How squirrels play with their genetics

How squirrels play with their genetics

For the tiny red squirrels of North America, jumping from treetop to treetop isn’t the only daring feat these little rodents perform. The little creatures also take big reproductive risks that could lead to big rewards when it comes to genetic fitness. It’s all a game of how well they can pass on their genes to the next generation.

A study published January 19 in the journal Science found that red squirrels that gambled more in the breeding game outperformed their more cautious cousins, even if there were higher short-term costs. The bet? That next spring will give a lot of seeds, and having a lot of offspring will not be a burden, since there is plenty for everyone.

[Related: Scientists confirm that squirrels are amazing gymnasts.]

It’s a bit like knowing what the winning lottery numbers will be, but having no idea when they will be called. While spending money on lottery tickets with those numbers may cost you money in the short term, the rewards will be greater in the future when you hit the jackpot. According to study co-author and University of Michigan biopsychology researcher Lauren Petrullo, natural selection favors female squirrels that have large litters in years when there is plenty of food to go around.

“We were surprised to find that some females have large litters in years when there will not be enough food for their young to survive the winter,” Petrullo said in a statement. “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 study looked at red squirrels living in the Canadian Yukon for a mast year. These are booms in a major food source every four to seven years. For squirrels, this buffet comes in the form of seeds from the cones of white firs. Squirrels can guess if it’s going to be a year before it happens and increase litter size and reproductive efforts in the months before so that there is a better future for their babies to survive and better genetic fitness for them.

Squirrels play too, but with their genes
North American red squirrel cubs in the Yukon, Canada. They are about 25 days old. CREDIT: Erin Syracuse

“There is a constant tug-of-war between the trees and the squirrels at our study sites with each player trying to trick the other into improving their own fitness,” Petrullo said.

The study used data from the Kluane Red Squirrel Project, a 34-year collaborative field study involving the University of Michigan, the University of Colorado, the University of Alberta, and the University of Saskatchewan. They observed 1,000 female squirrels from the project, collecting data on litter sizes and how many fir cones the squirrels eat.

“Each year, we collect data on how many babies squirrels produce and how many fir cones squirrels eat,” said Ben Dantzer, co-author and associate professor of psychology and ecology and evolutionary biology at the University of Michigan, in a statement.

The team watched female squirrels reproduce during food booms and busts and found differences in their genetic fitness whether or not they played with their reproductive strategy.

Some played it safe by keeping litter sizes small during leaner years, while others took a more optimistic approach and kept large litters even during leaner years. Squirrels plus “glass half full” had better lifetime fitness if they got to experience a mast year, the research showed.

However, it turned out to be a bet that 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 at all. This could be quite expensive.”

Not betting on the reproductive game does not seem to be a viable option for squirrels. If they lose their chance to win the jackpot, those who didn’t play could pay a heavy price.

“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.

According to the team, the best bet is for the squirrels to take a chance and suffer short-term fitness costs to avoid the huge cost of missing out on the genetic fitness jackpot entirely.

[Related: Why counting Central Park’s squirrels isn’t nuts.]

It is not yet clear how squirrels can forecast future food production in their environments. According to Dantzer, it may be that they are eating parts of the fir trees that affect their physiology and change the number of offspring they produce.

“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,” Dantzer said.

Other animals, such as some migratory songbirds, also use clues about the amount of food in their environment to make reproductive decisions. Climate change is causing the reliability of these signals to decline, and scientists aren’t sure how the costs of such errors will change the best reproductive strategy for squirrels.

“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 as squirrels are preyed on by many predators.”

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