Eastern Chipmunks Brave the Ice
July 2004
For the ancestors of the eastern chipmunk (Tamias striatus) there was no place like home, even if that meant sticking it out through the last ice age. As a huge sheet of glacial ice moved south from Canada, reaching its peak in the United States about 18,000 years ago, most temperate animals fled south in search of warmer climes, or so most scientists believed.
But a new study suggests that chipmunks living in Wisconsin and Illinois are related to animals that survived the ice in isolated patches of northern forest that scientists call “refugia.” The results, reported in the online edition of the Proceedings of the National Academy of Sciences, are based on an examination of mitochondrial DNA samples from 244 eastern chipmunks living in Wisconsin, Illinois, Indiana, and Michigan.
The DNA analysis uncovered 95 chipmunk haplotypes, or groups of individuals with similar genetic sequences. Seventy-eight of these haplotypes were traced to ancestral chipmunk survivors from the west and north. Rather than recolonizing northern regions as ice sheets retreated—the common scenario for the ancestors of many animals—the chipmunk DNA evidence suggests that T. striatus populations expanded southward from a northern glacial refuge.
Scientists were also surprised that, while chipmunks in Illinois and Wisconsin are closely related, they are only distantly related to those in Michigan and Indiana. Because there is no clear geographical barrier to explain these findings, the authors of the July 2004 study suggest that the Midwest may have been colonized from chipmunk populations living in separate ice age refugia.
Evidence for such ice-free forested patches comes from geological formations in a part of the central U.S. known as the “driftless region.” In this area, which includes parts of Minnesota, Iowa, Wisconsin, and Illinois, ancient land surfaces escaped the leveling effects of glacial scouring and scraping. Traditionally, scientists believed that small, shrubby tundra or taiga-like vegetation probably grew in the driftless region. But with the use of modern techniques, scientists have uncovered trace amounts of pollen from deciduous trees such as oaks in the region, suggesting that small stands of trees did indeed exist. And acorn-producing oaks would have provided a storable food source for isolated chipmunk populations. An analysis of chloroplast DNA from deciduous trees living near the southern limit of the ancient ice sheet also indicates that separate tree lineages may have existed closer to the ice than previously thought.
Taken together, the evidence suggests that hardy T. striatus were undeterred by cool glacial landscapes, settling instead on isolated forest pockets for their ice age homes.
Sources: Proceedings of the National Academy of Sciences 101:10355-10359 (2004), University of Illinois at Urbana-Champaign News Bureau.
