Ancient Iguanas Floated 5,000 Miles Across The Pacific

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman.

If you picture iguanas, you might imagine them sunbathing on the hot sand in the Caribbean or skittering around the Mojave Desert. But far, far away from where these iguanas are found is another group of iguanas living on the islands of Fiji and Tonga in the South Pacific, pretty close to New Zealand. And it begs the question, how did these iguanas end up all alone on the other side of the ocean?

In the journal Proceedings of the National Academy of Sciences, scientists suggest that millions of years ago, iguanas hitched a ride on a raft and accidentally sailed all the way across the ocean before washing ashore and starting a new life. Here to discuss this intrepid adventure is Dr. Simon Scarpetta, evolutionary biologist and assistant professor at the University of San Francisco in California. Simon, welcome to Science Friday.

SIMON SCARPETTA: Hi, Flora. Thanks for having me. Really excited to be here.

FLORA LICHTMAN: Simon, how far would these iguanas have traveled?

SIMON SCARPETTA: So in the modern day, the “as the crow flies” distance between the western coast of North America and Fiji is somewhere between 8,000 and 9,000 kilometers, so 5,000-ish miles, something around that.

FLORA LICHTMAN: That feels long to me.

SIMON SCARPETTA: Yeah. [CHUCKLES] It’s really long. It’s quite long.

FLORA LICHTMAN: What should I be picturing?

SIMON SCARPETTA: In my mind, when I think about it, it’s a big mat of vegetation or some downed trees that get knocked over by a storm or potentially even a tsunami. And there maybe are some iguanas in the trees or next to the trees. But however it happened, they or their eggs get caught up in all this vegetation. And then a series of ocean currents drift them out far into the ocean, and eventually they make their way to the South Pacific.

FLORA LICHTMAN: This feels like a Disney movie, but instead of an intrepid girl with her chicken, it’s just iguanas.

SIMON SCARPETTA: It’s iguanas making their way on their little flotilla from one place to the next.

FLORA LICHTMAN: Is this a world sailing record for a land animal?

SIMON SCARPETTA: It is. So, yeah, of all of the known over water dispersals– of which there are definitely others known, including in several other iguanas– this is the longest hypothesized one, and by a decent amount, too– a few thousand miles or so. And so really, what it gives us is information about just what is possible in the tree of life, just what animals and lizards in particular can do. And it’s really, really incredible.

FLORA LICHTMAN: This feels so improbable to me. How did you land on this hypothesis?

SIMON SCARPETTA: Yeah. Iguanas are a group of lizards that are currently found in the Caribbean and then in North America, especially in Central America, and then a bit into northern South America. And pretty much all of their close relatives are only found in the Western Hemisphere, too.

And then you have this group of these Fijian iguanas that are found all the way out in the South Pacific, really, really, really far removed. And so it begs the question, how could they have possibly gotten there? And one of the first results that I found that really caught my attention and then prompted further work, was that the Fijian iguanas are most closely related to the North American Desert Iguanas that we have in Southern California and Arizona and then into Northern Mexico.

FLORA LICHTMAN: Their closest relatives are 5,000 miles away.

SIMON SCARPETTA: Exactly, yeah. And on the western coast of the United States and Mexico specifically. [CHUCKLES]

FLORA LICHTMAN: Wow. OK, so that’s just genetics. It’s like if their close relatives are over here, they must have come from over here. Is that it?

SIMON SCARPETTA: Well, that’s definitely the first step towards getting there. So in addition to the genetic aspect, there’s, then, dating the evolutionary tree of iguanas to try and figure out what’s the most probable ancestral place from which the Fiji iguanas came, and then how is the mechanism by which they got there?

And so those models found pretty conclusively that the most probable ancestral range would have been North America, like you’d expect. And then the most probable mechanism would have been dispersal directly from North America to Fiji, rather than moving through some other route, like going through South America and then Antarctica and then up, or through Beringia, which is Alaska and eastern Russia, and then going all the way down through East Asia that way.

FLORA LICHTMAN: Do these iguanas in the South Pacific– do they resemble the iguanas. I would see in the desert in the Southwest?

