Page 22 - 2016Spring

Henry E. Heffner and Rickye S. Heffner
Postal:
Department of Psychology, MS 948 University of Toledo Toledo, OH 43606 USA
Email:
Henry.Heffner@utoledo.edu; Rickye.Heffner@utoledo.edu
The Evolution of Mammalian Sound Localization
Mammals evolved better high-frequency hearing than non-mammals, and they use it for localizing sound. Does this mean they localize sound better than other vertebrates?
Introduction
The ability to localize sound is useful for survival because it allows an animal to determine the location of sound sources, which, in nature, are usually other ani- mals. Indeed, the need to localize sound explains why terrestrial vertebrates have two ears, as differences in the time of arrival and the intensity of a sound at the two ears gives information about the direction of its source. However, unlike other ter- restrial vertebrates (amphibians, reptiles, and birds), mammals have evolved the ability to hear high-frequency sounds. Specifically, amphibians and reptiles rarely hear above 5 kHz, and birds do not hear much above 10 kHz; however, almost all mammals hear far higher, with over half of mammals tested so far able to hear above 50 kHz. As we show, the primary source of selective pressure for the evolu- tion of mammalian high-frequency hearing has come from the need to localize
sound.
There are three sound-localization cues available to mammals. The first is the binaural time-difference cue in which the audi- tory system com- pares the arrival time of a sound at the two ears to de- termine its azimuth (Figure 1A). This cue can be used to localize any low- frequency sound as well as high- frequency complex sounds such as clicks and noise.
Figure 1. A: Differences in the time of arrival and intensity of a sound at the two ears are used to determine the azimuth of a sound source. However, these cues do not indicate the elevation of a sound source nor whether it is in front or behind the observer. B: The direc- tionality of the pinnae to high frequency sounds enables an animal to determine if a sound source is in front or behind, as well as the elevation of the source.
The second is the binaural intensity-difference cue in which the difference in the intensity of a sound at the two ears is used to determine azimuth (Figure 1A). Be- cause low-frequency sounds (long wavelengths) can bend around an animal’s head with little or no attenuation, a mammal with a small head must hear frequencies that are high enough to be shadowed by its head to produce an intensity difference
20 | Acoustics Today | Spring 2016 | volume 12, issue 1 ©2016 Acoustical Society of America. All rights reserved.