Mastoid: a vestigial function in humans?

At the present time, the function of the mastoid remains unknown. One of the main hypotheses accredited in the literature interprets the mastoid as a pressure buffer. Other theories underline the role of the mastoid mucosa in pressure regulation by transmucosal gas exchanges. The question is what advantage does air reabsorption and the creation of a certain degree of negative pressure that mastoid seems to produce, bring to the middle ear and hearing? In the authors' opinion, it is possible that the mastoid, or, in general, every kind of mucosa contained in the middle ear of mammals, would act to create a quite constant, although slight, negative pressure to obtain favorable compliance and impedance conditions in the middle ear to hear and transmit high frequency sounds and ultrasounds. The Eustachian tube, in this perspective, would compensate excessive values of negative pressure. Clearly, that function of mastoid pneumatization in humans would have lost its role, due to the absence of a sensorineural system to analyse ultrasounds.