The upper frequency limit of hearing is negatively correlated with head size in mammals

My favorite graph in auditory science shows that the upper frequency limit of hearing is negatively correlated with the head size in mammals (defined as the functional interaural distance). It shows that sound localization is such an important requirement for survival that it was probably the driving factor that led to all the mammalian ear specializations required for the sensitivity and discrimination of high-frequency sounds. This was shown across 60 terrestrial mammals in several publications by Heffner and Heffner (e.g., 2016 Acoust Today). The wavelength of sound is inversely proportional to frequency. As frequency increases and the wavelength becomes shorter than a given head size, there is a level difference between the two ears. As head size decreases for smaller animals like mice, higher frequency hearing was required to exploit interaural spectral intensity differences needed to localize sounds. In order support such high frequency capabilities, the pinna, the middle ear, cochlea, and central mechanisms evolved together. Heffner and Heffner (1992) proposed that another important function of sound localization is to direct the eyes to the sound source and show a beautiful relationship between auditory and visual acuity. Hearing aids are limited to 3–4 kHz and, thus, do not make use of these high frequency capabilities. I have successfully argued to investors the importance of making wideband hearing aids and this culminated in the Earlens light driven wideband hearing aid (Levy et al. 2015 and Puria et al. 2016).