Chapter 1: Why Don’t Zebras Get Ulcers?

Sapolsky traces the scientific understanding of stress through foundational research, beginning with Hans Selye's accidental discovery of the General Adaptation Syndrome when his rat experiments revealed unexpected pathological changes, and Walter Cannon's characterization of the fight-or-flight response as an evolutionary mechanism that prepares organisms for survival threats. The chapter explains how stress physiology operates through disruption of homeostasis, the stable internal environment the body maintains under normal conditions. When a stressor activates the stress response, the body redirects resources toward immediate survival by mobilizing stored energy, increasing cardiovascular output, suppressing digestion and reproductive function, dampening immune defenses, and heightening sensory awareness and memory consolidation. These mechanisms evolved because they provide significant advantages in acute emergencies where physical action determines survival. However, the pathological consequence emerges when humans repeatedly or chronically activate these same physiological systems for psychological or social reasons—concerns about job performance, relationship conflicts, financial uncertainty, or health anxieties that never resolve. This chronic activation causes the protective mechanisms to damage the very systems they were designed to support, resulting in hypertension, metabolic dysfunction, reproductive suppression, immune vulnerability, gastrointestinal disorders, and accelerated neurological aging. The chapter establishes that the stress response itself is essential and adaptive, as evidenced by rare disorders like Addison's disease and Shy-Drager syndrome where individuals cannot properly activate stress responses and face severe health consequences. Yet Sapolsky emphasizes that modern human psychology creates vulnerability through the unique human capacity to perceive threat in abstract, non-immediate circumstances and to sustain that perception over extended periods, fundamentally distinguishing our experience of stress from that of other species.