SCIENCE LAB SCALES : FLOATING TO FIJI

Why did iguanas cross the ocean? They just got a little carried away. For decades, the native iguanas of Fiji and Tonga have presented an evolutionary mystery.

Every other living iguanas species dwells in the Americas. So how could a handful of reptilian transplants have ended up on two islands in the South Pacific, a vast ocean away?

In research published Monday in the Proceedings of the National Academy of Sciences, Dr. Scarpetta and his colleagues make the case that the ancestors of Fiji’s iguanas crossed on mats of floating vegetation.

Such a voyage across nearly 5,000 miles of open ocean would be the longest known by a nonhuman vertebrate.

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 in which s small creatures on land reach islands, said Hamish D. Spencer, an evolutionary geneticist in New Zealand who was not involved in the study.

Usually those are invertebrates, whose small size allows them to survive a long journey in an uprooted tree trunk.

Iguana species have made shorter crossings. In 1995 at least 15 iguanas were seen rafting 200 miles on hurricane debris from one Caribbean island to another.

And ancestors of the Galapagos island to another. And ancestors of the Galapagos Islands iguanas made the 600-mile trip from South America on bobbing vegetation.

But a crossing to Fiji is a much harder challenge. Dr. Scarpetta's team concluded that the Fijian iguana species - which belong to a distinct genus, Brachylophus - split off from their closest relatives around 30 million years ago, about the time volcanoes birthed the Fijian archipelago.

'' North America is the most probable area of origin for iguanas in Fiji, and overwater rafting is the best supported mechanism,'' Dr. Scarpetta said.

The World Students Society thanks Asher Elbein.