Stress is a common part of modern life, and everyone has experienced its negative effects. Many people even suffer health impacts from chronic stress. So why do we respond this way to challenges in the first place?

To answer this question, it’s important to first understand that stress responses are not unique to humans—they’re shared by almost all animals. And it’s also important to understand how stress works in the body. In vertebrates, stress responses are coordinated by hormones called glucocorticoids. When an animal perceives a stressor, a brain region called the hypothalamus sends a signal to the adrenal glands to release hormones. These hormones then enter the bloodstream, and travel throughout the body.

Along the way, they find and bind to receptors located on many different tissues throughout the body. By binding at these receptors, hormones turn on or off the expression of genes. This can change many processes in the body, and even change how an animal behaves. Eventually, the stress response is turned off through a process called negative feedback.

This happens when glucocorticoids bind to receptors in the brain, putting a stop to the secretion of further hormones, and eventually returning gene expression to normal levels. In this way, tiny glucocorticoids have far-reaching effects for the whole body.

At high levels, they can increase wariness, mobilize energy, and boost some types of immune function. These kinds of changes help us to cope effectively with major threats to survival—like getting out of the way of a predator. But while this kind of response is important when facing challenges, activating it too often, too strongly, or at the wrong time can be costly.

That’s because during a stress response, energy and resources are devoted to coping with the immediate threat instead of other important things like caring for offspring or searching for food. Over time, responding to many brief challenges can cause an individual to become chronically stressed. Some individuals respond to challenges differently from others depending on how strongly or for how long their bodies release hormones. This can be good or bad depending on the environments that they are in.

For example, in dangerous environments, birds that respond more strongly to challenges are more likely to survive. Over time, this results in the population evolving a stronger stress response. On the other hand, in relatively safe environments, responding strongly to stressors can be bad. In these safe environments, natural selection favors less stress-responsive individuals. Over time, this results in the evolution of a weaker stress response.

But the “best” way to respond to stress depends not just on how dangerous the environment is, but also on what else might be affected by responding too strongly. For example, when birds have new chicks, they have more than themselves to think about. If they are too cautious, and respond to predators that don’t pose an immediate threat by leaving their nest, the stress response can actually backfire and put the chicks at risk. Trade-offs like this can affect how stress responses evolve, and how they change across seasons, life stages, and environments.

This evolutionary history might help to explain why some species deal with new challenges better than others. Studying how birds and other animals respond to stress helps us understand why we respond to challenges in the ways that we do. Responding strongly to challenges probably helped humans survive throughout much of our evolutionary history.

But now, that same machinery is activated in response to the challenges of modern life. Research on birds and other animals has revealed a lot about why we respond to stress this way.

But what will happen next? Now that our environments are very different from the ones we have faced in the past, will our stress responses evolve too? Will we ever stop stressing the little things? There are still so many questions out there to answer!