In a new University of Michigan study, researchers found out that red squirrels who “gamble” while producing offspring gained an evolutionary edge over their counterparts in the long term, even if it affected them negatively in the short run. Animals that live in fickle environments often play a lottery with natural selection based on how well they are able to pass on their genes. Typically, natural selection favours female squirrels that produce large litters when food is abundant because this means that they get to contribute a lot of babies to the gene pool, according to Lauren Petrullo, lead author of the study published in the journal Science. “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. Because it’s biologically expensive to produce offspring, we wanted to know why these females make what appears to be an error in their reproductive strategy,” said Petrullo, who is a postdoctoral research fellow at the university, in a press statement. The red squirrels that were studied lived in the Canadian Yukon region. An important source of food for them is the seeds from the cones of white spruce trees. Once every four to seven years, these trees have a “mast year: where they produce a much larger amount of seeds than usual. If the squirrels accurately forecast this mast year happening and increase their litter size in the preceding months, it would ensure better future survival for the babies and also the fitness of the mothers themselves. To investigate this, Petrullo and colleagues used data collected by the Kluane Red Squirrel Project and quantified the reproduction of female squirrels during both food booms and busts. They discovered that while some squirrels play it safe and keep their litter sizes small each year, some took a more audacious approach where they had large litters even when food was scarce. Interestingly, whenever the latter group got to experience a mast year, they enjoyed much greater lifetime fitness. “In some ways, this strategy of gambling with litter sizes is like playing with fire. Because the average squirrel lifespan is 3.5 years and masts only happen every four to seven, a female could potentially be sabotaging her fitness by having too many babies in low-food years, hoping for a mast when she may die before she ever gets to experience a mast at all. This could be pretty costly,” explained Petrullo. On the flip side, the cost of not gambling at all might lead to an insurmountable cost in the game of reproduction if those squirrels are not able to hit the mast year jackpot. When females increased their litter size in low-food years, it did affect their fitness in the short term. But if and when they experienced a mast, they won the greater prize of lifetime reproductive success.
