The arrival of iguanas in the South Pacific can only be explained, a team of biologists have argued, if they caught a lift on a natural raft from the Americas. That’s a journey of 8,000 kilometers (5,000 miles), even if there was no backsliding or getting caught in ocean gyres – a fifth of the way around the planet. This would be the largest transoceanic voyage known to have been performed by any land animal with a spine, at least until hitchhiking on human shipping became possible.
Even if they can swim, land-dwelling creatures cannot make it across oceans under their own steam. Nevertheless, sometimes clearly related animals turn up vast distances apart, with no plausible route to go round. The most famous example of this is the way monkeys suddenly appear in the South American fossil record 36 million years ago, apparently having floated out to sea on some vegetation and made it across the Atlantic without dying of thirst. To top it off, the new world monkey family tree suggests another bunch did it again million of years later.
Iguanas are a highly successful family through much of the Americas, with 45 surviving species. They are, however, not known from other continents. Yet somehow there are four living species in Fiji and Tonga, along with one extinct one. The obvious answer is that they came via Asia, perhaps having got there at a time when the continents were closer together. Alternatively, before Antarctica was covered in ice, iguanas could have used it as a bridge to Australia, and then the Western Pacific. However, there is no fossil evidence for either of these explanations.
When Simon Scarpetta of the University of San Francisco and co-authors explored the genetics of the iguana family tree, and the Fijian genus Brachylophus’s place within it, these stories became even less plausible.
“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,” Scarpetta said in a statement.
If you want to settle new lands, you’ll need a female for the crossing like this B. fasciatus from the Lau Islands.
Image credit: Robert Fisher, USGS
That timing is not much younger than the age of the volcanic islands that make up Fiji.
Instead, Scarpetta and co-authors argue, Brachylophus crossed the Pacific Ocean on rafts washed out to sea in storms. We’ve seen them making shorter journeys between Caribbean Islands, and are confident this is how they got to the Galapagos, but that’s all pretty local compared to sailing most of the width of the world’s largest ocean. That’s even leaving aside the impressiveness of hitting a small island at the end of their voyage.
“That they reached Fiji directly from North America seems crazy,” said University of California, Berkeley professor Jimmy McGuire. However, when you have ruled out the impossible, the improbable must be accepted. “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,” McGuire continued. “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.”
If it were not for these sort of ocean crossings, volcanic islands would be largely barren, other than what birds and bats bring.
However, the sheer length of this journey, in time as well as distance, makes it stand out. Trapdoor spiders have crossed the Indian Ocean from Africa to Australia, but much of the journey was probably helped by the roaring 40s. The iguanas had to make it through the doldrums, although there is some evidence the currents were more favorable 34 million years ago. The capacity of desert species to go for long periods without water would have come in handy, but the authors note it is likely at the time North America’s iguanas lived in wetter habitats at the time.
You’d smile like this B. bulabula if your ancestors has just been recognized as the world’s greatest navigators.
Image credit: Robert Fisher, USGS
“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.
“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.
The study is published in Proceedings of the National Academy of Sciences.
