Pod Insights Short Wave The Iguanas That Rafted To Fiji About this episode 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 https://www.pnas.org/doi/10.1073/pnas.2318622122 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 https://indianapublicmedia.org/amomentofscience/green-iguanas-on-anguilla.php away from Guadeloupe to the territory of Anguilla.
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0:00 Speaker 00 This message comes from the Southern Environmental Law Center. No one voted for dirty air and water. That's why S E L C is fighting back to defend the healthy environment everyone depends on. Because it's not political, it's fundamental. Learn more at selc.org.
0:18 Regina Barber? You're listening to Short Wave from NPR.
0:23 Regina Barber? Hey, Short Wavers. Regina Barber here. And today I'm joined by NPR's Jonathan Lambert. Hey, Jon.
0:28 Speaker 02 Hey, Gina.
0:29 Regina Barber? So today you're bringing us a story that you say is sort of like half mystery, half swashbuckling adventure.
0:35 Speaker 02 Yes, it's seafaring, exploration, intrigue.
0:39 Regina Barber? Oh, my gosh, this is giving me very like One Piece vibes.
0:42 Speaker 02 Yeah. 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.
0:54 Regina Barber? Why was it a mystery?
0:57 Speaker 02 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.
1:09 Regina Barber? Ooh, OK, so like, how did these like landbound lizards wind up like all the way across the Pacific?
1:15 Speaker 02 According to a new paper, they floated.
1:18 Regina Barber? What? What did they float on?
1:20 Speaker 02 Likely some big clump of downed trees and other vegetation that became a raft of sorts.
1:25 Regina Barber? OK, OK.
1:26 Speaker 02 This new study argues that some intrepid, well, probably inadvertently intrepid group of iguanas set off from North America to float like 5,000 miles away to reach the island of Fiji.
1:38 Regina Barber? That is so far.
1:39 Speaker 02 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 transoceanic dispersal event of any land animal except for humans.
1:53 Regina Barber? Wow. OK, so today on the show, how scientists figured out the Fijian iguana origin story.
1:59 Speaker 02 And how iguanas might have survived such a long time at sea.
2:03 Regina Barber? You're listening to Short Wave, the science podcast from NPR.
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3:32 Regina Barber? 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.
3:40 Speaker 02 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.
3:52 Regina Barber? Wow.
3:52 Speaker 02 Over land bridges to modern day Asia or Australia.
3:57 Speaker 02 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.
4:05 Regina Barber? But there aren't iguanas in the eastern hemisphere now, right? Like, like Asia, Australia, or anywhere besides Fiji?
4:11 Speaker 02 Right, right. Yeah, this idea assumes that Fijian iguanas are the sole surviving ancestors of that larger group that made this trip.
4:18 Regina Barber? Ooh, okay. So what's the second hypothesis?
4:21 Speaker 02 The second idea is that they sailed there on some kind of raft of vegetation.
4:26 Speaker 05 From somewhere in the Americas, all the way across the Pacific. You know, through the doldrums of the equator, through the southern equatorial current, and then made their way to Fiji.
4:35 Speaker 02 That's Simon Scarpetta, a biologist at the University of San Francisco.
4:39 Regina Barber? And that's the one that they point to in the paper, right? That's the hypothesis.
4:43 Speaker 02 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.
4:51 Regina Barber? So what was stopping them? Like, why couldn't they tell which one it was up until now?
4:55 Speaker 02 Well, a couple of reasons.
4:56 Speaker 05 We didn't really know how the Fiji and Iguanas were related to other Iguanas. And then we also didn't really know how old they were.
5:03 Regina Barber? OK, so like, they're trying to figure out when these populations broke off and like, from what other species to help figure out like, how these Iguanas got to Fiji, right?
5:11 Speaker 02 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.
5:24 Regina Barber? OK, but I'm assuming we wouldn't be talking about like, this study unless they like, figured both of these things out. Like, so how did they get closer to an answer?
5:32 Speaker 02 Basically, by figuring out the Iguana family tree.
5:35 Regina Barber? OK.
5:35 Speaker 02 They used genomic data from 14 different Iguana species to see who was most closely related to whom. And the results surprised Scarpetta.
5:43 Speaker 05 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.
5:52 Regina Barber? Wait, desert Iguanas?
5:53 Speaker 02 Yeah, the kind that currently lives in the southwestern US.
5:57 Regina Barber? Wow.
5:57 Speaker 02 And they estimated that these Iguanas split a little over 30 million years ago. Now, that timeline doesn't quite line up with the land bridge 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.
6:16 Regina Barber? OK, cool. Tell me more.
6:17 Speaker 02 Yeah, so it's also around the same time that Fiji itself formed from an underwater volcano.
6:22 Regina Barber? What?
6:22 Speaker 02 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.
6:35 Regina Barber? Wow, OK. 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?
6:48 Speaker 02 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.
7:04 Speaker 06 Homes were torn apart and airplanes tossed aside and towers toppled. At least three people are reported dead and 100 more.
7:11 Speaker 02 Wrecked 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.
7:27 Regina Barber? Wow, OK, but Fiji is like way further away. This still seems really surprising.
7:33 Speaker 02 It is still really surprising.
7:34 Regina Barber? Yeah.
7:34 Speaker 02 I talked with Iguana biologist Christina de Jesus Villanueva about the paper, and here's what she had to say.
7:40 Speaker 07 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.
7:55 Regina Barber? I mean, I agree. It sounds really, really difficult.
7:58 Speaker 02 Yeah, these iguanas had so much going against them. It's really, really far, like 5,000 miles of floating across the vast wasteland of the Pacific.
8:08 Regina Barber? Wow.
8:09 Speaker 02 No fresh water, the beating sun.
8:11 Speaker 07 It just brings to mind the images of Tom Hanks in Cast Away, 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.
8:28 Regina Barber? And how long did this journey take? Like months, I imagine.
8:32 Speaker 02 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.
8:49 Regina Barber? It's desert iguanas.
8:50 Speaker 02 Yes.
8:52 Speaker 07 They seem to love to sunbathe really extra compared to other iguana species.
8:57 Speaker 02 So they could have been totally fine in the sun. And they can also survive a really long time without fresh water or food.
9:04 Regina Barber? I was just thinking about food this whole time.
9:06 Speaker 02 Yeah, yeah, yeah. And I mean, they are herbivorous, so they could have potentially snacked on the raft, too, if they needed to.
9:12 Regina Barber? That makes sense. That makes sense.
9:13 Speaker 07 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.
9:24 Regina Barber? It is so remarkable. But as the skeptic here, like, how sure are these scientists? Like, have they absolutely ruled out those other hypotheses or these other solutions to this mystery?
9:36 Speaker 02 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 Fijian iguanas rafted there. But if, say, some new fossil popped up in Southeast Asia that seemed more closely 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.
9:59 Regina Barber? Interesting. OK, so for now, iguanas are still like the reigning non-human transoceanic dispersal champions, right?
10:07 Speaker 02 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 happened quite often over the span of evolutionary time. And they really shape the biological world around us.
10:25 Regina Barber? That's beautiful, John. Thank you so much for bringing us this reporting.
10:28 Speaker 02 Yeah, thank you.
10:30 Regina Barber? This episode was produced by Hannah Chin and edited by our showrunner Rebecca Ramirez. John Lambert and Tyler Jones checked the facts. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber.
10:44 Speaker 02 And I'm Jonathan Lambert.
10:46 Regina Barber? Thank you for listening to Short Wave, the science podcast from NPR.
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