The animal used in Ivan Pavlov’s classic conditioning experiments was the dog.
Pavlov was studying digestion when he noticed that laboratory dogs began salivating not only when food arrived, but also when they encountered cues associated with feeding, such as the presence of an attendant. He then systematically paired a previously neutral sound with food. After repeated pairings, the sound alone could trigger salivation.
Food was the unconditioned stimulus because it naturally produced salivation. The sound began as a neutral stimulus and became a conditioned stimulus; salivation in response to that sound became the conditioned response. Pavlov published experimental results on the phenomenon in 1897.
The dogs were not being trained to perform a voluntary trick, which distinguishes classical conditioning from operant conditioning. Pigeons and rats became important in later learning research, especially in operant-conditioning studies, but they were not the animals associated with Pavlov’s famous digestive experiments.