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Monkey Ears

Just the other day I was talking to Ziggy, my Helping Hands capuchin monkey, and she looked at me quizzically and said, “Huh? Speak up!”

I have been operating under the assumption that her eyesight and hearing was equal to or better than ours. What made me think that? Well, we live at the top of a hill and, as a result, cars coming up the steep incline can generally can be heard lowering into a heftier gear just before their approach. When K-9, our Dalmatian was alive, even though she was a bright dog, Ziggy used to bark the arrival of an approaching vehicle before K-9 did. Therefore, I’d just assumed that the monkey’s ears were keener. Now a new study comes out from some researchers at the Michigan State University telling me I’m wrong. That monkeys’ hearing is “discernibly less acute than that of people for the frequency range in which human speech is expressed and heard.” In fact, the clinical truth of this has been known for a long time, but a fundamental explanation as to why has forever been lacking. Until now.

Physics is a field dealing with the properties and interactions of matter and energy. Currently, a new subfield of physics, biological physics is providing answers to questions such as why monkey ears, while so similar to our own, work differently.

Michael Harrison, a Michigan State University physicist, has written a paper for the American Physical Society outlining, for the first time, his results explaining this phenomenon. And apparently size is the all important key.

To begin, Harrison tells us that we can think of our ears as holding pens for all matter of sound. Human ears register pure tones, which our brain eventually translates into meaningful sound such as speech or music, but the tones must fight their way through a lot of noise. The noise is created from the amount of air that is found inside the ear canal, under certain ambient air temperature. In other words, Harrison explains it like this: “Air molecules are like people moving around in a crowded room at a cocktail party. The warmer it is, the more molecules—or cocktail guests—run around, and it creates noise. With this random noise, it’s harder to hear an individual conversation.”

The constant ambient air temperature is the physical mechanism which, in random fashion, creates sound waves that resonate within the air column leading to the eardrum. It follows then, that these incoherent sound waves create a “resonant pressure” on the eardrum, similar to what it is like when you hold a seashell to your ear and the sound waves bounce around. The resonant pressure fluctuates and increases the random firing of nerve cells in the auditory system. Transmitted from the auditory system to the brain, these random firings result in noise that masks or obscures a signal that contains speech or other useful information.