U. of Michigan study: Red squirrels win gamble on evolutionary fitness

Red squirrels that gamble with their reproduction strategy outperform their counterparts in terms of Darwinian fitness, even if it costs them in the short term, according to a recent study by researchers at the University of Michigan.

The study was published in “Science” and co-authored by Stan Boutin from the University of Alberta, Andrew McAdam from the University of Colorado, and Jeff Lane from the University of Saskatchewan, according to a University of Michigan news release.

The researchers studied red squirrels in the Canadian Yukon, that experience a "mast year" once every four to seven years, during which their main food source, white spruce tree seeds, booms, the release stated. The squirrels forecast the large mast crop of food and increase litter sizes in the months prior, which enhances survival for their babies and better fitness for themselves.

"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," said the study's lead author, Lauren Petrullo, a National Science Foundation postdoctoral research fellow in biopsychology at the University of Michigan. 

"Because it's biologically expensive to produce offspring," he added, "we wanted to know why these females make what appears to be an error in their reproductive strategy."

The researchers found that females who increased litter sizes in low-food years took a short-term hit to their fitness. Yet they were more likely to increase litter sizes if and when they experienced a mast, taking home the ultimate prize of greater lifetime reproductive success.

Meanwhile, some squirrels played it safe by keeping litter sizes small each year.

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

The scientists quantified the reproduction of female squirrels during both food booms and busts, discovering differences in their fitness, whether they gambled with their reproductive strategy or not, according to the release. 

The researchers suggest that, for squirrels, the cost of not gambling at all in the game of reproduction can be insurmountable if they miss their shot at the jackpot. 

The squirrels' best bet, according to the researchers, is to take their chances and suffer short-term fitness costs in order to avoid the unmatched cost of missing the fitness jackpot completely.

Scientists remain unsure exactly how the squirrels can forecast future food production. The animals may be eating parts of the spruce trees that affect their physiology and alter the number of babies they produce, Dantzer said.

"This is exciting because it suggests that squirrels are eavesdropping on the trees, but we still have much more to do to solve this puzzle," Dantzer said.

Due to global climate change, many animals' cues about matters like food in their environment are becoming less reliable, which raises questions about how the costs of these types of errors will alter what the best reproductive strategy is, the release said.