Stressed moms give babies a boost, squirrel study suggests

Having a stressed-out mom may give baby squirrels a competitive edge, a new study suggests.

Red squirrels who were stressed out during pregnancy had babies that out-competed their peers by growing significantly faster without any extra food, reported the study, published online in Science Express.

"What that suggests is that they're first able to predict what sort of environment their offspring will encounter… and they're preparing them for what their offspring are going to face," said Ben Dantzer, lead author of the study he worked on while he was a Ph.D. student at Michigan State University under the supervision of Guelph University biologist Andrew McAdam.

'The hardest part of the study was spending hundreds of hours grinding up fecal samples.' —Ben Dantzer, researcher

Further investigation uncovered a link between faster growth among the baby squirrels  and higher levels of stress hormones in their mothers during the pregnancies.

That link may explain how environmental conditions cue the animals to respond and adapt.

Canadian researchers, including Stan Boutin at the University of Alberta, Murray Humphreys at McGill University in Montreal and McAdam at the University of Guelph, had been studying red squirrels near Kluane Lake, Yukon, for 22 years to find out how they are affected by changes in resources such as food over time.

They had noticed that in the years when food such as white spruce seeds were more widely available, there was a much higher population density of squirrels — crowded conditions that the territorial squirrels find stressful. Among the babies born in those years, only the fastest growing ones survived.

That left researchers wondering whether it was the higher population density that prompted the baby squirrels to grow faster. The tricky part was being able to show that any changes in growth rates weren't caused by greater availability of food.

Squirrels tricked with recordings

To tackle that problem, Dantzer played recordings of calls that squirrels use to defend their territories, tricking the squirrels into thinking their forest was more crowded than it actually was. For these squirrels, the growth rates of their offspring were compared to the growth rates of those whose mothers didn't hear the extra audio recordings.

To confirm whether the recordings stressed the squirrels out, Dantzer, with the help of University of Toronto researcher Rudy Boonstra, collected squirrel feces and analyzed them for the stress hormone cortisol.

"I would say the hardest part of the study was spending hundreds of hours grinding up fecal samples," recalled Dantzer, who is now a postdoctoral researcher at the University of Cambridge.

As expected, squirrels who thought they were living in crowded conditions had higher levels of cortisol in their feces.

The researchers also fed peanut butter laced with cortisol to some pregnant squirrels to boost their levels of the hormone, and found that their babies grew 41 per cent faster than the babies of squirrels that were fed plain peanut butter.

That bolstered the evidence that it was the increased cortisol levels that had caused the offspring of the stressed mother squirrels to grow faster.

Dantzer said the results came as something of a surprise because researchers had previously thought baby animals always grow as fast as they can according to the amount of food available. The data suggest that baby animals usually aren't growing as fast as they possibly can, but only do that when they really have to.

He added that researchers aren't sure why that is the case, but the results suggest that the faster growth rate may have a cost to either the mother or the babies. For example, there is some evidence that reproducing more often can cause some physiological stress to mother squirrels, and that offspring born in years when the squirrel density is high don't live as long.