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Mason, M.J. (2016) Internally coupled ears in living mammals. Biological Cybernetics 110: 345-358.
Many non-mammalian tetrapods have "internally coupled" ears, in which there is a wide, patent communication between the two middle ear cavities, passing through the head. This renders the response of each tympanic membrane directionally-sensitive, which allows accurate sound localization at low frequencies. In contrast, the right and left middle ears of mammals are generally regarded as being acoustically isolated from each other, i.e. sound entering one ear will not excite the contralateral ear, and sound localisation is therefore based on different mechanisms. In this paper, I argue that the right and left middle ears of the platypus and certain subterranean mammals may in fact be internally coupled. In the platypus, this involves wide, patent pharyngo-tympanic communications, resembling those of non-mammals, whereas some moles have middle ear cavities in direct communication through the pneumatised basisphenoid bone. Internally coupled ears should, at least in principle, improve sound localisation at low frequencies, which might be particularly important for these unusual mammals given that their hearing is likely restricted to low frequencies and they lack pinnae. Further experimental work is needed to test this hypothesis.
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