The Mystery of Fiji and Tonga's Iguanas

The presence of native iguanas in Fiji and Tonga has puzzled scientists, as nearly all other iguana species are found in the Americas. New research by Simon G Scarpetta and colleagues proposes a possible solution to this mystery.

Rafting Hypothesis

• Rafting: The proposed method by which Fiji's iguanas arrived is known as rafting, where animals travel on floating vegetation across oceans. 
  • While common among small creatures and invertebrates, rafting is less so among vertebrates.
  • Lizards and snakes raft more effectively than mammals due to slower metabolism.

Evidence of Rafting

• In 1995, green iguanas were observed rafting over 300 km in the Caribbean.
• The ancestors of Galápagos iguanas likely made a 1,000 km journey from South America.

Alternative Theories

• Some scientists believe Fiji's iguanas might be remnants of an extinct group that traveled over land from the Americas to Asia or Australia.

Genetic Analysis and Timing

•  Scarpetta’s research involved genetic sampling of 14 living iguana species. 
  • Fijian iguanas are closely related to the genus Dipsosaurus, found in the American Southwest and northwestern Mexico.
•  The divergence of Brachylophus and Dipsosaurus is estimated to have occurred 30-34 million years ago, coinciding with: 
  • The formation of the Fijian archipelago.
  • Climate conditions that would prevent iguanas from reaching Asia or Australia over land.

Supporting Factors

•  A three- to four-month ocean crossing aligns with iguana winter hibernation periods, allowing survival without starvation. 
•  Floating vegetation would provide a consistent food source for the iguanas. 

Conclusion and Significance

This study supports the theory of overwater rafting as the most likely mechanism for iguana dispersal to Fiji, aligning with growing evidence that long-distance dispersal plays a significant role in the evolutionary history of many species.