Visiting Assistant Professor Oliver Pergams at the University of Illinois-Chicago and population geneticist Robert Lacy of the Chicago Zoological Society compared the genetic makeup of 115 white-footed mice in the Volo Bog State Natural Area northwest of Chicago using mitochondrial DNA taken from collection samples as old as 150 years and mice collected in recent years.

They found a new type of mouse replaced the old type in Volo Bog between 1976 and 2001.

"The new mice were genetically very different," said Pergams.

He and University of Illinois-Chicago Professor Mary Ashley reported in 2001 about similar morphological changes in size and shape occurring during the past century involving deer mice on three different California Channel Islands and black rats from two Galapagos Islands.

While Pergams found the coincidental changes surprising, he said it is too soon to say if that is somehow related to world climate change.

Pergams and Lacy report the new findings in the journal Molecular Ecology, available now online and in print in late December.