Voyage of the iguanas: How they travelled from Americas to Fiji

Such a voyage across nearly 8,000 km of open ocean would be the longest known by a non-human vertebrate.

An incredible challenge

Rafting — the term scientists use for hitching a ride across oceans on uprooted trees or tangles of plants — has long been recognised as a way for small creatures to reach islands. But this is generally seen among invertebrates, whose small size means they can survive a long way in an uprooted tree trunk. Among vertebrates, lizards and snakes seem to be able to raft farther than mammals, perhaps because their slower metabolism allows them to fast for a long time.

Iguana species have proved adept at making shorter crossings. In 1995, scientists observed at least 15 green iguanas rafting more than 300 km on hurricane debris from one Caribbean island to another. And researchers have long agreed that the ancestors of the iguanas of the Galápagos Islands made the nearly 1,000 km trip from South America on bobbing vegetation.

A crossing to the South Pacific islands, however, represents an almost unimaginable challenge. This has led many scientists to argue that the iguanas were the remnant of an extinct group, one that had possibly crossed over land from the Americas to Asia or Australia, and then made the relatively easier crossing to Fiji and Tonga.

Timing is everything

Scarpetta’s team tackled the question by trying to work out when Fijian iguana species — which belong to a distinct genus, Brachylophus — split off from their closest relatives.

After sampling the genetics from 14 living iguana species, the researchers found that the Fijian species’ closest living relatives were the genus Dipsosaurus, a group of desert iguanas found in the American Southwest and northwestern Mexico. The team’s analysis suggested that the two genuses split around 30 million and 34 million years ago.

That timing is important. This was roughly the time when the Fijian archipelago was born. More importantly, the cold and ice around the poles at that time would have made it impossible for any lineage of temperature-sensitive iguanas to make it to Asia or Australia from the Americas, and then hop to the Pacific islands.

This means that overwater rafting is the “best supported” mechanism for explaining how the American reptile landed in Fiji.

This argument is further bolstered by the fact that a three- to four-month crossing would have been roughly the length of creatures’ winter hibernation, meaning the lizards could have made the voyage without starving. Also, travelling on vegetation would have provided the herbivores with a steady source of food.

Evolutionary genetecist Hamish G Spencer of the University of Otago said the study adds to a growing body of research suggesting that “long-distance dispersal is far more important in the evolutionary history of many animal groups than had previously been appreciated.”