Published online 15 October 1998 | Nature | doi:10.1038/news981015-3


Iguanas ride the waves

Raft-riding green iguanas that reached the Caribbean island of Anguilla in the wake of a hurricane have provided ecologists with rare proof of one of the most debated theories of animal colonization of islands. As Ellen J. Censky from the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania and colleagues report in the 8 October edition of Nature, at least 15 green iguanas (Iguana iguana) rived on the eastern beaches of Anguilla on a large mat of logs and uprooted trees, shortly after autumn hurricanes in 1995.

The iguanas were first spotted by fishermen on Anguilla on 4 October, coming ashore from a raft of uprooted trees, some of which were more than 30 feet long. The best guess for the iguanas’ origin is the island of Guadeloupe, a few hundred miles to the southeast, where they occur naturally. Hurricanes Luis and Marilyn swept up through the Caribbean in September 1995, passing Guadeloupe on their way to Anguilla. Given the generally north-west set of the currents in the region, Guadeloupe seems a more likely origin than the nearer but more northerly Virgin Islands.

How various terrestrial animals have become dispersed around the Caribbean islands has been a controversial topic since the turn of the century, with land bridges and over-water transport both being invoked. Many believed that over-water transport - which would mean rafting on floating debris - would be a highly improbable means of dispersal for animals larger than insects. The few accounts of larger vertebrate animals found on sea-going rafts have been of single individuals, who could not have established a new population even if they did reach land.

Green iguanas are found throughout South America and in the more southerly islands of the Lesser Antilles, but Anguilla itself had no indigenous green iguanas so this is a rare natural experiment in animal distribution. In particular, the rafting refugees formed a big enough group to be able to colonize the island. When surveyed by the ecologists a month after their arrival, both males and females were found, so the group could potentially breed. The most recent sighting was in March 1998, nearly 30 months after the iguanas first arrived, and was of a female in reproductive condition.