The response or reaction of an animal to a situation perceived as a threat to its survival, which involves physiological changes in the animal body through the action of the sympathetic nervous system, priming the animal to deal with danger by staying, fighting or running away.
This concept was first described by Walter Cannon in his theory called Cannon's theory in 1929. Based on his theory, in the presence of a threatening stimulus, a part of the brain regulates the metabolic and autonomic functions to prepare the muscles for any subsequent violent action, i.e. to either run away or fight. Example of an autonomic reaction is the increased release of adrenaline in the body and some of the physical manifestations include increased blood pressure and heart rate (as a result of higher concentrations of adrenaline in the body).
See also: Cannons theory