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between the ears. Thus, the smaller the animal, the higher it must hear in order to use the binaural intensity cue.
Finally, mammals evolved external ears, or pinnae, that alter the spectrum of a sound as a function of the location of the sound source (Figure 1B); monaural pinna cues enable an animal to avoid front-back confusions and localize sound in the vertical plane. Indeed, pinna cues also serve to localize sound in the horizontal plane and are especially important when a sound is audible in only one ear (Butler, 1999). For the pinnae to provide directional information, the sound must have a complex spectrum (such as clicks or noise), as opposed to pure tones, and it must contain frequencies high enough for its spectrum to be modified by the pinna. Thus, both the binaural intensity cue and monaural pinna cues re- quire that animals be able to hear high frequencies.
The following is a description of what we know about mam- malian sound localization and the role that high-frequency hearing plays in it. Among the questions we ask are: What is the evidence that mammals evolved high-frequency hearing for sound localization? Do mammals localize as accurately as the physical cues available to them allow or are other fac- tors involved? Finally, do mammals have better sound-local- ization abilities than non-mammals?
High-Frequency Hearing in Mammals
By the late 19th century, it was known that mammals dif- fered in their ability to hear high-frequency sounds and that some species could hear higher frequencies than humans. This was established by Francis Galton, who used a high-fre- quency whistle attached to his cane and operated by a rub- ber bulb to observe the unconditioned responses of animals to high-frequency sounds. Of the animals he observed, he found cats had the best high-frequency hearing, which he attributed to their need to hear the high-frequency sounds made by mice and other small prey (Galton, 1883). Thus, by this time, it was apparent that mammals varied in their abil- ity to hear high frequencies, with the variation attributed to the individual needs of each species.
The study of mammalian hearing began to progress by the second half of the 20th century when reliable procedures for determining the sensory abilities of animals were developed (e.g., Stebbins, 1970). At first the results did not challenge the notion that the high-frequency hearing of animals was adapted to their individual needs. For example, the high- frequency hearing ability of bats was seen as an adaption to their use of high-frequency echolocation signals, whereas
the good high-frequency hearing of mice was believed to be linked to their use of ultrasonic vocalizations, and the in- ability of humans to hear above 20 kHz was attributed to the importance of speech, which is primarily low frequency (Sales and Pye, 1974).
Systematic Variation in High-Frequency Hearing and Sound Localization
The discovery of the link between high-frequency hearing and sound localization was made in the 1960s when it was noticed that smaller mammals had better high-frequency hearing than larger ones (Masterton et al., 1969). Others, such as von Békésy and Rosenblith (1951), had already no- ticed the relation between size and high-frequency hearing, attributing it to an unspecified need for large mammals to hear low frequencies. However, noting that the magnitude of the binaural time-difference cue depends on the size of an animal’s head, Masterton suggested that the smaller an animal’s head, the smaller the maximum binaural time-dif- ference would be and the more dependent the species would be on the binaural intensity-difference cue. Because small heads do not block low frequencies as effectively as they block higher frequencies, an animal must hear frequencies high enough to be attenuated by its head and pinnae in or- der to use the intensity-difference cue. Thus, it was proposed that the smaller a mammal’s head, the higher it would have to hear to use the binaural intensity-difference cue.
To quantify the relationship between head size and high- frequency hearing, it was necessary to adopt a definition of those measures. This was done by defining head size, in units of time, as the maximum interaural time delay an animal might experience, which is the time it takes for sound to travel around the head from one ear to the other—referred to as functional head size. For terrestrial mammals, functional head size is determined by dividing the distance around the head (from the opening of one ear canal to the opening of the other) by the speed of sound in air. For marine mam- mals, functional head size is determined by dividing the distance between the middle ears (as measured through the head) by the speed of sound in water, as this is the path that water-borne sound takes when traveling from one ear to the other. High-frequency hearing ability is defined as the high- est frequency audible at a particular level, typically, 60 dB sound pressure level (SPL).
The first statistical analysis of the variation in high-frequency hearing, which included body size as well as functional head
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