Ben Dantzer, formerly with Michigan State University's zoology department and now a postdoctoral researcher at the University of Cambridge in England, said the study showed for the first time how females' use social cues to correctly prepare their offspring for life outside the nest.
The study, published in the journal Science, confirmed red squirrel mothers boosted stress hormone production during pregnancy, which increased the size and the chances of survival of their pups.
"Natural selection favors faster-growing offspring, and female red squirrels react accordingly to increase their pups' chances of survival," Dantzer said in a statement. "Surprisingly, squirrels could produce these faster growing offspring even though they didn't have access to additional food resources."
The research team based much of its study on the Kluane Red Squirrel Project, a 22-year-long study on North American red squirrels living in the Yukon led by researchers from the University of Alberta, University of Guelph and McGill University in Montreal.
In the field, researchers used recordings of territorial vocalizations, or rattles, to create the illusion of a big population of squirrels. Females reacted to the increased commotion by producing more stress hormones while pregnant and their pups grew faster.