Final answer:
The stimulus that initiates a fixed action pattern is called a sign or releasing stimuli. These are instinctual triggers that result in an automatic and completed sequence of behavior, such as the aggression displayed by male three-spined sticklebacks in response to red objects during breeding season.
Step-by-step explanation:
The stimulus that initiates a fixed action pattern may also be called a sign or releasing stimuli. A fixed action pattern is a programmed, instinctive behavioral response that an animal will carry out to completion once initiated, regardless of changes in the environment. For example, male three-spined sticklebacks show a fixed action pattern of aggressive behavior when they see the color red on the underside of an object during breeding season. This occurs even if the object does not resemble another fish. The stimuli, in this case, is the color red which serves as a 'sign' or 'releasing' stimulus triggering the fixed action pattern of aggression.
In the case of associative learning, especially classical conditioning, the pairing of stimuli can lead to a conditioned response, such as Pavlov's dogs salivating to the sound of a bell after it had been repeatedly associated with feeding. While operant conditioning involves a behavior being modified by its consequences, fixed action patterns are innate and not conditioned by consequences but triggered by specific stimuli.