Zoology: of rats and men - a tale of extinction

Zoologists have turned a scientific spotlight on what they call the "forgotten mass extinction of mammals" - the loss within the past 500 years of 15 to 20 diverse rat-like species living on Caribbean islands, some of which grew as big as cats. All died out as a result of European colonisation.

Researchers from the Natural History Museum and Zoological Society of London used the latest DNA technology to analyse remains of these rodents in museums and archaeological collections. The results, published in Proceedings of the Royal Society B, demonstrate a richer and more varied evolutionary history than anyone had expected.

The rodents, known as rice rats, or oryzomyines, reached the West Indian islands in two main waves within the past 10 million years - probably by floating on rafts of vegetation washed out to sea from rivers on the northeast shoulder of the South American continent. Once there, they underwent rapid evolutionary change in what biologists call an insular radiation. Some expanded to cat-like dimensions, others shrank to mouse size.

On many islands, rice rats were the only land mammals. They survived low-density pre-European settlement. "Archaeological evidence shows that some species were a widespread source of food for the Amerindians," says Sam Turvey of London Zoo.

But Europeans were too much for them, transforming the landscape through deforestation, sugar cultivation and urbanisation - and, worse still, introducing alien competitors such as black and brown rats and mongooses.

The last Caribbean rice rats died out at the beginning of the 20th century. Turvey says their mass extinction rivals in its magnitude the better-known, post-settlement loss of Australian mammals.

"We might not expect rodents, which are widespread, fast-breeding and adaptable, to be vulnerable to extinction," he says. "However, while rice rats were once found on many islands in the Caribbean and were even more diverse than we originally thought, they were not able to survive the threats caused by human activity. Their complete disappearance highlights the vulnerability of many species that have evolved on islands, and demonstrates that huge amounts of biodiversity are potentially at risk of human-caused extinction."

Selina Brace of the Natural History Museum adds: "Hot climates damage ancient DNA, so working with extinct Caribbean rodent samples was incredibly challenging. Despite the pain, it was worth it when we found such an unexpected level of diversity in this group, meaning we could highlight this forgotten radiation of extinct rodents."

Photograph: Paul Gervais/Natural History Museum

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