After a 5,000-mile float, these iguanas probably set an ocean record

Yahoo

17-03-2025

About 34 million years ago, a group of iguanas went on an epic journey. This lofty band of reptiles traveled about 5,000 miles from the western coast of North America all the way to Fiji. Biologists believe that this is the longest known transoceanic dispersal of any land-based vertebrate. The findings are detailed in a study published March 17 in the journal Proceedings of the National Academy of Sciences (PNAS).
There are more than 2,100 species in the suborder Iguania. This large group includes other reptiles including chameleons, bearded dragons, and horned lizards. The Western Hemisphere family of lizards are the green ones that most people think of when they picture an iguana. There are 45 different species of Iguanidae in the Caribbean and the tropical, subtropical and desert areas of North, Central, and South America, including the marine iguanas of the Galapágos and the chuckwallas in the American Southwest.
However, the four iguana species found on the Pacific islands of Fiji and Tonga are a bit of an outlier. They sit there in the middle of the Pacific Ocean and scientists have long debated how they got there.
Overwater dispersal–where terrestrial organisms go from one land mass to another via a body of water–is the primary way that newly formed islands are populated with plants, animals, and even humans. This process often leads to the evolution of new species and ecosystems.
This study suggests that the original ancestors of Fiji's iguanas coincided with islands' formation by volcanoes. Scientists estimate their arrival at roughly 34 million years ago based on genetic evidence. Fiji iguanas (Brachylophus) and their closest relatives, the North American desert iguanas (Dipsosaurus) show signs of genetic divergence.
Biologists initially proposed that the Fiji iguanas may have descended from an older lineage that was initially more widespread around the Pacific, but has since died out. Another theory was that iguanas traveled from tropical parts of South America and then through Antarctica or even Australia. While there is no genetic or fossil evidence to support these earlier theories, a new genetic analysis does.
'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, either post-dating or at about the same time that there was volcanic activity that could have produced land,' Simon Scarpetta, a study co-author and University of San Francisco paleontologist and herpetologist, said in a statement.
'That they reached Fiji directly from North America seems crazy,' study co-author and University of California, Berkeley herpetologist Jimmy McGuire said in a statement. 'But 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. 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.'
Today's sailors can typically reach Fiji from California in about one month. However, it would take a group of iguanas a bit longer. The reptiles must hop on some flotsam, ride through the doldrums, and across the equator to Fiji and Tonga.
Fortunately, iguanas are large and herbivores and can go long periods of time without food and water. Their 'rafts' were also made from uprooted trees that would have provided them with food to eat along the way.
'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.
Based on some fossils found in eastern Asia, biologists believed that some now extinct populations of iguanids lived around the Pacific Rim and island-hopped their way to the middle of the Pacific Ocean. They may have used the Bering Land Bridge to journey to cross over from North America and then through Indonesia and Australia, or followed the Humboldt Current along the Pacific coast of the Americas. Earlier genetic analyses of some iguanid lizard genes were inconclusive about how the Fiji iguanas are related to those found elsewhere.
'Different relationships have been inferred in these various analyses, none with particularly strong support,' McGuire said. 'So there was still this uncertainty about where Brachylophus really fits within the iguanid phylogeny. Simon's data really nailed this thing.'
Scarpetta collected genome-wide sequence DNA from more than 4,000 genes from the tissues of over 200 iguanian specimens from museum collections. The genetic data showed that the Fiji iguanas are most closely related to the iguanas in the genus Dipsosaurus. The most widespread species within the genus is the North American desert iguana, which is adapted to life in the searing heat of the deserts of the American Southwest and northern Mexico. Other species within this genus are native to Santa Catalina Island in the Sea of Cortez.
'Iguanas and desert iguanas, in particular, are resistant to starvation and dehydration, so 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 [4,970 mile] journey across the Pacific on a mass of vegetation, a desert iguana-like ancestor would be the one,' Scarpetta said.
The genetic analysis determined that both lineages–Brachylophus and Dipsosaurus–diverged about 34 million years ago. This revised analysis does not align with the earlier theories of the origin of the Fiji iguanas.
[ Related: Pink Iguana hatchlings spotted for the first time on the Galápagos in decades. ]
'When you don't really know where Brachylophus fits at the base of the tree, then where they came from can also be almost anywhere,' McGuire said. 'So it was much easier to imagine that Brachylophus originated from South America, since we already have marine and land iguanas in the Galapagos that almost certainly dispersed to the islands from the mainland.'
This new analysis rules out the idea that the iguanas originated in South America. Additionally, because the Fiji Islands themselves emerged from the sea also about 34 million years ago, the iguanas might have landed on the islands just in time not long after. Other Pacific islands aside from Fiji and Tonga may have also harbored iguanas. However, volcanic islands disappear as quickly as they appear, so some evidence of other Pacific Island iguanas may have been lost.
The team will continue to analyze genome-wide data for Iguanian lizards to better understand their evolutionary relationships and learn more about their interactions through time and space.