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sound-localization acuity is driven by the need to direct the eyes to the source of a sound, not by the magnitude of physi- cal cues. Although physical cues may set limits on acuity, we know of no clear example where this has occurred. However, we currently know little about the ability of birds to local- ize sound and almost nothing about the localization acuity of amphibians and reptiles. As with mammals, the ability of other terrestrial vertebrates may be strongly influenced by factors other than the magnitude of physical locus cues.
Finally, it is interesting to consider that the evolution of high-frequency locus cues in mammals might not have been driven by the need for more accurate localization but in- stead to compensate for the loss of an ability possessed by non-mammals. Specifically, the two ears of most non-mam- malian vertebrates are interconnected, resulting in what is referred to as pressure-gradient receiver (Christensen-Dals- gaard, 2011). In amphibians and reptiles the connection is through the mouth; in the case of birds, the connection is through an interaural canal, which is a network of passage ways through the skull. As a result, sound reaching one ear can pass through to the other ear, an arrangement that ap- pears to aid sound localization by enhancing the binaural time and intensity differences at low frequencies (Chris- tensen-Dalsgaard, 2011). The ears of mammals, however, are acoustically isolated; why they gave up the interconnection is unknown, but one possibility is that it may have been to prevent breathing sounds from entering the ears and mask- ing external sound.
Reptiles and amphibians breathe intermittently (Milsom, 1991), so such breathing noises would not be the disadvan- tage it is in mammals that breathe continuously. That con- tinuous breathers need to isolate their ears from the throat is supported by the observation that birds, also continuous breathers, get around the problem of breathing sounds by connecting their ears through their interaural canal with the Eustachian tubes coming out in the roof of the mouth. By doing this, birds avoid the problem of breathing noise while retaining the directional advantages of acoustically connect- ing the two ears. Mammals, on the other hand, have isolated ears and thus may have evolved high-frequency hearing to compensate for the loss of coupled ears.
Acknowledgment
We thank Gimseong Koay for his help in preparing this article.
Biosketches
Henry Heffner and Rickye Heffner are professors of Psychology at the Uni- versity of Toledo. After receiving their doctoral degrees from Florida State University, they worked at the Univer- sity of Birmingham (England) and Johns Hopkins University before moving to
the University of Kansas in 1973, where they took over the animal laboratory at Parsons State Hospital and Training Center. Working at first on auditory cortical mechanisms and mental retardation, they began their comparative audi- tory research by studying the hearing and sound localiza- tion abilities of horses, cattle, and an elephant. In 1987, they moved their Laboratory of Comparative Hearing to the Uni- versity of Toledo, where they have continued their compara- tive studies.
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