Crossing five thousand miles of open ocean is not an easy task, but this is exactly what iguanas living on isolated Pacific islands appeared to have accomplished. Now scientists have shown that iguanas living on the islands of Fiji and Tonga in the Pacific Ocean may have walked there when the islands were still connected to the mainland.
Iguanas. These ones don't look active enough swim that far... Image: Hypothesisnow
The alternative – drifting across the Pacific from South America on rafts of rotting vegetation– would require incredible luck and endurance. The new theory provides an enticing alternative, although ‘rafting’, as it is known, cannot be completely ruled out.
Fiji and Tonga were once part of the supercontinent Gondwana, which also included Australia, Antarctica, Africa, and parts of Asia. Fossil iguanas from Mongolia confirm that Gondwana did possess populations of iguanas, so it would have been easy for some to become stranded on islands when the supercontinent broke up.
This hypothesis was strengthened when researchers looked at the DNA of the iguanas living on Fiji and Tonga. They found that the iguanas split from other iguana species around 60 million years ago – before the islands were isolated from the continental mainland.
This just leaves one puzzling question: why don’t we find any iguanas on other Pacific islands which once belonged to the supercontinent?
The answer to this is much closer to home. When humans arrived on the other islands they adapted to local food sources – and ate the iguanas into extinction. Fiji and Tonga were settled much more recently, so their iguana populations have survived intact. This is supported by fossil evidence from other islands: sudden iguana extinction occurs just as people began to colonise the islands!
Noonan, B.P. and Sites Jr., J.W. (2010). Tracing the Origins of Iguanid Lizards and Boine Snakes of the Pacific. The American Naturalist, 175:1, p 61–72. DOI: 10.1086/648607
