As parents, we set our children up for life’s challenges by feeding them, caring for their health, and sending them to school. Many animals also provide for their young, and some do it in inadvertent and surprising ways. Take the red squirrel. The first year of this cute creature’s life is marked by intense competition and imminent death. Its goal is simple: get a territory before winter sets in, or face death by starvation. That goal becomes tougher if there are lots of other competitors around, but a mother squirrel has ways of preparing her pups for these trials. If she hears the sounds of crowded forest, stress hormones surge through her body and begin affecting her pups even before they are born. When they finally pop into the world, they start growing faster. The hormones are a chemical message from mum: Live fast, so you don’t die young.

Red squirrels have a diverse diet, but the seeds of the white spruce are among the most important items on their menu. These trees are “mast seeders” meaning that they all produce tons of seeds every 2 to 6 years and very few seeds in others. In the bonanza years, the squirrels have plenty of spruce seeds to bury in the autumn and snack on through the harsh winter. This means that a bumper autumn for spruce is always followed by a crowded spring for squirrels. That leads to conflict. Red squirrels defend large areas surrounding a central store of spruce cones. They’ll fight for this real estate, because if the youngsters don’t establish a territory before winter sets in, they’ll find themselves bereft of buried grains, and almost certainly die. A team of Canadian scientists has been studying the fates of red squirrels in the Yukon, Canada for the last 22 years. Back in 2006, they showed that the squirrels can actually anticipate gluts of spruce seeds, and produce more young in anticipation. Now, the same team have found that females can also adjust how quickly their young grow up depending on how much competition they will face.

The team, led by Ben Dantzer, now at the University of Cambridge, found that in competitive years following a spruce bonanza, the fastest-growing squirrels fare best and are more likely to survive their first winter. In normal years, these fast-growers have no advantage. This explains why the mothers don’t always give birth to fast-growing young. It comes at a price—squirrels tend to have shorter lifespans if they are born in crowded years, so their early spurt seems to cost them later on in life. It’s a cost that’s only worth paying if there’s some advantage to be gained. To test this link between density and growth, Dantzer’s team carried out a wonderfully simple but beautiful experiment where they simulated the sounds of a crowd. They recorded the loud “rattles” that squirrels make to defend their territories, and played them back to the creatures at two levels—one representing six times more squirrels than the other. If the females heard what sounded like squirrel-infested woods, they gave birth to pups that grew much faster.

How? The answer lies in the mothers’ hormones. The more crowded an area is, the higher the levels of cortisol - a stress hormone - in the squirrels’ bodies. Indeed, if Dantzer’s team created more crowded forests - either genuinely so by feeding the squirrels and boosting their numbers, or dishonestly so by playing the rattle recordings - the females’ cortisol levels went up. Mums with higher cortisol levels gave birth to pups that grew faster. This wasn’t just a correlation. The team also injected some females with extra cortisol, and saw that their offspring grew up 41 percent faster.

Why? This part of the story is less clear. We know from other studies that and early dose of stress hormones can steer the development of young animals, affecting their ability to learn or handle stressful situations. Think of the hormones as an fortune-telling system, plugged into the mother’s senses. It provides the young squirrel with omens of the conditions it will face in the outside world, and prepares its brain and body to deal with those challenges.