Published April 18, 2013
The wilds of the Yukon may seem like a strange place to study stress.
But a long-running project in a spruce forest near Kluane National Park has found that when it comes to survival of the fittest, stress can be a good thing.
Females squirrels, when stressed by life in the woods, improve their pup’s odds of survival by making them grow faster, according to the latest study from the Yukon “squirrel camp” published in the journal Science on Thursday.
“Despite the widespread perception that being stressed is bad, we found that high stress hormone levels in mothers can actually help their offspring,” said lead author Ben Dantzer, of Britain’s University of Cambridge.
He and his colleagues from several universities found pups of stressed mothers grew 41 per cent faster than the offspring of unstressed squirrels. And incredibly, the pups grew faster even tough they didn’t have access to extra food. “It was quite surprising,” said Dantzer.
The red squirrels in the Yukon have proven to be ideal research subjects. Unlike mice and lab rats, they are still wild creatures.
But the squirrels stick close to home – “they generally settle within 100 metres of where they are born,” said Dantzer, who spent many summers as a PhD student at the camp getting to know the creatures.
The squirrels are amenable to being handled, and sport metal tags and coloured pipe cleaners on their ears that make them easy to identify and follow around.
Research teams, led by biologist Stan Boutin at the University of Alberta, have been able to create a “pedigree” of the thousands of squirrels that have lived in the forests over the last 22 years. It has enabled the biologists to track the family dynamics and the population in the research area, which varies from 400 to 800 squirrels depending on the year.
One study published in 2006 showed the squirrels are so finely tuned to the ecosystem that they ramp up reproduction in years when spruce trees, their source of food, are going to have a bumper crop of cones. That way there are lots of extra pups to feast on the bounty.
The biologists have also long puzzled over another curious phenomenon – in years when there is a high density of squirrels the females produce pups that grow faster and have a better chance of surviving the first year of life.
Dantzer, who worked at the camp as a graduate student from the Michigan State University, was intrigued. He decided to run a social experiment for his PhD project to try tease out what was making pups grow faster when the woods got more crowded.
He strung loud speakers in the spruce trees and started broadcasting the “rattles” or territorial calls of other squirrels throughout the day. This created the illusion that there had been an invasion of new squirrels.
Then he and his colleagues studied the impact on females that were already pregnant when the racket started. With babies on the way and what seemed to be a lot of new noisy neighbours threatening to gobble up the food supply, the scientists found the females’ stress hormone, called glucocorticoid, started to climb.
To assess the impact on the pups the researchers put radio collars on pregnant females and followed them back to the nests.
When the pups were born the researchers gingerly took them from their nests and weighed them on the first day or two of life. The pups were weighed again at 25 days, just before they left their nest for the first time. Data on almost 60 females and almost 200 pups revealed that pups born to stressed mothers with elevated glucocorticoid levels grew 41 per cent faster than pups born to unstressed mothers with normal glucocorticoid levels. The fast growth early in life gave the pups a big advantage once they got out in the world, and more survived their first year than slow-growing pups.
“In this case stress is incredibly good as it essentially prepares the babies for the environment they are going to experience,” saidstudy co-author Rudy Boonstra, of the University of Toronto.
Exactly how the stress hormone speeds up the pup growth is still not clear but the brains of some of the young squirrels are now in Boonstra’s lab in Toronto undergoing testing.
The researchers said other animals likely make similar adjustments in stressful conditions to increase the growth rates of their young to boost their chance of survival early in life.
But the scientists said there is a downside. The fast-growing squirrels have a better odds of surviving their first year, but they don’t tend to live as long as slow-growing squirrels born in low-density years.
“This suggests that their successful early sprint burns a few matches that would have helped them in the marathon of life,” the researchers said.