Morphing mice respond to warming climate

Fifty years ago, two species of mice looked different than they do today, and the likely cause is the warmer winters they’re experiencing, according to new research.

In the forested woodlands south of Montreal, researchers looked at white-footed mice (Peromyscus leucopus) and deer mice (Peromyscus maniculatus), two of the most common mouse species across North America. They compared data collected and archived in a collection at the Redpath Museum at McGill University as far back as 50 years ago to mice that they trapped recently.

Both species changed the shape of their skulls, said Virginie Millien, an associate professor at McGill University and the lead author of the recent study published in Evolutionary Ecology.

“There are parallel changes in the two species,” she said. “That’s why we think that it’s the same common driver, and the most obvious culprit is climate change.”

It is an extremely short amount of time to see such distinct changes in the species, Millien said, and it’s likely also happening in other taxa as well as they shift their ranges northward to adapt to climate changes. 

As part of the study, Millien and her colleagues took pictures of the species’ skulls —old museum specimens as well as newly collected samples — under microscopes and measured their shape, including how wide, tall and thick each skull was.

She found that both species now have longer noses than they did 50 years ago. The position of their teeth also changed.

Why would climate change result in longer noses? The nose doesn’t just help mice smell, Millien said. It also contains light bones that help regulate their body temperature. A longer nose could help them adjust to warmer days.

The position of the mice’s teeth also changed, she said, shifting backward relative to the skull. This change, she said, is likely due to changes in their chewing or biting. Since climate warming has an effect on vegetation, Millien said vegetation changes might have led to changes in their teeth positions over 50 years. “The fact that they both do it tells me it’s in response to changes in their habitat and their food,” she said.

The changes in skull structure have made it easier to tell apart the two species than it was 50 years ago, Millien said, due to their increased direct competition. “When you put two species that are very similar they compete for the same resources,” she said. “They tend to evolve morphologically so they can reduce the competition pressure between them.”

The proportions of deer mice and white-footed mice present in the area also seem to have switched. In the 1970s, researchers noted that they collected 90 percent deer mice and 10 percent white-footed mice. Millien found the opposite. “It looks like the white-footed mouse is moving up north,” she said.

“They’re all shifting their range north and it’s happening fast,” Millien said. “In the short term, we’re gaining species, but there’s a biodiversity paradox. We all know that’s not sustainable. So the question is, what species is going to have to accommodate for that?”

Header Image: Deer mice are one of two species that have shown changes in their skull shape over the past 50 years. ©Gregory “Slobirdr” Smith