Final answer:
The amygdala within the limbic system is responsible for animals' emotional responses, such as fear, leading to behaviors like a cat arching its back or a dog snarling. It is involved in recognizing fearful situations and activating the sympathetic nervous system, initiating the fight-or-flight response.
Step-by-step explanation:
When a cat arches its back in fear or a dog snarls aggressively, we can deduce that the part of the animal's limbic system known as the amygdala is active. The limbic system is responsible for regulating emotion, including behaviors related to fear and motivation. It includes parts of the thalamus and hypothalamus, as well as the hippocampus and the amygdala. The amygdala is especially significant for the sensation of fear and for recognizing fearful faces. It is through the amygdala that animals process emotional responses and initiate the sympathetic nervous system which is closely associated with the fight-or-flight response, characterized by an increase in heart rate, respiration, and energy mobilization, facilitating either combat or escape.
The hypothalamus also plays an integral role as it stimulates the release of stress hormones in response to input from the amygdala. Through classical conditioning, the amygdala learns to associate specific stimuli with danger, influencing the animal's behavior in fearful situations.