Many scientists believe that global warming will prompt animals and plants of the Northern Hemisphere to shift their ranges northward (or, in the Southern Hemisphere, southward). But the history of the eastern chipmunk, Tamias striatus, shows that such climate-driven shifts can follow unpredictable patterns, says Ken Paige, an animal biologist at the University of Illinois at Urbana-Champaign.
Paige and his colleagues found that when glaciers rolled through the American Midwest some 18,000 years ago, the eastern chipmunk stayed put rather than migrating southward to warmer areas. The animal inhabited a large driftless region of forestland that went unscathed at the time.
The evidence came from mitochondrial DNA that researchers collected from 244 modern chipmunks in Wisconsin, Illinois, Indiana, and Michigan. Analysis showed that the Wisconsin and Illinois chipmunks were most closely related; nonetheless, the Wisconsin population showed the greatest genetic diversity, suggesting they had stood their ground through the Ice Age, whereas the Illinois chipmunks reached that region more recently.
The more distant ties to chipmunks in Indiana and Michigan suggested a merging of multiple distinct populations about 200,000 years ago, most likely because of habitat change. However, their origins and their migrations remain unknown. “There were very complex patterns of movement,” says Paige. “Some were north. Some were south. Some were from Lord-knows-where.”
Why the Wisconsin chipmunks sat through the Ice Age is equally unclear. The murkiness points to the difficulty of forecasting precisely what effects global warming will have. “The predictability just isn’t there,” Paige says. “Probably anything goes.”