When the woods get crowded, female squirrels improve their offspring's odds of survival by ramping up how fast their offspring grow. In a study led by Michigan State University and the University of Guelph (Canada), researchers showed for the first time how females use social cues to correctly prepare their offspring for life outside the nest. The results, published in the current issue of Science, confirm that 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," said Ben Dantzer, formerly with MSU's zoology department and now a postdoctoral researcher at the University of Cambridge (United Kingdom). "Surprisingly, squirrels could produce these faster growing offspring even though they didn't have access to additional food resources."

Proving that food availability isn't always the universal variable affecting population dynamics took a true team effort. It was equal parts field physiology, experimental ecology and longitudinal studies of natural selection that led to these findings, he added.

The 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 (Canada), Guelph and McGill University (Canada). Out 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. Dantzer, along with Amy Newman of the University of Guelph, also manipulated stress hormone levels in mothers to conclusively show that stress hormones were the cause of the faster growth rates of their pups.

"Despite the widespread perception that being stressed is bad, our study shows that high stress hormone levels in mothers can actually help their offspring," Dantzer said.