Volitional control of vocal production is an essential ability for vocal usage learning in animal calls. Operant conditioning of vocal production is one of the most direct experimental approaches for assessing volitional vocal control in animals. In this case study, we attempted operant conditioning of the vocalizations of an immature female white-handed gibbon (Hylobates lar). For the operant conditioning of vocal production, the gibbon was required to immediately vocalize in response to a V-sign cue by the human experimenter's hand. During the 2-month period of intensive training for conditioning, the gibbon successfully learned to produce the contingent response of vocalizations. We again tested the conditioning with a 30-day interval after the completion of conditioning and were able to immediately show memorization of vocal conditioning. These results directly suggest greater volitional control of vocal production in gibbons than previously considered. Our study implies the possibility of upper level volitional control in this species, which involves differentiation of several call types in response to specific arbitrary stimuli over the motivational state.
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