WASHINGTON (AP) – Chipmunks may not be big, but some of them were pretty hardy. New research indicates a group of the minute mammals toughed it out through the last ice age rather than migrating south.

A study of chipmunks living in Illinois and Wisconsin indicates that most of them descended from ancestors who survived the glaciers in isolated pockets of northern forest.

Kevin C. Rowe, a doctoral student at the University of Illinois, said the researchers were surprised at the findings. Their work, based on samples of DNA from 244 chipmunks, is reported Monday in the online edition of Proceedings of the National Academy of Sciences.

The mitochondrial DNA, which is inherited from the mother, indicated that the animals came from 95 groups and that 78 of those groups descended from ancestors living in the north and west.

Scientific theory has held that most animals would flee southward to escape the encroaching glaciers, but that appeared to be the case for only a minority of the chipmunks.

During the last glaciation there were pockets of tundra and forest that were bypassed by the ice, Rowe explained, leaving potential homes for animals.

They would have had to survive there for a substantial time, he noted, perhaps 5,000 years. The last glaciation reached its greatest extent about 18,000 years ago.

Those pockets are identified by rock formations that indicate the region was never covered with glaciers. They are in an area called the driftless region, which includes parts of Minnesota, Iowa, Wisconsin and Illinois.

Rowe, lead researcher for the paper, said it is also surprising that the chipmunks in Illinois and Wisconsin are closely related to one another, but are only distantly related to chipmunks in Indiana and Michigan.

The research was funded by the American Museum of Natural History, the American Society of Mammologists and the National Science Foundation.