Q: Do squirrels lose weight during the winter?

A: There are a lot of factors in play, but according to a couple experts, squirrels likely will lose weight during the winter.

Ed Heske, a retired mammalian ecologist at the Illinois Natural History Survey, said, “I’m sure they lose weight.”

“Body mass change = energy in (from food) – energy out (for thermoregulation, activity, maintenance, etc.),” Heske wrote in an email from New Mexico.

Steve Sullivan, director of the Hefner Museum of Natural History at Miami University in Ohio, runs projectsquirrel.org.

“All squirrels try to get as fat as possible in the fall,” he said. “They are usually much skinnier in the spring than they were in the fall.”

But it depends on several variables.

“A tree squirrel’s spring weight depends on the number and diversity of tree species in its neighborhood, the precipitation of the previous year, the number of competing squirrels and other nut eaters (like deer and turkey), the distribution and number of warm nests, the predator population and their parasite load,” he said.

Squirrels store acorns and walnuts to survive the winter, as they have fat, protein and other nutrients squirrels need.

“Tree squirrels use this fat as energy as well as insulation, and it has to be maintained by eating the cached nuts from earlier in the year,” Sullivan said. “They might skip a day’s eating if it is too wet or windy, but for the most part, they need to eat and drink each day.”

Squirrel weight can depend on the type of trees in the area.

“Not all nuts are the same,” Sullivan said. “For example, work by Steele et al. shows that white oak acorns germinate in the fall and are low in chemical defenses, so squirrels eat them immediately. However, red oaks are tannin-rich and germinate in the spring, so they can be cached for later use.”

When it’s cold, Sullivan said squirrels may share a nest to stay warm, but that can also spread disease.

After a warm winter, squirrels will likely be fatter than they otherwise would be, but Sullivan said warm weather could also cause the stored nuts and acorns to germinate early or rot.

“Also, a warm winter one year can lead to a poor nut crop the next. So fatter squirrels this spring might be followed by skinnier or fewer next spring,” he said.