Experiments on the auditory and vocal functions of the chimpanzee were reported. The first main focus was on the basic auditory functions such as auditory sensitivity and difference thresholds, on the auditory cognition such as auditory-visual intermodal integration, and on the speech perception such as the prototype and context effects. A chimpanzee showed difficulties in the auditory-visual inter-modal integration. The second focus was on the close auditory-vocal interrelationship. It was reported that the chimpanzee had a W-shaped auditory sensitivity. The perception experiments showed that the second formant was not important for the perception of human vowels and vowel-like vocal sounds of the chimpanzee. The acoustic analyses showed that the second formant was constant between these vowel-like vocal sounds. These results indicated that the auditory and vocal functions were closely inter-related each other. The third focus was on the early vocal development of an infant chimpanzee. A comparison in vocal development between chimpanzee and human infant suggested that the chimpanzee infant did not reach to the Oller's Expansion Stage of the human vocal development. The high larynx and the small pharinx were suggested to be partially responsible for the limitation of the vocal development and the repertoire of vowel-like vocal sounds in the chimpanzee.
