Feb. 3, 2006 — Humans who smell a pillow, shirt, shoe or other object that was in close contact with another person may be reminded of a certain someone. New research suggests squirrels have a similar ability to not only associate smells with particular squirrels, but to also create mental images of them.

The study, published in this month's Animal Behavior, represents the first time the ability has been demonstrated in rodents. A second, not-yet-published study by other researchers indicates hamsters also have the skill.

Like humans, squirrels must first be familiar with an individual before an odor can become associated with that other animal. A husband, for example, could smell his wife's perfume in an elevator and be reminded of her, but a perfume he has never smelled before could trigger no such memories.

"Squirrels need to be familiar with others to be able to put all of an individual's odors into a representation of that individual, as if repeated interactions make that individual meaningful, and thus worthy of remembering at this level," explained Jill Mateo, who conducted the research.

Mateo, who is an assistant professor in the Department of Comparative Human Development at the University of Chicago, told Animal Planet News that squirrels not only scent mark where they have been, but they also release odors from the mouth, ears, back, the bottom of their feet, and from their anal scent glands.

She first presented Belding's ground squirrels, Spermophilus beldingi, with each of these odors from different squirrels. Over time, squirrels became familiar with an odor and stopped sniffing it. When presented with a new odor from a different squirrel, the squirrel's interest piqued again.

Mateo explained that this means squirrels "produce unique odors and other squirrels can detect this difference."

Surprisingly, urine was not found to not be individually distinct. In other words, all of the other odors appear to convey information about individuals, but urine does not appear to carry this level of data, possibly because it contains too much waste material.

Mateo then presented her test squirrels with more than one odor from the same individual. After smelling one odor from that squirrel, the sniffers spent very little time investigating all other odors from that same animal, which indicates they could associate all of the smells with the one individual.

Mateo explained how humans do the same thing with all sorts of sensory cues.

"Think of someone you know," said Mateo. "Their eyes are familiar to you, their voice is familiar, their mouth may look familiar, but if you see or hear these stimuli, you don't think 'Sue's eyes' or 'Sue's voice,' you think 'Sue.' Due to your relationship with Sue, you put all of her characteristics into one representation in your mind — Sue."

She theorizes that animals besides squirrels and other rodents, such as birds, reptiles and other mammals, possess similar abilities.

For squirrels, smells seem to be more important when two animals are close to each other, but visual information kicks in over distances.

"Squirrels seem to know when someone cruising by is 'good' or 'bad,'" she said, and explained that squirrels might remember visual attributes, similar to how humans tend to remember faces.

Bob Johnston is a professor of psychology who directs the Johnston Lab at Cornell University. He worked on the upcoming hamster study. Johnston told Animal Planet News he thinks the new squirrel research is "very thorough and excellent."

He said, "I do agree that these experiments suggest mental representations of other individuals in hamsters and squirrels. Jill's paper is the first that I am aware of that shows this for rodents."

He hopes people will begin to recognize "the importance of social intelligence in animals, even ones that might seem to be rather simple and 'stupid.'"