Gambling squirrels win big when it comes to evolutionary fitness

Imagine hearing the winning numbers for the Powerball lottery, but you didn’t know when those numbers would be called, just that sometime in the next 10 years or so, they would be. Despite the financial cost of playing those numbers on a daily basis during that period, the reward is great enough to be worth it.

Animals that live in highly variable environments play a similar lottery when it comes to their Darwinian fitness, or how well they can pass on their genes. In a new study led by the University of Michigan, scientists found that red squirrels that gambled on the breeding game outperformed their counterparts, even if it cost them in the short term.

Natural selection favors female squirrels that have large litters in years when food is plentiful because they contribute many babies to the gene pool, said Lauren Petrullo, lead author and a National Science Foundation postdoctoral researcher in biopsychology at the University of California. Michigan.

“We were surprised to find that some females have large litters in years when there won’t be enough food for their babies to survive the winter,” he said. “Because it is biologically expensive to produce offspring, we wanted to know why these females make what appears to be a mistake in their reproductive strategy.”

The red squirrels studied live in the Canadian Yukon and experience a “mast year” or boom on their main food source, the seeds in the cones of white fir trees, once every four to seven years. Squirrels forecast the big food harvest before it occurs and increase litter sizes in the months before, ensuring better future survival for their babies and better fitness for themselves.

“There is a constant tug-of-war between the trees and the squirrels at our study site,” Petrullo said, “with each player trying to trick the other for their own physical gain.”

Petrullo and Ben Dantzer, a UM associate professor of psychology and of ecology and evolutionary biology, used data collected by the Kluane Red Squirrel Project, a 34-year collaborative field study involving UM, the University of Colorado, the University of Alberta and the University of Saskatchewan.

“Every year, we collect data on how many babies the squirrels produce and how many fir cones the squirrels eat,” Dantzer said.

The scientists quantified the reproduction of female squirrels during food booms and busts, and discovered differences in their physical state, whether they played with their reproductive strategy or not. While some squirrels played it safe by keeping litter sizes small each year, those that took a “pie-in-the-sky” approach by having large litters, even when food was scarce, enjoyed increased fitness for life if they got to experience a year of mast, the research showed. .

Unlike the Powerball example, however, the squirrels are not guaranteed to eventually win.

“In some ways, this strategy of betting on litter size is like playing with fire,” Petrullo said. “Because the average lifespan of a squirrel is 3.5 years and masts only occur every four to seven, a female could be sabotaging her fitness by having too many babies in lean years, hoping to a mast when you can die before you get to experience a mast. This could be quite expensive.”

Alternatively, for the squirrels, the cost of not betting on the replay game at all can be insurmountable if they end up missing out on the chance to hit the jackpot.

“It’s essentially impossible for a female to recoup the fitness costs of not increasing reproduction in a mast year, so the stakes are high,” Petrullo said.

Females that increased litter sizes in years of food scarcity suffered a short-term impact on their physical condition. But they were more likely to increase litter sizes if and when they experienced a mast, taking home the ultimate prize of increased lifetime reproductive success, she said.

The squirrels’ best bet, according to the researchers, is to take a chance and suffer short-term fitness costs to avoid the unmatched cost of missing out on the fitness jackpot entirely.

“Determining the relative costs of different types of errors is key to understanding why animals make what we think are mistakes,” Petrullo said.

Scientists aren’t yet sure how squirrels can forecast future food production. The animals may be eating parts of the fir trees that affect their physiology and alter the number of babies they produce, Dantzer said.

“This is exciting because it suggests that the squirrels are eavesdropping on the trees, but we still have a lot more to do to solve this puzzle,” he said.

Because many animals use cues about things like food in their environment to make reproductive decisions, and the reliability of these cues is declining due to global climate change, scientists also wonder how the costs of these types of errors will alter what is the best reproductive strategy. .

“If the predictability of a food boom is reduced and squirrels can no longer predict the future, this could affect the number of squirrels in the boreal forest,” Dantzer said. “This could be problematic given that squirrels are preyed on by many predators.”

The research, “Phenotype-environment mismatch errors enhance lifelong fitness in wild red squirrels,” appears in Science.

Study co-authors are Stan Boutin, University of Alberta; Andrew McAdam, University of Colorado; and Jeff Lane, University of Saskatchewan.