Why Zebras Don’t Get Ulcers: The Acclaimed Guide to Stress, Stress-Related Diseases, and Coping: Summary & Key Insights

Stress begins deep in the brain, in a structure called the hypothalamus. When the brain perceives a threat—whether real or imagined—it rapidly sets off a cascade called the hypothalamic–pituitary–adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone (CRH), signaling the pituitary gland to release adrenocorticotropic hormone (ACTH). This, in turn, tells the adrenal cortex to pour glucocorticoids—most notably cortisol in humans—into the bloodstream.

These hormones orchestrate the body's emergency mode. Heart rate increases, blood is diverted from digestion to muscles, glucose floods the system for quick energy, and immune responses are temporarily suppressed. In the wild, these changes are lifesaving. They prime an organism to flee, fight, or cope with sudden danger. Once the threat passes, feedback loops push hormone levels back to baseline. A zebra that outruns a lion goes back to grazing without biochemical residue from the ordeal.

Humans, however, can keep those same stress hormones circulating for days, months, even years. No predator need be present; we generate threats through thought alone. The HPA axis is not designed for perpetual activation. What began as an elegant survival mechanism turns destructive when it never shuts down. The story of chronic stress, therefore, is the story of a misused biological masterpiece.

Evolutionarily, stress serves a clear purpose. Short-term stress responses prepare an animal to meet acute challenges. A zebra encountering a predator uses stress hormones to mobilize energy, sharpen perception, and increase survival odds. The same responses conserved through evolution work wonders in emergencies.

Humans, though, have taken this ancient system into an environment of abstract worry. Modern stressors rarely demand physical action; they are psychological, social, or economic. Yet our bodies treat each invoice or traffic jam as if it were a lion. Chronic psychological activation keeps heart pressure elevated, blood sugar erratic, and immune processes dysregulated.

What’s adaptive for a creature that occasionally flees becomes maladaptive for one that perpetually anticipates danger. The irony is profound: our intellect—the same faculty that allows us to plan, predict, and prevent—creates anxieties that our bodies endure as if every thought were life or death. That mismatch is at the root of modern stress-related disease.