For sound energy to reach the inner ear, an impedance matching device is needed between the surrounding medium and the inner ear fluid. The mammalian middle ear performs two tasks: impedance matching and sound amplification. For terrestrial mammals, the middle ear ossicular inertia is the main factor limiting the high- frequency hearing, and for animals ranging from bats to elephants, the high-frequency hearing limit (HFHL) is inversely proportional to the cubic root of the ossicular mass (Hemilä et al. 1995). The middle ear size grows with animal size, and for smallest mammals able to hear very high frequencies, additional constraints are posed by the cochlear sensitivity. The co- existence of inertial and cochlear constraints is seen among pinnipeds where the ossicular inertia in air, and cochlear sensitivity in water are the limiting factors for phocids, but the cochlear sensitivity alone limits hearing in both media among otariids (Hemilä et al. 2006). The origin of cetaceans is one of the best examples of macroevolutionary changes in life's history and shows how the hearing organ changed while whales became obligately marine (Nummela et al. 2004). Due to different acoustics of air and water, sound transmission from the surrounding water to the inner ear demands a reversed impedance matching than is required on land. This kind of mechanism has evolved in odontocetes, and despite a much larger ear size can transmit equally high frequencies as small bats hear in air. Additionally, the sensitivity is 50–100 times better than in man. Hemilä, S., Nummela, S. & Reuter, T. 1995. Hear Res 85, 31–44. Hemilä, S., Nummela, S., Berta, A. & Reuter, T. 2006. J Acoust Soc Am 120, 3463–3466. Nummela, S., Thewissen, J.G.M., Bajpai, S., Hussain, S.T. & Kumar, K. 2004. Nature 430, 776–778.