When the woods get crowded, female squirrels improve their offspring's odds¹ of survival by ramping² 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 hormone³ production during pregnancy⁴, 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, formerly⁵ with MSU's zoology⁶ 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 dynamics⁷ took a true team effort. It was equal parts field physiology⁸, 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 recordings⁹ of territorial¹⁰ vocalizations, or rattles¹¹, to create the illusion of a big population of squirrels.

 

Females reacted to the increased commotion¹² by producing more stress hormones¹³ 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 conclusively¹⁴ 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.

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