Island Rodents Naturally Selected for Size
Observations of mainland rodents and their nearby island counterparts show one obvious difference: size. Now research confirms what islanders have often seen firsthand – giant mice and rats, who often become predatory.
This phenomenon occurs when rodents are left on islands previously free of their kind. They quickly evolve to much larger sizes. For example, mice living on remote Gough Island, located roughly 3,000 miles off the South African coast, were introduced there by British sailors in the 1800s. In the relatively short span of two hundred years, their average size has doubled.
Researchers from Duke University, led by biologist Paul Durst, found a few cases of rodent species becoming smaller but the overall trend was that island life equals monstrous rats and mice.
Dr. Durst noted a possible natural selection process at work, after analyzing over 1,000 rodent populations of 60 different species.
“The predominance of extremes of larger size could suggest that a form of ‘immigrant selection’ was at work, with greater success of large-bodied individuals surviving dispersal over water,” he noted.
In some areas of the world, the large rodents are a problem. Lacking natural predators and living in a food-rich environment, they can become a threat to larger mammals.
The differences in rodent size on various islands probably have to do with food resources. Mice that have evolved to be tiny on some island outposts may not have had access to much food.