There's a reason people compare garbled voicemails to listening to someone talk underwater—our ears just aren't built for a watery world. But animals like whales and dolphins use sound all the time to hunt down dinner or to serenade a mate. Now, new research is highlighting just how marine mammals evolved to listen underwater.
Whales and dolphins had land-based ancestors that made their way into the ocean millions of years ago. Part of that transition involved modifying their ears so that they could clearly hear sounds underwater and tell where they were coming from, says Nick Pyenson, curator of fossil mammals at Smithsonian's National Museum of Natural History in Washington, D.C. (Read about whale evolution in National Geographic magazine.)
So while marine mammals still have many of the same ear parts as people do, they've been modified to enable them to function in the ocean, says Pyenson, a National Geographic grantee.
Yamato talked to us about some of those differences between human and whale ears—and why everything sounds so muffled to us underwater.
Sound is basically vibrations in the air or water that people or animals perceive as noise, right?
So why can't we hear underwater?
We can hear underwater, we just can't hear well. Our ears are designed for hearing in air. We have an air-filled ear canal—at the end there is an eardrum that vibrates in response to sound, which sets up a chain reaction of movement in the ear bones. The vibrations are translated to the inner ear, where the mechanical energy is translated to electrical pulses to the brain.
Underwater, our whole skull vibrates in response to sound and so both of our ears are affected at the same time. And what that means is that it's really hard to figure out where that sound is coming from. (Find out how superhuman hearing could one day be possible.)
How does having our head vibrate keep us from localizing sound?
When your whole skull is vibrating—the bones housing your middle and inner ear are integrated into your skull—both of your ears are vibrating with the skull. That doesn't happen in air.
In whales, the bones housing the middle and inner ears are separated from the skull, and that's part of the mechanism that isolates each ear from each other and from the skull, and they're able to figure out where sounds are coming from much better.
So animals need sounds to arrive at their ears at different times to localize sound?
There's a time delay part and a loudness part. So because of the rest of our head, the sound doesn't sound the same from one side to the other. The sound is arriving faster at one ear, also louder.
How do our ears compare with whale ears?
In land mammals like us, we have the fleshy pinna or the outside [projecting] part of the ear that helps collect sound and funnel it into the ear. That works because the acoustical properties of the air and your flesh are different, so sound is affected by [us] having that fleshy pinna and gets channeled into the ear canal.
[Once there] it vibrates the eardrum and the ossicles [or ear bones].
But in water, the acoustical properties of your flesh and water are pretty similar, so it doesn't really make a difference if you have this fleshy outside part of the ear.
We still don't know that much about hearing in baleen whales, but in toothed whales, instead of sound coming in through the ear canal, sound comes in through these fatty tissues in the jaws.
The fats attach to this part of the ear called the acoustic funnel, and we think that then the ossicles vibrate and [that's translated to] the fluid-filled inner ear.
The main difference is that whales don't have an ear canal that works and the bones in the ear aren't attached to the skull. Also, there are sinuses around each ear that help to isolate each ear. (Learn about the unique ears of deep-sea fish.)
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