The secret of how seals can hear in air and water has been revealed using the Natural History Museum’s collection.
Seals evolved a unique ear structure for hearing in both air and water.
Over 200 museum scans of living and fossil specimens revealed how this developed.
Understanding how seals hear could help to protect them from noise pollution.
The secret of how seals can hear in air and water has been revealed, thanks to pioneering research led by Natural History Museum scientists.
Humans and other land mammals hear by transferring sound waves from the eardrum to the cochlea, where vibrations are converted into neural signals. Underwater, however, this system fails because the air pocket behind the eardrum reflects almost all sound.
Seals are the only mammals that hear equally well in both air and water, prompting scientists to investigate how they achieve this.
By micro-CT scanning more than 200 living and fossil seal specimens, researchers found the answer lies in the seals’ cavernous tissue. This structure swells with blood to protect the ear from being damaged by the extreme pressure changes that happen when a seal dives.
As blood has a similar density to seawater, most of the sound is carried through the middle ear to the cochlea – allowing seals to hear and communicate underwater.
Dr James Rule, the study’s lead author, is a Scientific Associate at the Natural History Museum, London, and a Postdoctoral Research Fellow at Monash University. He says: “We know that seals are masters of sound. They have eerie songs that sound like something out of a sci-fi film, can keep a beat and can even mimic human speech. But how they manage to hear in air and water has been a puzzle for decades.”
“We now know that the cavernous tissue in their ears holds the answer. Fossils of seals and their relatives suggest that this extraordinary mechanism evolved more than 26.7 million years ago, with all modern seals inheriting their amphibious hearing from this one event.”
While ancient relatives of seals, such as Potamotherium and Puijila, probably only heard well in air, animals like Enaliarctos would have been among the first to have amphibious hearing. It’s likely this transition was driven by the ancestors of seals moving into the sea where hearing underwater would have offered distinct advantages.
The hearing abilities of later seals changes depending on their lifestyle, with early true seals like Devinophoca being better at hearing underwater while the first eared seals appear to have been more adapted to listening in air.
Dr Natalie Cooper, Merit Researcher at the Natural History Museum, London, adds: “The Natural History Museum has some of the best seal collections in the world, and this study really shows its value. This research simply wouldn’t have been possible without these specimens, which have been collected across hundreds of years from all over the world.”
“It’s important that we know how seals hear because noise pollution in the ocean is growing. As the seas get louder, it’s harder for seals to find mates and communicate with each other – putting their survival at risk. By understanding how these animals hear, we can start to put solutions in place.”
