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Evolution of Mammalian Sound Localization
On the other hand, some mammals cannot localize high- frequency tones, indicating that they do not use the bin- aural intensity-difference cue. As seen in Figure 7, the ani- mals that do not use the intensity-difference cue are hoofed mammals, such as horses, cattle, pigs, and alpacas (Vicugna pacos). It should be noted that although some mammals cannot localize high-frequency pure tones, they can local- ize high frequencies that are amplitude modulated at a low frequency or that contain a band of high frequencies that produce a fluctuating envelope. This is because the envelope itself (rather than the carrier signal) provides a basis for a time comparison at the two ears. For example, alpacas could not localize a 4-kHz pure tone but could localize it well when it was modulated at 250 or 500 Hz. As a result, most natural sounds can be localized using the binaural time cue even if they contain only high frequencies because they naturally have an irregular envelope. So far, all mammals known to have relinquished the binaural intensity cue have large func- tional head sizes. Thus, with their large heads, some, but not all, large mammals have relinquished the binaural intensi- ty cue, relying instead on the binaural time difference and monaural pinna cues to localize sound.
Pinna Cues
The ability to use pinna cues for locus is demonstrated by discriminating between two sound sources for which binau- ral cues are the same. This occurs for sources that are cen- tered symmetrically around 90 degrees to one side, such as 60 and 120 degrees to the right, placing the speakers on what is known as the cone of confusion (Figure 8). In this case, the signal to be localized is a band of noise because locus infor- mation is derived from the way the pinna modifies the spec- trum of high-frequency noise. Figure 8 shows an alpaca that could perform a front-back locus discrimination but only if the sound contained high frequencies, in this case a 3-kHz high-pass noise. The fact that the animal could not perform the discrimination when a 2-kHz low-pass noise was pre- sented demonstrates that pinna cues rely on the presence of high frequencies. Thus, although animals like alpacas may relinquish the binaural intensity-difference cue, they retain high-frequency hearing because it is required for using the pinna locus cues.
Why Do Mammals Use
High-Frequency Locus Cues?
Did the evolution of high-frequency hearing and its use for binaural intensity and pinnae cues give mammals better lo- calization ability than that of amphibians, reptiles, and birds?
Figure 8. The pinna cues that enable mammals to discriminate sounds coming from the front versus those coming from the back re- quire high frequencies. This is why alpacas can perform a front–back discrimination when a high-frequency sound (3 kHz high-Pass) is presented but not when a low-frequency sound (2 kHz low-Pass) is presented. From Heffner et al. (2014).
So far, no one has been able to train amphibians or reptiles to respond reliably to sound, and so we know little about their behavioral hearing abilities. Although birds readily learn to respond to sound, there is information on the ability of only seven species to localize brief sounds (Feinkohl and Klump, 2013). With the exception of owls, the MAA of birds ranges from 17° for starlings to 101° for zebra finches, suggesting that most mammals may exceed birds in left-right localiza- tion acuity.
The barn owl, on the other hand, appears to have exception- ally good sound-localization ability. However, the barn owl has been tested by training it to orient to the source of a sound and then determining its average error, resulting in a threshold of 3-5°. Just how that measure compares to MAA is not known, but it nevertheless appears that the barn owl’s localization acuity may exceed that of most mammals. Inter- estingly, the barn owl possesses the equivalent of a mamma- lian external ear in the form of feathers around the opening to its ears and, although its ability to perform a front-back locus discrimination has not been tested, it is able to localize in the vertical plane, a discrimination for which mammals require pinna cues (Knudsen and Konishi, 1979).
The question of whether the evolution of high-frequency hearing in mammals resulted in better sound-localization abilities than non-mammals cannot be answered for the simple reason that sound-localization acuity is determined not by the available physical cues but by how accurate an an- imal needs to localize sound. As we have seen, mammalian
26 | Acoustics Today | Spring 2016