Stressed-out mothers raise stronger, heartier offspring – at least among squirrels.
In a new study, international researchers – including University of Guelph biologists – say squirrels tailor their parenting to meet the varied conditions facing their young.
For pups born during crowded, stressful times, mama squirrels kick maternalism into high gear. By the time they leave the nest, these offspring are significantly larger than pups raised under less stressful conditions.
The study was published Thursday in the journal Science.
Female squirrels listen to social cues during pregnancy and while tending their young, said Andrew McAdam, a professor in Guelph's Department of Integrative Biology. He conducted the study with Guelph biology professor Amy Newman and lead author Ben Dantzer.
Females listen to sounds of other squirrels. As crowding increases, territorial defence "rattles" get louder and more frequent. That causes mother squirrels to make more stress hormones, which makes their pups grow faster. "If they know the population is exploding, they must do what they can to produce fitter offspring, so that they can make it under such conditions," McAdam said.
Red squirrels' territorial behaviour enables them to survive through the winter. When there are a lot of squirrels around, it's harder to find vacant homes, McAdam said.
"When there are lots of squirrels around only the fastest-growing squirrels survive ," he said. "But when population density is low, all squirrels survive well, so how quickly they grow doesn't matter."
For the study, Newman and Dantzer, a post-doctoral researcher at the University of Cambridge and McAdam's former graduate student, tricked mother squirrels into believing that they had more neighbours than they really did by playing squirrel calls recorded in the forest over loudspeakers.
Dantzer and Newman then tested the effects of the stress hormones by feeding some mother squirrels peanut butter laced with stress hormones. Those mothers also raised faster- growing pups than control females.
"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.
The team studied groups of North American red squirrels over six years.
"What was remarkable," Newman said, "Is that the perception of high density and elevated maternal stress hormones boosted pups' growth rates as much as if the mothers had been fed extra food."
"It proves that complex ecological and physiological factors – and not simply resources – affect reproduction and maternal behaviour," she said.
However, McAdam added, squirrels ncrease their investment in their offspring only during crowding.
Similar principles probably apply to other animals. "In a changeable world, they need to be flexible in their parenting and adjust to current conditions," McAdam said.
Other researchers in the study were Rudy Boonstra, University of Toronto-Scarborough; Rupert Palme, University of Veterinary Medicine in Austria; Stan Boutin, University of Alberta; and Murray Humphries, McGill University.
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