Though cute and furry on the outside, North American red squirrels have the souls of warriors. Each adult will claim a small section of forest, and fiercely defend its territory against all others. And the most successful fighters owe it all to stressed-out mothers.

Human mothers are advised to avoid stress, but that advice doesn't translate well to the world of squirrels. New research shows that red squirrels (Tamiasciurus hudsonicus) born to mothers whose systems are pumped up with stress hormones are able to outcompete other pups for food and territories.


It's quite easy to tell when you've stumbled upon a red squirrel's domain — if you're not immediately accosted by the defensive rodent's vocalizations (called "rattles"), you'll likely see its midden, or huge cache of white spruce cones. The red squirrel's territoriality likely stems from this precious food cache, which it loves above all else and needs to survive the harsh winter. In fact, the animal will only bother to socialize when it's time to make babies.

When a female is in heat, males flock to her territory. "Then they have what we have called a 'mating chase,'" says Ben Dantzer, an evolutionary ecologist at the University of Cambridge in the U.K. "The female plays games and things like that, and either she lets them mate with her or not."

Thirty-five days later she gives birth to a litter. About 70 days after that, the now-juvenile squirrels are already out on their own, battling it out with other juveniles for vacant territories. Squirrels that don't secure a domain and stockpile food inevitably die of starvation when winter comes around.


Red squirrels love their white spruce cones. Courtesy of Ryan W. Taylor.

As you can imagine, things get dicier when there are a lot of other squirrels to compete with, which happens from time to time. White spruce trees are "mast seeders," meaning that they produce a lot of seeds in some years and very few seeds in other years. Somehow, the squirrels are able to anticipate high-production years and will have more babies accordingly, Dantzer told io9. But here's where get really interesting: Pups born in years where there's a large population of other squirrels nearby will grow up faster than pups born in low-density years.


But what exactly is going on here? Dantzer and his colleagues, who have been studying these squirrels in Canada for 22 years, hypothesized that the squirrel mothers somehow set their children up for success during pregnancy. And given that you see quick-growing pups mostly in high-density years, the mothers must be able to tell when there are a lot of other squirrels around.

To unravel the mystery, the team began by tricking the mothers: They simulated high-density conditions by playing rattles through loudspeakers. Compared with non-manipulated mothers, mothers who heard the fake rattles believed they were surrounded by a large number of other squirrels (about six times as many squirrels than in control conditions), causing them to give birth to faster-growing pups in the absence of extra food.


The red squirrel's territorial rattle. The researchers simulated a high density of squirrels by playing these sounds through speakers. Courtesy of Andrew McAdam.

And when the researchers looked at the females' levels of glucocorticoids, they found that the tricked mothers were producing a lot more of the stress hormones than their non-stressed peers. In other tests, the scientists fed the squirrels the stress hormones and found that those mothers produced babies that grew 41 percent faster than babies produced by normal mothers.

To sum it up: Crowded forests stress out red squirrels, causing pregnant females to produce more glucocorticoids, which in turn help their offspring grow up faster. Interestingly, the faster growing rate is only beneficial in high-density years, when there's more competition for food and territories. In low-density years, quick-growing pups are no more likely to survive to the next year than their slow-growing competition.


"What we've also seen is that those offspring that are born in high-density years tend to have shorter lifespans," Dantzer says. So although the stress hormones help the juveniles survive their first winter, it cuts down the number of winters they'll see in their lifetime.

It's not yet known just how the stress hormones are causing this physiological effect. Whatever the case, the mothers are helping their children thrive, not completely unlike human mothers. "They anticipated what types of characteristics would lead to the highest survival for their offspring, and they prepared their offspring for that type of environment," Dantzer says. "And they were able to do all that without additional access to food."

The scientists' work was published recently in the journal Science.

Top image via Ryan W. Taylor.