Chipmunks in Wisconsin toughed out ice age.

At the beginning of the last ice age about 40,000 years ago, when most animals were heading south, some chipmunk populations stayed in northern refuges, scientists say.

Researchers tallied the mutations in mitochondrial DNA extracted from small tissue samples from 244 eastern chipmunks, Tamias striatus, captured at 25 sites in Wisconsin, Illinois, Indiana, and Michigan. Results suggest that the most recent common ancestor of all those chipmunks lived about 200,000 years ago, says Ken N. Paige, an evolutionary biologist at the University of Illinois at Urbana-Champaign.

However, the DNA analyses indicate that chipmunks living in the westernmost swath of the four-state region are more closely related than those living at eastern sites. The most recent ancestor of the western-zone chipmunks probably lived less than 50,000 years ago, says Paige.

In the eastern area, chipmunk populations weren't related closely enough for rates of DNA mutation to indicate migration patterns of the animals' ancestors. But in the western swath, the genetic variability in the chipmunks is telling, and it's lower at southern sites than at northern ones.

That pattern, among others, hints that the chipmunks repopulated that area from north to south when ice retreated at the end of the last ice age. Such an emigration, says Paige, suggests that some chipmunks rode out the last ice age in a hospitable zone in Wisconsin and then moved south. He and his colleagues report their findings in the July 13 Proceedings of the National Academy of Sciences.--S.P.