Squirreling It Away

How Does a Furry, Gray Forest Creature Remember Where It Hid All Those Nuts?

by Kathleen Scalise

If finding your keys in the morning is a challenge, be glad you're not a gray squirrel, which must locate the hundreds of nuts it buries each winter. Just how this small creature keeps track of its far-flung stash has perplexed scientists for decades and is the focus of a new Berkeley neuroscience study.

"These squirrels are not putting any flag there; they are not smelling the nuts; they are really remembering the exact location of their nuts," said neuroscience researcher Pierre Lavenex. "They use information from the environment, such as the relative position of trees and buildings, and they triangulate, relying on the angles and distances between these distant landmarks and their caches."

Such a feat would not be possible for humans, said Lavenex. "People can do this for a few sites, maybe six or seven, but not for nearly as many as squirrels do," he said.

Researchers on the project looked for neurological changes in a species called the American gray squirrel, Sciurus carolinensis, and reported their findings Oct. 25 at a meeting of the Society for Neuroscience in New Orleans.

"Every fall when the trees begin to turn red in the Northern hemisphere, many animals cache a generous food supply on which they will rely to pass the long, harsh winter," said Lavenex. "Some species, such as the American gray squirrel, forage throughout the forest looking for nuts they will store. One peculiarity of the gray squirrel, however, is that it does not place all of its nuts into a 'single basket,' its nest for example." Although easy to remember and relocate, such a rich hoard would need to be guarded from pilferers.

Instead, each autumn these squirrels bury nuts singly, never more than one per cache, and end up with food stashed in hundreds of different sites.

"The squirrel thus ensures it will not lose everything in a single theft, but it must pay the price by having to remember where all those nuts were cached," said Lavenex, who collaborated on the study with Assistant Professor of Psychology Lucia Jacobs and M. A. Steele of Wilkes University in Pennsylvania.

Some birds, such as the black-capped chickadee, exhibit similar behavior when storing seeds for the winter.

Scientists had previously discovered that in these birds the size of the hippocampus, a brain structure critical for memory, varies by season, growing larger in the fall when caching is due to begin.

However, after examining 30 wild adult gray squirrels from Pennsylvania, the Berkeley researchers found no similar growth in the brains of squirrels.

"Our results suggest that mammals have evolved a different strategy to cope with long-term, repeated increases in memory load," said Lavenex.

"The avian model may represent a particular case of adult brain plasticity related to learning that is not expressed in mammals."

What that mammal model might be is still in question, but the hippo-campus does tend to be significantly larger in rodent species that choose to bury their nuts in many different locations rather than one or a few big hoards.

However, more detailed analysis of the hippocampus, now under way on the project, may show subtle anatomical differences from one season to the next, unrelated to overall size.

For instance, the number of neurons or the density of the connections between neurons might account for the memory feats of squirrels.

Lavenex and his colleagues believe squirrels can teach humans something about how memory works.

Though the exact role of the hippocampus remains relatively poorly understood in humans, damage has been shown to result in amnesia for some stroke victims and in memory loss for those with Alzheimer's disease.



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