Eastern chipmunks (Tamias striatus) have upset the apple cart of assumptions on glacier-driven population migrations. Based on a mitochondrial DNA analysis of 244 chipmunks, it seems the majority of them living in Illinois and Wisconsin today descend from ancestors who survived the last North American ice age in what researchers believe were isolated pockets of forestland amid the cold tundra.
The findings -- reported online this week (July 12-16) ahead of regular publication by the Proceedings of the National Academy of Sciences -- came as a surprise to researchers at the University of Illinois at Urbana-Champaign and the Illinois Natural History Survey.
They found that 78 of the 95 haplotypes (groups of individuals with similar sequences of base pairs of genetic material) identified in mostly the Wisconsin and Illinois populations clearly descend from survivors in the west and north, closer to the Wisconsin glaciation. Over time, these chipmunks migrated south from the colder region, merging with chipmunks that migrated into the region from the warmer east and south.
"This is counter-intuitive given that organisms would be expected to respond to glacial expansion by shifting their ranges to more suitable climates most often in a southern refuge followed by a northward recolonization as the glaciers receded," said Kevin C. Rowe, lead author and doctoral student in the evolutionary/molecular biology laboratory at the U. of I.
"It also is particularly surprising that while chipmunks in Illinois and Wisconsin are closely related, they are distantly related to chipmunks in Indiana and Michigan," he said. "There really is no clear geographical barrier at present that should lead to their isolation, so chipmunk history may be responsible. From our data, this history appears to include colonization of the Midwest from multiple sources such as separate ice-age refugia."