When a pig is being chased by a dog, its body goes into "fight or flight" mode. This is a natural response to danger that is triggered by the release of hormones from the adrenal glands. These hormones, including adrenaline and cortisol, cause the heart rate and breathing to increase, the muscles to tense up, and the senses to become more acute. This allows the pig to either fight the dog or run away to safety.