighboring Tonga, where these iguanas live. 

Luckily, iguanas are large herbivores adept at going without food or water for extended periods – and vegetation rafts may have provided them with nourishment.

“You could imagine some kind of cyclone knocking over trees where there were a bunch of iguanas and maybe their eggs, and then they caught the ocean currents and rafted over,” Scarpetta said.

Fiji iguanas as an evolutionary outlier

Altogether, more than 2,100 species populate the suborder Iguania, comprising creatures such as chameleons, anoles, bearded dragons, and horned lizards. 

The term “iguanas” typically refers to the Western Hemisphere lizards in the family Iguanidae, which includes the familiar green iguana. Forty-five iguanid species live in the Caribbean and in tropical, subtropical, and desert regions of the Americas – including the marine iguanas of the Galapagos and the chuckwallas of the American Southwest.

The Fiji iguanas stand apart, marooned in the Pacific. Four species live on Fiji and Tonga, all listed as endangered because of habitat destruction, invasive rats, and illegal exploitation for the pet trade.

In the past, some researchers suggested that a now-lost iguana population once ranged widely around the Pacific Rim, eventually reaching the central Pacific through island-hopping. 

They may have traveled by land and sea along the Pacific coast of the Americas, passing through Antarctica, or perhaps they rafted on the Humboldt Current from South America into the South Pacific.

Genetic origins of Fiji iguanas 

Previous genetic analyses of a few selected genes proved inconclusive about how Fiji iguanas relate to other iguanids. Then, while a postdoctoral fellow with McGuire, Scarpetta launched a comprehensive survey of all genera in Iguania to clarify the group’s phylogeny.

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

Collecting genome-wide sequences from over 4,000 genes, along with tissue samples from more than 200 iguanian specimens archived in museums worldwide, Scarpetta found that Fiji iguanas are most closely aligned with the Dipsosaurus genus. 

Their best-known member, the North American desert iguana, Dipsosaurus dorsalis, endures the scorching conditions of the American Southwest and northern Mexico, with another related species confined to Santa Catalina Island in the Sea of Cortez.

“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,” Scarpetta said.

Genomic data reveals dispersal patterns

This genetic evidence clarifies that Brachylophus and Dipsosaurus diverged around 34 million years ago – ruling out earlier conjectures about older Pacific lineages or a South American origin.

Moreover, since the Fiji Islands also formed near that time, the iguanas’ extraordinary journey aligns neatly with the emergence of volcanic land.

“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,” McGuire added.

“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.”

With the new analysis, a South American path can be dismissed. And as the volcanic Fiji Islands appeared roughly 34 million years ago, iguanas possibly found them not long afterward.

Other South Pacific islands might have supported iguanas at some point, but ephemeral volcanic islands can vanish as fast as they appear, erasing any trace of past populations.

Scarpetta, who has studied reptiles and amphibians from a young age, is continuing to utilize genome-wide data to further unravel the evolutionary relationships and dispersal patterns of iguanian lizards over time.

Image Credit: Nicholas Hess

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