Squirrels that gamble on reproduction often end up winning the bet

In this episode of Michigan Minds, U-M associate professor of psychology and ecology &  evolutionary biology Ben Dantzer, PhD and National Science Foundation postdoctoral biopsychology research fellow at U-M Lauren Petrullo, PhD detail their research project, which studies how squirrels reproduce in years in which food is abundant and when it is not, and the long-term impact of those behaviors. The researchers found that some female red squirrels have large litters in years when there won’t be enough food for all their pups to survive the winter and wanted to understand why. 

Ben Dantzer and Lauren Petrullo on Michigan Minds

“This study was born out of a desire to really understand why we sometimes see animals make what we consider to be mistakes, things like breeding too early or too late in a particular year or searson or having too many or too few offspring in a particular year or season,” Petrullo, lead author of the study, says. “We expect natural selection will be constantly working toward promoting the most appropriate or favorable response an animal can make, and so it is a little confusing when we see animals do the ‘wrong’ thing and we wanted to try to figure out why this happens.”

For this project, the scientists studied squirrels in the Canadian Yukon, a highly fluctuating environment that causes the animals to have to cope with extreme changes in their preferred food source: seeds from the cones of white spruce trees. Dantzer and Petrullo explain that while the seeds are only available in the fall, squirrels have their litters in the spring and summer months, leading to a guessing game for the adult animals. There are indications, the researchers say, that squirrels are able to predict times of food surplus or ‘mast’ years by eating different parts of the trees and essentially “eavesdropping on the trees.”

“Squirrels are a model system in ecology and evolution primarily because we can accurately measure Darwinian fitness — the survival and reproduction of animals — and this is often tricky to do. With squirrels, we can track individuals over their entire lifetime and we can measure fluctuations in their natural environment.”


“Over time, I think that all of us grow to appreciate squirrels and have this strong fondness for our animals.” 

The research ultimately indicated that it is better for a squirrel to take a chance on having a large litter, even if she isn’t sure if it will be a mast year, since her lifespan likely only allows for the chance at one abundant food year. 

“What our work shows is that sometimes, those mistakes — the instances in which squirrels produce large litters but it is NOT a mast year — can actually be beneficial in that it makes them more likely to get it right if the future environment becomes favorable,” Petrullo says.

“The squirrels are gambling with their reproduction, and it’s really cool to see this kind of ‘pie in the sky’ strategy in nature.”


Dantzer adds that squirrels are an essential component of many ecosystems, and studying them allows scientists to track individual animals over their entire lifespan to determine why some squirrels may live longer and why some produce more offspring. 

“Understanding the causes of variability in squirrel populations is important to understand what the potential consequences are for ecosystem health,” he says. 

Dantzer and Petrullo conducted the study, which was published in Science, with co-authors Stan Boutin, University of Alberta; Andrew McAdam, University of Colorado; and Jeff Lane, University of Saskatchewan.