Squirrels winning at outwitting trees' survival strategy

In the Dec. 22 edition of Science, Andrew McAdam, an assistant professor of fisheries and wildlife at Michigan State University, outlines how red squirrels have figured out a way around the elaborate ruse trees have used to protect their crops of tasty seeds.

The remarkable part: The squirrels are divining the arrival of bumper crops of spruce cones months before the cones ever materialize and then betting on those crops with the most expensive evolutionary collateral – a second litter of pups.

“We’ve been watching a co-evolutionary arms race where the trees and the squirrels are constantly trying to outwit each other,” McAdam said. “The trees’ strategy to outwit the squirrels has been documented, but now we’ve documented a counter strike by the squirrels.”

The study, funded in part by the National Science Foundation and the Natural Sciences and Engineering Research Council of Canada, allows scientists to track the push and pull of evolution against an unforgiving struggle for survival.

McAdam, Stan Boutin from the University of Alberta in Edmonton, Canada, and other international collaborators have been studying two different types of squirrels, which share the ‘red squirrel’ name but are only distantly related, in the forests of Canada, Belgium and Italy. The study has been going on for 20 years, with each squirrel carefully tracked.

The red squirrels on this side of the ocean dine almost exclusively on the seeds of spruce cones, which they gnaw off before the seeds ripen. The squirrels are thus a major enemy of the spruce trees. If the squirrels become too successful, their hoarding and gorging on the cones thwarts the trees’ ability to cast seeds and reproduce.

So the battle continues.

Over the years, the trees devised a strategy called masting in which they unpredictably – every few years ­– produce an overabundance of cones. Boutin, the study’s lead author, said it’s called a “swamp and starve” strategy to take predators by surprise, flooding the market with more seeds than the squirrels could harvest. The starve part of the strategy comes from the few cones that are produced in the years between bumper crops.

Swamp and starve is designed to play to the squirrels’ weakness. Squirrels are territorial and need not only food to feed growing pups, but also a place for their offspring to call their own. When there are lots of cones, there is plenty to feed pups, but the catch is that adults also survive well so there aren’t many vacancies for young squirrels searching for a home.

“If you produce lots of pups after the mast, there’s no place for them to go.” McAdam said. “To be successful they need to produce the pups ahead of time.”

What the latest paper shows is that the squirrels are on to the trees’ game, McAdam said. Prior to masting seasons, the squirrels are producing second litters – an act that would be, well, squirrelly in seasons of low food supply.

But a population boom that coincides with a seed boom is brilliant. More food means more places for juvenile squirrels to set up shop.

“What we’ve shown is that the squirrels also are playing an adaptive strategy,” McAdam said. “They’re not just following along; they’re making reproductive decisions that are best for them. This is about how red squirrels make a living in a variable world and how they make reproductive decisions in what seems like unpredictability. And given the currency they use, the stakes are high.”

Boutin speculates that buds the spruce trees put out the summer before cones develop may hold the clues to a masting event. These buds differentiate in the summer; some become tree branches, some develop into cones. The squirrels eat the buds and may detect which destiny they’ll follow thus getting a tip-off to an upcoming masting event.

“I hope this gives people a better appreciation that these squirrels are not just taking what they get,” McAdam said. “Both players – the squirrels and the trees – are following these strategies to try to maximize their fitness.”