Short Wave - The Iguanas That Rafted To Fiji

Episode Date: April 1, 2025

Most iguanas are indigenous to the Americas. So how did the Fijian species end up on the island, nearly 5000 miles away in the South Pacific? According to a new study in the journal PNAS, it was probably via raft ... that is, on clump of floating trees. And this rafting hypothesis isn't entirely unprecedented. After hurricanes Luis and Marilyn hit the Caribbean in the 1990s, researchers found that a group of iguanas had floated over 180 miles away from Guadeloupe to the territory of Anguilla. Want to hear more about iguanas? Or rafts? Or evolutionary biology? Email us at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, ShoreWavers, Regina Barber here. And today I'm joined by NPR's Jonathan Lambert. Hey, John. Hey, Gina. So today you're bringing us a story that you say is sort of like half mystery, half swashbuckling adventure. Yes, it's seafaring, exploration, intrigue.

Oh my gosh. This is giving me very like one-piece vibes. And on top of all of that, it's about iguanas. specifically iguanas native to the tropical island of Fiji and how they got to this super isolated island has always been a bit of a mystery. Why was it a mystery? So most iguanas are native to the Americas,

with some in the Caribbean and some on the Galapagos Islands. Fiji is like one-fifth of the way around the globe, and while some iguanas can swim, they can't swim that far. Okay, so like how did these land-bound lizards wind up all the way across Pacific? According to a new paper, they floated. What? What did they float on? Likely some big clump of downed trees and other vegetation that became a raft of sorts. Okay, okay.

This new study argues that some intrepid, well, probably inadvertently intrepid. Okay. Group of iguanas set off from North America to float like 5,000 miles away to reach the island of Fiji. That is so far. Yeah, it's wild. Biologists call this kind of journey where an animal travels to a new place and sets up permanent camp, a dispersal event. And this would be the longest known trans-oceanic dispersal event of any land animal except for humans. Wow. Okay, so today on the show, how scientists figured out the Fijian iguana origin story.

And how iguanas might have survived such a long time at sea. You're listening to Shortwave, the science podcast from NPR. Okay, John, so let's like back up a second. Tell me more about like this mystery and why scientists didn't really know how iguanas ended up on Fiji. So there were basically two ideas for how iguanas got to Fiji. The first is that they got there gradually. Over like 50 million years ago, iguanas might have walked over the course of lots of generations. Wow. Over land bridges to modern day Asia or Australia.

Those land bridges are now underwater, but they might once have allowed iguanas to spread in the eastern hemisphere and sort of island hop the rest of the way to Fiji. But there aren't iguanas in the eastern hemisphere now, right? like Asia, Australia, or anywhere besides Fiji. Right, right. Yeah, this idea assumes that Fiji and iguanas are the sole surviving ancestors of that larger group that made this trip. Oh, okay. So what's the second hypothesis? The second idea is that they sailed there on some kind of raft of vegetation.

From somewhere in the Americas, all the way across the Pacific, you know, through the doldrums of the equator, through the southern equator, and then made their way to Fiji. That's Simon Scarpetta, a biologist at the University of the United States. University of San Francisco. And that's the one that they point to in the paper, right? That's the hypothesis. Yeah, yeah. But until this paper, scientists hadn't really been able to confirm it either way, whether they made it there very gradually or all in one go.

So what was stopping them? Like, why couldn't they tell which one it was up until now? Well, a couple of reasons. We didn't really know how the Fijian iguanas were related to other iguanas, and then we also didn't really know how old they were. Okay, so like they're trying to figure out when these populations broke off and like from and what other species to help figure out, like, how these iguanas got to Fiji, right? Yeah, yeah.

So understanding what else Fiji and iguanas are closely related to could give scientists clues about where they came from. And knowing how old they are could indicate how likely it is that they walked over these ancient land bridges. Okay, but I'm assuming we wouldn't be talking about, like, this study unless they, like, figured both of these things out. So how did they get closer to an answer? Basically, by figuring out the iguana family tree.

