Stressed Squirrels Produce Faster Growing Offspring
April 19, 2013

Stressed Squirrels Produce Faster Growing Offspring

Alan McStravick for — Your Universe Online

Expectant mothers are often advised to limit their exposure to stressful situations. It´s been long documented how the introduction of stress hormones to the human infant in the gestational period can severely affect the pregnancy, producing earlier births and lowered birth weights.

But, and this may come as no surprise, squirrels are not humans. What was surprising for an international group of researchers, led by University of Cambridge post-doctoral researcher Ben Dantzer, was how an increase in stress for a squirrel mother led to offspring that are decidedly stronger and heartier.

The new study, conducted over six years and published recently in the journal Science Express, states the parenting of squirrels is tailored specifically to the varied environmental situations the mothers find themselves and their offspring in.

Pregnant and recently pregnant female squirrels pay particular attention to social cues, according to Andrew McAdam, a professor in the University of Guelph´s Department of Integrative Biology.

“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. Females can predict increased crowding by listening for territorial defense “rattles,” which get louder and more frequent as the prevalence of available nesting gets more limited. In this situation, a squirrel mother will produce more stress hormones that, in turn, causes their pups to grow faster.

“When there are lots of squirrels around, only the fastest-growing squirrels survive,” continued McAdam. “But when population density is low, all squirrels survive well, so how quickly they grow doesn´t matter.”

Also contributing to the study was University of Alberta biologist Stan Boutin. “You try to prepare your kids for what they´ll face when they get out in the big, real world,” stated Boutin. “Female squirrels in our study were doing that too for their offspring.”

In order to conduct this study, the researchers essentially tricked the squirrels into believing they had more neighbors than they actually did. This was achieved by playing recorded squirrel calls over loudspeakers throughout the forest. Boutin contends an increase in communication among squirrels is correlative to an increase in population density and, therefore, competition to establish and maintain a specific territory.

Additional testing of the hypothesis was achieved when the team provided peanut butter for the squirrel mothers that had been laced with stress hormones. Mothers who consumed the peanut butter also raised faster-growing pups when compared to the study´s control females.

Researchers were able to ascertain the increase in the stress hormone cortisol by determining an increase in levels of the hormone in the droppings of females who had been exposed to the audio recordings.

“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. Boutin continued, “A key challenge for squirrels is the population density of other squirrels and the food supply they all depend on.”

Professor Amy Newman, also of the University of Guelph, commented, “What was remarkable 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.” Continuing, she said, “It proves that complex ecological and physiological factors — and not simply resources — affect reproduction and maternal behavior.”

McAdam points out this phenomenon only occurs when expectant mothers perceive future environmental crowding. “In a changeable world, they need to be flexible in their parenting and adjust to current conditions,” he stated.

The team claims their findings go against the long-held theories regarding stress and pregnancy. Until this study, the belief was that increased stress in a mother could only be negative, yielding lower birth rates, smaller litters and smaller babies within those litters.

Boutin finishes by describing the fieldwork conducted for this study as elegant. “The females are doing something for the future,” he says. “It´s an adaptive and predictive response showing females are running the show in terms of preparing their offspring for what they´re going to face.”

Other researchers contributing to this study were Rudy Boonstra of the University of Toronto-Scarborough, Rupert Palme of the University of Veterinary Medicine in Austria, and Murray Humphries of McGill University.