To give her offspring better odds in life, a mom can contribute all sorts of advantages: good genes, healthy milk, protection from predators, and more. Red squirrels—cousins of the more robust gray squirrels—are no exception. But new research in the journal Science shows that for these rodents, one such maternal boost stems from a surprising source: stress.

The research was conducted by a group of scientists who have spent the last 22 years studying a group of wild red squirrels in the Yukon. The ecosystem there follows an episodic pattern: every few years, when the spruce trees produce seeds en masse, the squirrel population booms in response. The team’s previous research had shown that in the years when squirrel numbers are especially high, fast-growing offspring tend to fare better than those that grow more slowly, presumably since they are better equipped to outcompete their rivals.

But how, exactly, do some squirrels end up growing so quickly?

Since female squirrels feed and care for their young, the researchers focused on how they responded to a booming squirrel population. They broadcast squirrel vocalizations over loudspeakers, signaling to wild squirrels that the environment was getting crowded. Once the female squirrels gave birth that season, the researchers monitored the offspring’s development by weighing the pups just after birth and again about 25 days later.

The offspring of female squirrels that had been exposed to the loudspeakers grew much faster than the offspring of control squirrels, which didn’t hear any extra squirrel vocalizations. In other words, a mom hearing extra squirrel calls before her pups were born somehow sped up their development.

Increases in growth rates are often attributed to concurrent increases in food availability, but that wasn’t the case here—this effect was seen even without the extra food resources that often accompany times of high population density. Instead, a mere social cue appeared to trigger the boost in the pups’ growth rate.

The key, the researchers realized, was that the females’ stress hormones—called glucocorticoids—were rising, thanks to the perceived overcrowding. A separate experiment, in which the scientists tested the effect of crowded conditions on stress levels, showed a positive relationship between population density and glucocorticoid levels: the more crowded the environment, the more stressed pregnant squirrels became.

And this increase in stress levels, according to the researchers, can affect pups’ development. To confirm this link, they experimentally boosted the stress levels of pregnant female squirrels by feeding them cortisol. The offspring of these stressed-out females grew 41 percent faster than the offspring of control females that hadn’t been fed the hormone, confirming that a mom’s stress—specifically, her glucocorticoid levels—can vastly change how fast her pups develop.

Stress is often considered to be a bad thing; we constantly hear about the health consequences of stress, and doctors often advise pregnant women to avoid stressful situations. But for red squirrels, stress brought on by social cues may be important to help offspring start off on the right foot. Stress plays an important role in many animals' lives, spurring them to seek out new territory or run from predators, among other things. But until now, there has been little evidence that a mother's stress can actually benefit her offspring.

There is no evidence that this stress harms the moms in this scenario; female squirrels that gave birth to fast-growing young didn’t appear to suffer any survival costs. However, there do appear to be ramifications for the pups. Fast-growing squirrels tend to die earlier than their slow-growing counterparts, suggesting that there are consequences for this advantage early in life.

Researchers don't yet know exactly how glucocorticoid levels in utero affect growth rate or why fast-growing pups don’t live as long. What is clear, however, is that stress may have a more intricate relationship with fitness than scientists previously thought.

Science, 2013. DOI: 10.1126/science.1235765 (About DOIs).