Spiral Cochlea Aids Hearing

Swirling colors give an impression of sound in the inner ear

Biological Strategy

Humans

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Sense Sound and Other Vibrations From the Environment

For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.

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Transduce/Convert Signals

Sometimes a signal, such as sound or vibration, reaches a living system and needs to be converted into a different, usable form. This requires converting from one type of energy form to another. For example, organisms with eyes have photoreceptors at the back of the eyes that convert light into electrical pulses. These pulses travel to the brain and allows it to register color, shape, and detail.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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The cochlea of the human ear helps us hear deep vibrations by directing low-frequency waves into the tightest turns of its spiral.

“Deep inside your ear, the pea-size, spiral-shaped cochlea helps translate reverberations from the outside world into neurological signals that we perceive as sound. The cochlea’s coil has traditionally been regarded as little more than the body’s way of packing a lot of membrane into a small space—a mechanical that did not affect hearing. Not any more.

Last March a team of engineers found a function for the cochlea’s shape. Using a mathematical model, they determined that the tight coil at the cochlea’s center steers low-frequency waves into its tightest turns, helping us hear deep vibrations. Previous models had treated sound waves as if they traveled in a straight line, an assumption that failed to take into account how the cochlea’s shape affects the waves’ path. ‘It’s the curvature that’s critical,’ says biophysicist Richard Chadwick at the National Institutes of Health, a collaborator on the project. ‘The more the curvature changes, the more focused the energy gets. It’s behaving something like a whispering gallery, but even better.'” (Ornes 2006)