The effect of acoustic shielding the mental foramens of a bottlenose dolphin (Tursiops truncatus) on its auditory thresholds has experimentally been studied using the method of instrumental conditioned reflexes with food reinforcement. The detection thresholds of short broadband acoustic pulses deteriorated significantly (by 30–50 dB), under conditions of acoustic shielding the region of the mental foramens over the whole frequency band of a dolphin’s hearing. Therefore, the mental foramens of its lower jaw take part in reception and conducting the sounds into the mandibular fat body in the entire frequency range of a dolphin’s hearing. The obtained results give an experimental proof for the assumption that the morphological structures of the lower jaw play a role of the peripheral part of a dolphin’s hearing. Now there are grounds to assume that Odontoceti have the similar peripheral part of their hearing. This assumption is based on similarity of their morphology.