Stomach butterflies are associated with the body’s fight-or-flight response. When the brain perceives a potential threat to survival, it increases alertness by raising heart rate, blood pressure, and breathing rate. At the same time, the nervous system stimulates the adrenal glands, which release hormones including adrenaline and cortisol that can turn the body into a tense, sweaty mess. Muscle tension (especially in the stomach) helps keep us alert; perspiration helps cool the body down. The smooth stomach muscles are also extra-sensitive during the fight-or-flight response, and the added sensitivity may be partly to blame for that fluttery sensation. Some researchers refer to the stomach as the “second brain,” based on findings that the gut contains 100 million neurons linking it to the brain, known as the brain-gut axis . So when we feel nervous before a stage debut, the brain communicates that anxiety to the gut, sometimes causing a case of the butterflies. But to fully understand the reasons behind the butterflies, we may have to look back hundreds of thousands of years.