The evolution of mammalian hearing

In amphibians, reptiles and birds, the ears are internally coupled either through the mouth or through an interaural canal, an arrangement that makes the ears directional and enhances sound-localization cues. In the evolution of mammals, however, the two ears became isolated. Having lost the directionality of coupled ears, mammals evolved both the ability to hear sounds well above 10 kHz and external ears. This allowed them to use two high-frequency cues for localizing: the difference in the intensity of a sound at the two ears, and the directionality induced by the pinnae (another mammalian invention). Because the magnitude of the high-frequency locus cues depends on the size of the head and pinnae relative to the wavelength of the sound, smaller mammals hear higher frequencies than larger mammals in order to use these cues. Localization acuity, however, is related not to the magnitude of the locus cues available to an animal, but to the accuracy needed to direct the eyes to a sound source. The result is that mammals with relatively narrow fields of best vision (e.g., humans and cats) require more accurate localization acuity to direct their gaze than do animals with broad fields of best vision (e.g., gerbils and cattle). So why did mammals give up the directional advantage of coupled ears? We suggest that mammals, because they breathe continuously, needed to isolate their ears from their mouth to prevent breathing sounds from masking external sounds. Amphibians and reptiles do not require such isolation because they are intermittent breathers. Birds, which, like mammals, are continuous breathers, isolated their ears from their mouth by evolving a canal that connects their ears through the skull, allowing them to retain the advantages of coupled ears.In amphibians, reptiles and birds, the ears are internally coupled either through the mouth or through an interaural canal, an arrangement that makes the ears directional and enhances sound-localization cues. In the evolution of mammals, however, the two ears became isolated. Having lost the directionality of coupled ears, mammals evolved both the ability to hear sounds well above 10 kHz and external ears. This allowed them to use two high-frequency cues for localizing: the difference in the intensity of a sound at the two ears, and the directionality induced by the pinnae (another mammalian invention). Because the magnitude of the high-frequency locus cues depends on the size of the head and pinnae relative to the wavelength of the sound, smaller mammals hear higher frequencies than larger mammals in order to use these cues. Localization acuity, however, is related not to the magnitude of the locus cues available to an animal, but to the accuracy needed to direct the eyes to a sound source. The result is...