Imagine listening to people speak through the din of a seashell's roar. This, in fact, is how monkeys hear human speech, according to new research. Describing his findings today at a meeting of the American Physical Society, Michigan State University physicist Michael Harrison offered his explanation of the long-observed discrepancy between our own auditory ability and that of our primate cousins.

Externally, monkey ears are shaped much like human ears. But they differ in the size of the canal leading from the eardrum to the outer part of the external ear; in monkeys the canal is smaller. As a result, the temperature of the air enclosed in the canal is warmer, which can more easily generate noise that masks auditory signals. "Air molecules are like people moving around in a crowded room at a cocktail party," Harrison explains. "The warmer it is, the more moleculesor cocktail guestsrun around, and it creates noise. With this random noise, it's harder to hear an individual conversation."

These incoherent sound waves exert a "resonant pressure" on the eardrum, yielding an effect akin to that experienced when holding a seashell to one's ear. In contrast to the monkey condition, "normal, healthy human ears can detect signals so weak that they are barely able to emerge from random noise that exists around us all the time," Harrison notes. "The human ear is a remarkable thing."