Scaly sailors may have made a record-setting oceanic voyage. Tens of millions of years ago, iguanas in North America floated across the Pacific Ocean. Clinging to vegetation washed into the sea, they traveled one-fifth of the way around the world, eventually disembarking and settling in the islands of the South Pacific, researchers report March 17 in Proceedings of the National Academy of Sciences. The seafaring lizards’ trip may be the longest known transocean emigration among land vertebrates.
“It’s just a truly incredible example of long-distance dispersal,” says Ethan Gyllenhaal, an evolutionary biologist at Texas Tech University in Lubbock who was not involved with the research.
All members of the iguana evolutionary family are found in the Americas, with one glaring exception: the Fijian iguanas (Brachylophus). The origins of the four living iguana species in Fiji have been a mystery for researchers, considering all the reptiles’ closest relatives are an ocean away.
There were two main hypotheses, says Simon Scarpetta, an evolutionary biologist at the University of San Francisco: Researchers thought the iguanas either drifted on vegetation rafts to Fiji from the Americas, or extinct ancestors migrated a shorter distance from Asia or Australia.
Scarpetta and his colleagues got some clues when studying the evolutionary relationships of over 200 species of iguanas and related lizards, comparing thousands of points in their genetic instruction books. Fijian iguanas are most closely related to desert iguanas (Dipsosaurus dorsalis), which live in Mexico and the American Southwest, the team found.
These genetic results, combined with the geography of where iguana fossils have previously been found, point to ancestral iguanas rafting across the ocean from the shores of North America. The team thinks the lizards set sail between 31 million and 34 million years ago, possibly causing the evolutionary split between Brachylophus and Dipsosaurus.
That cruise across the Pacific would have been over 8,000 kilometers, the longest move among landlubber vertebrates. The closest contenders appear to be other lizard groups. Other animals, such as trapdoor spiders, have also been known to have made epic voyages.
Rayna Bell, an evolutionary biologist at the California Academy of Sciences in San Francisco, points out that it’s difficult to know the precise distance traveled, partially because other islands may have served as stepping-stones but since disappeared
“The deeper we go [back] in evolutionary time, we’re more limited in terms of what evidence is available to us, because the planet has changed so much,” she says.
Iguanas may be particularly well-suited for long trips at sea. Herbivorous iguanas may eat part of their raft while traveling or — like some species do today — go into a state of sluggishness and forgo eating altogether. They may also be resistant to heat and dehydration. Most iguana species alive today are also found on islands, and have been directly observed colonizing new islands via vegetation rafts in recent decades.
“If you had to pick a vertebrate group that could survive a rafting event across thousands of kilometers of open ocean, iguanas are a great choice,” says Scarpetta.
For Gyllenhaal, the findings illustrate how an incredibly rare, chance event — like iguanas successfully floating across an ocean — are more likely to play out as long as millions of years of time are available. “When you’re dealing with evolutionary time, you’ve got a lot of opportunity for these very small probability things to occur.”
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