Imma Perfetto
Cosmos science journalist
Iguanas are scattered across the Caribbean and Galápagos islands and throughout the tropical, subtropical and desert areas of North, Central and South America. How they managed to reach the isolated Fiji Islands, thousands of km away over the Pacific Ocean, has long puzzled scientists.
New genetic analysis has now confirmed that their ancestors drifted a fifth of the way around the world, likely on a raft of vegetation, from the western coast of North America.
“That they reached Fiji directly from North America seems crazy,” says Jimmy McGuire, professor of integrative biology at the University of California Berkeley in the US and senior author of a study describing the findings in PNAS.
“But alternative models involving colonisation 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.”
First author of the study, Simon Scarpetta, a herpetologist and palaeontologist at the University of San Francisco, collected DNA from more than 4,000 genes and tissues of more than 200 iguana specimens housed in museum collections around the world.
It revealed that the Fiji iguanas, genus Brachylophus, are most closely related the North American desert iguanas, genus Dipsosaurus.
“The lineage of Fiji iguanas split from their sister lineage relatively recently … either post-dating or at about the same time that there was volcanic activity that could have produced [the Fijian archipelago],” says Scarpetta.
“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.”
Iguanas are large herbivores which can survive for long periods without food and water, with desert iguanas being particularly resistant to starvation and dehydration. If the flotsam consisted of uprooted trees, the raft itself could have provided food.
“If there had to be any group of vertebrate, or any group of lizard, that could make an 8,000km journey across the Pacific on a mass of vegetation, a desert iguana-like ancestor would be the one,” says Scarpetta.
The findings conflict with previous ideas for the origin of the lineage. For example, biologists had speculated, based on a few fossils found in east Asia, that an ancestral and now extinct population of iguanids lived around the Pacific Rim and somehow made their way to Fiji.
“When you don’t really know where Brachylophus fits at the base of the [family] tree, then where they came from can also be almost anywhere,” McGuire says.
“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.”
Today, the 4 species in Fiji are listed as endangered or critically endangered due to habitat loss, predation by invasive rats, and the exotic pet trade.
