Washington, July 31 - The head shape and overall size of rodents have been changing over the years. Now, an ecologist has tied these changes to human population density and climate change.

Oliver Pergams, assistant professor at the University of Illinois, Chicago, said that such size-and-shape changes in mammals, occurring in less than a century, are quite substantial.

He had earlier done studies on a century's worth of anatomic changes between two geographically isolated rodents -- Channel Island deer mice from coastal California and white-footed mice northwest of Chicago -- and noted fast change among both.

'I suspected they weren't unique examples,' he said. 'I wondered whether these changes were occurring elsewhere, whether they were global in nature, and what some of the causes may be.'

Pergams examined specimen rodents from museums around the world, including the big collections held at Chicago's Field Museum and the Smithsonian in Washington.