SIMON SCARPETTA: They do, yeah. Yeah, no, they have a lot of surprising similarities to the desert iguanas that we have in the Southwest. Very interestingly, they have some similarities in the morphology of the teeth that you don’t generally see in other iguanas.

Yeah, there’s a lot of morphological similarities, but then some decided differences too. So relative to all of the other iguanas and chuckwallas that are known, they’re much more slender. They have these sort of a bit more compressed snouts, longer tails. The Fijian iguanas are these green, resplendent, beautiful creatures. Whereas the North American ones– also beautiful, in my opinion– but they tend to be a bit more drab.

FLORA LICHTMAN: More beautiful to an iguana scientist.

SIMON SCARPETTA: [CHUCKLES] More beautiful to an iguana scientist who likes going out in the desert and looking for lizards. [CHUCKLES]

FLORA LICHTMAN: OK, I got to get back to the voyage because I’m still mesmerized by it. How long would this voyage have taken, do you think?

SIMON SCARPETTA: There have been a number of different estimates proposed for that. But, maybe as short as two months and then maybe on the upper end, to a year.

FLORA LICHTMAN: What were they eating? How did they survive?

SIMON SCARPETTA: Yeah. [CHUCKLES] No, great question. And that’s one of the other really cool things about this story, is that if you had to pick one organism that could really, really do this, lizards in the general sense would be a great option. And then within lizards, if you had to pick one that would just be the best candidate, it would really be iguanas. So they have a lot of organismal traits that I think predispose them to being able to survive these long distance journeys.

FLORA LICHTMAN: Why?

SIMON SCARPETTA: Yeah. One is that they’re large. So they have a decent amount of mass to work with and to lose. Two is that their metabolism is something called ectothermic, which is often called cold blooded. And so basically what that means is that they don’t use a lot of energy to maintain a standing body heat like mammals and birds do. And so it means that they have to eat a lot less.

But there’s other stuff too. So iguanas are also predominantly herbivorous, though they do consume insects and small other things occasionally.

FLORA LICHTMAN: So they could eat their raft.

SIMON SCARPETTA: Exactly. That’s exactly it. They could potentially even eat their raft. And then on top of that, some species are pretty resistant to heat. They’re resistant to starvation. And then some of the species are also pretty resistant to dehydration too, so, again, especially those that live in the desert.

But that said, obviously they could get some fresh water from eating the plants. But they could also conceivably get some fresh water from dew and things like that, that would collect on a raft in the morning.

FLORA LICHTMAN: Man, they’re tough.

SIMON SCARPETTA: They’re tough creatures, yeah. They’re tough– tough as nails.

FLORA LICHTMAN: This story really changed my perception of iguanas.

SIMON SCARPETTA: [CHUCKLES] I’m glad to hear it.

FLORA LICHTMAN: I think I thought of them– I’m actually scared to say this to you as an iguana–

SIMON SCARPETTA: No, go for it.

FLORA LICHTMAN: –an iguana stan. I thought of them as, I don’t know, just kind of sunbathing, hanging in the sand, plopped out. You know what I mean?

SIMON SCARPETTA: Yeah. No, of course. [CHUCKLES]

FLORA LICHTMAN: And this study is making me think I’ve completely misunderstood them.

SIMON SCARPETTA: Well, you know, they do like to hang out in the sun. It’s true. And they do kind of plop, especially the bigger species, honestly– the green iguanas that probably most everyone is most familiar with. They’re kind of vibing a bit.

FLORA LICHTMAN: But it’s also like a work hard, play hard, or something. They go hard on vacation. But it seems like they– I need the right catchphrase for them.

SIMON SCARPETTA: No, no. I think that definitely works.

FLORA LICHTMAN: They don’t just plop, is what I’m learning.

SIMON SCARPETTA: Yeah. No, the plopping is interspersed with doing things that are difficult or incredible or surviving hard conditions. And then when the times are chill, they just get to hang out. Work hard, play hard– that’s how iguanas do it.

FLORA LICHTMAN: Simon, thanks so much for joining me today.

SIMON SCARPETTA: Yeah, of course. Thanks for having me, Flora. It’s been a pleasure.

FLORA LICHTMAN: Dr. Simon Scarpetta is an evolutionary biologist and assistant professor at the University of San Francisco in California.

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