Okay. They use genomic data from 14. different iguana species to see who is most closely related to whom. And the result surprised Garpeta. The iguanas that live on Fiji were most closely related to a group of iguanas that I knew very well from the United States called desert iguanas. Wait, desert iguanas? Yeah, the kind that currently lives in the southwestern U.S. Wow.

And they estimated that these iguanas split a little over 30 million years ago. Now, that timeline doesn't quite line up with the Landbridge idea because such bridges would have been either underwater or covered in ice or just too cold for cold-blooded lizards at the time. So there would have been no way for iguanas to walk there. Okay, cool. Tell me more. Yeah, so it's also around the same time that Fiji itself formed from an underwater volcano. Yeah.

And so to Scarpetta, that all suggests that at some point in the last 30 million years, some small group of iguanas just so happened to be on some raft of vegetation, and that raft drifted all the way to Fiji. Wow. Okay, so this sounds so wild to me. Like, has anything like this ever been observed before? Like, is this idea of, like, land animals floating on a raft just some, like, idea biologists had, or has it been observed? Both.

Biologists have long surmised that animals on islands could have gotten there by floating, but people have also actually seen it happen. In 1995, combined hurricanes hit the Caribbean. Here's a clip from the Weather Channel documenting the damage. Homes were torn apart. and airplanes tossed aside and towers toppled. At least three people are reported dead. Corrected buildings, fallen trees, and apparently displaced animals, because in the aftermath of these hurricanes,

researchers tracked a group of iguanas that floated on a raft of downed trees over 180 miles from the Caribbean island of Guadalupe to Anguilla. Wow, okay, but Fiji is like way further away. This still seems really surprising. It is still really surprising. Yeah. I talked with iguana biologist Christina de Jesus Villanueva about the paper, and here's what she had to say. I don't usually read a paper and go, wow.

That's a surprise. But I did do that when I was reading this one. You know, it just seems like that was the most difficult path in a way. I mean, I agree. It sounds really, really difficult. Yeah, these iguanas had so much going against them. It's really, really far, like five. thousand miles of floating across the vast wasteland of the Pacific.

Wow. No fresh water, the beating sun. It just brings to mind the images of Tom Hanks and Castleway and he's kind of like surviving there with Wilson and struggling and just thinking about like lizards, not only surviving these journeys, but also establishing themselves in a new place. And how long did this journey take, like months, I imagine? Yeah, so they don't know for sure. but some previous estimates have ranged from like two and a half months to four months or maybe even longer. And that's a really long time, but both Scarpetta and Villanueva told me that if any creature could do it, it's iguanas. And remember, it's not just any iguanas.

It's desert iguanas. Yes. They seem to love to sunbathe really extra compared to other iguana species. So they could have been totally fine in the sun. And they can also survive a really long time without fresh water. were food. I was just thinking about food this whole time. Yeah, yeah, yeah.

And I mean, they are herbivorous, so they could have potentially snacked on the raft, too, if they needed to. That makes sense. That makes sense. I don't think that there are other species as well equipped to survive such a journey. But it is a massive, massive feat, and it is remarkable. It is so remarkable.

But as the skeptic here, like, how sure are these scientists, like have they absolutely ruled out those other hyperboverbalt? or these other solutions to this mystery? They're not 100% sure, which Scarpetta and his colleagues acknowledge in the paper. They think given the available evidence, it's most likely that Fiji and iguanas rafted there. But if, say, some new fossil popped up in Southeast Asia that seemed more close related to the Fijian iguanas than desert iguanas, that might change the picture because it could imply that iguanas had been in the eastern hemisphere longer. Interesting. Okay. So for now, iguanas are still like the raining, non-human trans-oceanic dispersal champions, right?

They are. And I think this is just a really cool example of the resilience of some animals. And it also really shows how these events that can seem so improbable actually probably happen quite often over the span of evolutionary time. And they really shape the biological world around us. That's beautiful, John. Thank you so much for bringing us this reporting. Yeah, thank you.

This episode was produced by Hannah Chin and edited by our showrunner, Rebecca Ramirez. John Lambert and Tyler Jones check the facts. Beth Donovan is a senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. And I'm Jonathan Lambert. Thank you for listening to Shortwave, the science podcast from NPR.