How Do Animals' Ears Work: How Birds, Lizards and Frogs Hear with Directional Hearing

First Posted: Feb 23, 2016 07:26 AM EST

How does direction hearing work in animals? Humans use the time delay between the arrival of a sound wave at each ear to discern the direction of the source, but in frogs, lizards and birds, the distance between the ears is too small. Now, scientists have taken a closer look at this time of directional hearing.

Whether perceiving an encroaching predator or finding prey in the dark, precisely localizing the source of a sound is indispensable in the animal kingdom. Almost all mammals, including humans, localize sound sources horizontally via the delay in time in which sound signals arrive at each ear. Using this time difference the brain can calculate the direction from which the sound emanated.

However, frogs, many reptiles and birds do not have this option since the distance between their ears often measures merely a few centimeters. The time difference is so small that it cannot be processed by the brain. Now, scientists have found that these animals have an air-filled cavity that connects the eardrums of the two ears.

This cavity, which runs through the head, couples the eardrum. Known as "internally coupled ears," or ICE, this tunnel is clearly visible when light falls into one ear of a gecko; the light actually shines out the other ear.

Unlike humans, the animals perceive not only external signals, but also a superposition of external sound waves with those that are created internally through the coupling of two sides. Animals use the resulting signals for pinpointing sound sources.

In this latest study, the researchers took a closer look at what exactly happens in a coupled ear. They created a mathematical model that describes how sound waves propagate through the internally coupled ears.

"Our model is applicable to all animals with this kind of hearing system, regardless that the cavities between the eardrums of the various species look very different," said Leo van Hemmen, one of the researchers, in a news release. "We now understand what exactly happens inside the ears of these animals and can both explain and predict the results of experiments in all sorts of animals."

The findings are published in the journal Physical Review Letters.

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