Squirrels time litters to anticipate food booms

A food-chain fight between red squirrels and spruce trees demonstrates how evolutionary biology can win out over ecology, according to a study conducted in the Yukon.

Ecological models suggest that when food resources rise or fall, animals will produce more or less offspring the following season.

But red squirrels have developed an ability to anticipate a rise in spruce seed production before it happens, researchers found. This helps counterthe trees' strategy of varying seed production from year to year to starve predators. The squirrelsproduce an extra litter of babies in boom years.

University of Alberta professor Stan Boutin and his international team of researchers published their findingsFriday in the journal Science.

Boutin has been following the population dynamics of the squirrels since the 1980s and was startled one year to discover an explosion in both the number of squirrels and the number of spruce cones produced.

"In 1993 there was a big cone crop, we're talking thousands of thousands instead of hundreds of thousands," he told CBC News Online.

"And at the same time, the squirrels were breeding bigger litters, and there were all of these characters running around. I thought, 'there's something bizarre going on here.' "

Further research through the years revealed the same pattern. Each time the trees produced more reproductive buds than vegetative buds, the squirrels would pick up on some cue to reproduce more aggressively in time for the appearance of the spruce cones containing the seeds.

What that cue is remains a mystery, said Butin, because the pattern governing the boom-or-bust seed production strategy of the trees isn't well understood. The trees typically produce an abundance of seed-bearing cones every four to seven years.

Boutin speculates the squirrels might be eating the buds and picking up on some difference in taste between reproductive and vegetative buds. Visually, the buds are nearly identical, he said.

"They look exactly the same," he said. "I can't tell them apart unless I use a microscope."

Squirrel baby boom

The tree's cycle from producing the buds to dropping the cones containing the seeds is about 18 months, while squirrels can produce babies within 35 days of mating. The short gestation period allows the squirrels to produce two litters in a single season.

While the squirrels' strategy for anticipating the seed production works, it doesn't mean they are winning the war over the spruce trees, said Butin.

The trees' "swamp and starve" seed production trick still produces more in boom years than the squirrels and other seed eaters can consume, and over the years remains a more effective way topropagate its seeds. If it didn't work, said Butin, the trees using the strategy would not have spread at the expense of other trees.

The red squirrels were ideal candidates for the study in part because they are highly territorial and operate in daylight, making them easy to track.