Ears for Hearing
Air transmits sound waves less efficiently than water, so when the distant ancestors of mammals moved onto land, modifications were needed to help catch and amplify the weakened vibrations.
Most mammals have pinnae — exterior ears — for gathering and funneling sound inward. From there, the sound travels along a canal to the eardrum. The resonance of this pipelike ear canal, or auditory meatus, serves to amplify sound, almost doubling its force. When sound waves hit the taut membrane of the eardrum at the far end, they are converted to mechanical vibrations, which are then transferred to the malleus, the incus and the stapes, the tiny bones that make up the middle ear. These bones, or ossicles, work together — the malleus pushes the incus, which pushes the stapes — doubling or tripling the force of vibration in the process. Last in line, the stapes exerts pressure on the so-called oval window, a membrane covering the opening of the inner ear, a fluid-filled, spiral-shaped structure known as the cochlea. The concentration of force that occurs as the vibrations pass from the eardrum to the much smaller oval window produces an additional fifteen- to thirtyfold amplification. At this point, the vibrations are converted into hydraulic pressure waves that pass through the liquid of the inner ear, ultimately stimulating the nerve endings that generate electrical impulses to the brain.
Not surprisingly, the mammalian ear — an organ designed to transduce sound traveling from air to fluid — doesn't work as well underwater. Thus, marine mammals have had to make their own modifications. Cetaceans, for instance, have small holes located just behind their eyes instead of pinnae, which would obstruct the smooth passage of water along their streamlined body. In an additional adaptation, their middle and inner ears are suspended by ligaments, isolating them from their skull, which, along with their body, absorbs sound from the water and therefore would interfere with sound reception in the inner ear. Sound is transmitted to their inner ear along a fat deposit running the length of the lower jaw.
The frequency of sound is measured in cycles per second. Most mammals can hear a much wider range of sound frequency than they can produce. A dog, for example, can perceive frequencies between 15 and 50,000 cycles, but makes sounds only within the range of 452 to 1,080 cycles.
The hearing range of a mammal tends to correspond to its survival needs. For instance, although smaller mammals generally hear and emit sounds at higher frequencies than larger animals, the enlarged bulla (middle ear chamber)of the tiny desert kangaroo rat (Dipodomys deserti) increases its sensitivity to low frequencies, thus allowing it to perceive the low-intensity, low-frequency sounds made by predators, including the sidewinder rattlesnake and the barn owl. Other rodents living in dry environments, such as the lesser Egyptian jerboa, also exhibit this increased sensitivity, leading some researchers to believe that the adaptation may also have developed as compensation for the poor sound-carrying quality of dry desert air.
