Final answer:
A rapid heartbeat is not a primary bodily signal of hunger. Primary signals include stomach contractions (hunger pangs) and low blood glucose levels. Increased salivation is a preparatory response to food, not a direct hunger signal.
Step-by-step explanation:
The body has several physiological mechanisms that signal hunger. When our stomachs are empty, they contract, which typically causes a person to experience hunger pangs. These muscle contractions, often experienced as stomach growling, are a primary bodily signal of hunger. Additionally, when blood glucose levels drop, various chemical signals are produced by the pancreas and liver, inducing hunger. In response to food stimuli like the smell, sight, or thought of food, the salivary glands produce more saliva, and the stomach produces hydrochloric acid to aid digestion, but increased salivation is more a preparation for eating rather than a direct signal of hunger. When considering phases of gastric control, the appearance of food initiates the cephalic phase leading to salivation and stomach acidity preparations, which are not direct indicators of hunger but rather preparatory responses to food intake.
Therefore, the option 'c. Rapid heartbeat' is NOT a primary bodily signal of hunger, because an increased heart rate is typically associated with physical exertion, stress, or excitement, rather than the physiological mechanisms associated with hunger.