Smart Squirrels

Study: squirrels over spruces in evolutionary race
Matthew Miller
Lansing State Journal
January 18, 2007

In the evolutionary arms race between squirrel and spruce tree, scientists once thought trees had the upper hand.
But it turns out that the squirrels are no chumps.


That's according to a new study by Michigan State University professor Andrew McAdam and a team of international scientists, published in the Dec. 22 edition of Science.AdvertisementSpruce trees have evolved a sophisticated strategy for protecting their seeds from fluffy-tailed predators.The phenomenon is called "masting." Most years, the trees will produce only a few cones, starving the squirrels and giving themselves an evolutionary leg up.Then, unpredictably (to human scientists, anyway), every spruce tree in a given area will produce a bumper crop, too many for the squirrel population to gobble up in a single season.But McAdam and the other researchers, who looked at nearly two decades' worth of data on American red squirrels in the Yukon, found the squirrels are not so easily outwitted.They have learned to predict these bumper crops and respond by having a second litter of pups in those years."What people used to think is that the squirrels and the other seed predators were just sort of following along and being taken advantage of by this clever strategy on the part of the trees," McAdam said."What we've documented here is evidence that the squirrels have evolved a counterstrategy."
'Real-time' evolution
McAdam, a professor in the departments of Zoology and Fisheries and Wildlife, is interested in how evolution happens in "real time," not over millennia, but over decades.And what he and his colleagues have found is that squirrels produce their extra litters not when they can, but when it makes good evolutionary sense.McAdam and his colleagues tried providing squirrels with extra food to see if they could induce them to produce second litters. They couldn't.That, he said, is because it's not a question of the mothers having enough food to produce more offspring, but a question of the young squirrels' chances of survival down the road."The short story is the offspring survive much, much better when they're produced in one of these bumper food years, and they survive very poorly in the years following the bumper food years," he said.Extra food in a year without a bumper crop of cones might allow a female squirrel to "produce lots and lots of pups," he said, "but they have nowhere to go."
'Extremely exciting'
John Koprowski is a professor of wildlife conservation and management at the University of Arizona and directs efforts to monitor and recover the federally endangered Mt. Graham red squirrel.He called McAdam's finding "extremely exciting.""The complex co-evolutionary relationship that the ... research team uncovers between squirrels and their major food source once again demonstrates the incredible amount of interaction and interdependence between component members of our forests," he said.He added that the study also "points towards our need to fully understand the complexities of the ecosystems that we endeavor to manage and conserve."
Not 'passive players'
McAdam and his colleagues still don't know how the squirrels know when the trees will produce extra cones.They could be getting hormonal cues by feeding on the trees' buds, he said.They could have learned to decode whatever the cue is that allows trees to synchronize their increased cone production.But the simple fact that they know is significant enough."The important message," McAdam said, "is that the squirrels and presumably other seed predators are not just passive players in this game."