New research by McGill University biologists shows that milder winters have led to physical alterations in two species of mice in southern Quebec in the past 50 years – providing a textbook example of the consequences of climate change for small mammals.

The findings also reveal 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 McGill’s Gault Nature Reserve, about 40 kilometres east of Montreal in the St. Lawrence valley, biologist Virginie Millien for the past 10 years has been studying two similar, coexisting species: 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 collected by McGill researchers 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 11 kilometres 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% were white-footed, those proportions are now reversed, according to findings by Millien’s team, published recently in the journal Evolutionary Ecology.