What is the relationship between animals' heart rates and their physiological responses to stress?

The relationship between animals' heart rates and their physiological responses to stress is that when animals experience stress, their heart rates typically increase. This is a natural response to the body's fight-or-flight reaction, where the heart pumps more blood to deliver oxygen and nutrients to the muscles needed for a quick response to the stressor. This increased heart rate is a key indicator of the animal's physiological response to stress.