The importance of sounds (acoustics) to mammals should be obvious. As in vision, binaural cues are timing differences between the arrival of sounds at one ear before the other, and they assist in the localization of sources of sounds. Humans use acoustical information to recognize the voices of family and friends or to locate an accident from the wail of an emergency vehicle’s siren. In odontocetes, the ability to use binaural hearing is improved by an evolutionary shifting of the bones of the skull so that the hearing anatomy of the skull is asymmetrical. This makes odontocetes particularly sensitive to the direction of an incoming sound.
The auditory system of most mammals consists of the following five main components:
• the pinnae, an external structure that acts as a sound collector
• the ear drum, or tympanum, that converts vibrations in air (sounds) to mechanical vibrations
• an amplifying system, the auditory ossicles (malleus, incus, and stapes) or bones of the middle ear
• a transducer (the oval window), where mechanical vibrations are converted to vibrations in fluid in the inner ear
• sites for converting vibrations in fluid to electrical stimuli (hair cells attached to the basilar membrane in the cochlea)
Through these components, electronic representations of the sounds are generated and transmitted to the brain via the auditory nerve.
Fossorial mammals, those that live most of their lives underground, may lack pinnae (which would only collect dirt). Many, but not all, aquatic mammals also lack pinnae (which would collect water). In fact, as a mammal progresses from amphibious (otters, seals, walrus, and sea lions) to totally aquatic (whales and dolphins), the pinnae go from small and valvular to absent. In fossorial mammals, considerable fusion of the auditory ossciles has reduced sensitivity to high-
Sounds used by mammals can be of very different pitch or frequency, depending upon the species and situation. African elephants are sensitive to sounds at frequencies below 40 Hz; blue whales produce sounds as low as 20 Hz. These are referred to as “infrasounds,” because they are below the range of human hearing (arbitrarily, 40 Hz). Other mammals, notably many bats, most carnivores (Felidae, Canidae, Mustelidae, Viverridae), and dolphins (Delphinidae), use sounds that are well above the range of human hearing (these are ultrasounds, theoretically >20,000 Hz). Humans hear best at frequencies from about 100–5,000 Hz, while some bats and dolphins hear very well at more than 200,000 Hz. In general, low-