The Bigger Your Skull, The Less You Hear

Dog trainers have long known that dogs will respond to whistles that are too high-pitched for human ears to hear. Similarly, bats navigate and hunt utilizing a system of "radar" based on squeaks that they emit in flight, but which are beyond the audible range of human ears.

Technically, the upper limit of human hearing is about 20,000 hertz (cycles per second). Dogs are sensitive to sound at over 40,000 hertz, and bats, mice and many smaller animals can hear to over 80,000 hertz. At the opposite extreme, humans can hear sound at frequencies as low as 125 hertz, to which the smaller animals seem oblivious. The question naturally arises: does size have anything to do with the sounds that a creature can detect? The answer seems to be yes. The larger the animal, the lower in tone its hearing range is centered.

Rickye and Henry Heffner of the University of Kansas, in collaboration with Ned Stichman of the Independence (Kansas) zoo, have found that animals with a head larger than man's have the ability to hear lower sounds. The three investigators tested the hearing range of a seven-year-old Indian elephant at the Independence zoo. They found that while their subject, Lois, could not hear sounds higher than 12,000 hertz (well within the range of human hearing), she could hear sounds at frequencies as low as 16 hertz (well outside the range of human hearing).

The testing method used with Lois involved a reward when the elephant correctly punched a "yes" or "no" button with her trunk, depending on whether a sound was audible to her. In the course of their experiment, Stichman and the Heffners rewarded Lois with a total of eight gallons of watered-down Lemon-Lime Kool-Aid (her favorite), half a cup at a time.

The conclusion was that the ability for an animal to perceive sound is related to the distance between its ears. That is, slowly vibrating long-period (and long-wavelength) sound waves are best perceived by a skull with a larger resonant cavity, while rapidly vibrating short-period waves are more easily picked up by smaller craniums.