OREANDA-NEWS. Over the last 50 years, milder winters have led to physical changes in two species of mice in southern Quebec, Canada, researchers report. The findings provide a textbook example of the consequences of climate change for small mammals.

The research also reveals a stark reversal in the proportions of the two mice populations present in the area, adding to evidence that warming temperatures are driving wildlife north.

At the McGill University Gault Nature Reserve, about 25 miles east of Montreal in the St. Lawrence valley, biologist Virginie Millien has been studying two similar, coexisting species for the past 10 years: the deer mouse and the white-footed mouse. Both are common in eastern North America. But while the deer mouse can be found in Canada’s northern reaches, the white-footed mouse is a more southerly species, rarely found north of the St. Lawrence River.

By comparing data from the past decade with specimens researchers collected as far back as the 1950s, Millien’s team discovered that the skull shapes of both mouse species have changed over time.

The changes in the two species paralleled each other, but have been more pronounced in the white-footed mouse—with the result that the cranial shapes of the two species have become more distinct.

At the same time, the white-footed mouse has been moving farther north as winters get milder—at a rate of around 6.8 miles a year, the researchers estimate. While nine of 10 specimens caught in the reserve by researchers in the 1970s were deer mice and only 10 percent were white-footed, those proportions are now reversed, according to findings by Millien’s team.

‘Bones and teeth’

“Evolutionary theory predicts morphological changes in response to climate warming, but there is very little evidence for it so far in mammals,” Millien says.

These changes may be related to a dietary shift caused by climate change, combined with competition for food resources between the two species of mice, according to the researchers. A shift in the position of a molar tooth in both species, for example, could reflect changes in the type of food that the mice need to chew.

One question that remains to be settled is whether the changes are genetic, and will be passed on to future generations—actual evolution—or whether they represent “plasticity,” the capacity of some species to adjust to rapid environmental change.

In either case, the physical changes—although difficult for untrained observers to discern—are significant.

“We are talking about bones and teeth, hard structures that are not easy to bend,” Millien notes.

The findings add to the few documented cases of rapid responses by wildlife to climate change, such as Rosemary and Peter Grant’s studies of finches in the Galápagos archipelago for four decades starting in 1973.

By careful measurements of the population of two species on one tiny island over the course of major weather changes such as El Niño events and droughts, the Grants were able to show that evolutionary changes in beak size and body size can occur in as little as a couple of years.