Have you ever tried to talk to someone while underwater? No matter how loud you might shout, it always sounds muffled. But why? As with many things in our bodies, the answer lies with evolution.
Humans evolved on land, and our ears are adapted to collect and interpret sound travelling through the air. Unfortunately, this makes our ears pretty much useless underwater.
Most mammals can either hear well only in air (dogs, humans) or water (whales). But there is one group of mammals who can do both - seals. This includes true seals, eared seals, and walruses.
These marine mammals spend time both on land and at sea, and need to balance life in both worlds, including their hearing. Known as amphibious hearing , this ability is unique to seals, and therefore quite the superpower.
While we know that seals can hear amphibiously, what's long remained a mystery is exactly how they do so, and when this ability evolved. In our new paper, published today in Proceedings of the Royal Society B: Biological Sciences , we investigated amphibious hearing in seals to find answers to these questions.
One or the other
Theoretically, being able to hear well in both air and water should be impossible for mammals.
That's because for animals like humans, who live on land, sounds travel through the air in our ear canals to the middle ear. It then transmits sounds to the inner ear for processing.
In water, which has a similar density to our bones, sound travels four times faster. This means sound travels through muscles and bones directly to the inner ear, bypassing the middle ear.
This auditory shortcut means our middle ears can't filter out unnecessary sounds, muddling the signal - this is why things sound muffled underwater to us.
So how do seals manage both? And how long have they been doing it for?
Special tissue in the ears
To find out, we 3D scanned the ears of seal specimens from museum collections around the world. These scans revealed that the ear canals and middle ears of seals contain something called cavernous tissue.
This type of tissue is "spongy" and contains a lot of blood vessels. In seal ears, it fills with blood and expands as the seal dives, allowing them to equalise the air pressure inside their ears.
Importantly, the blood has a similar density to water and can fulfil a second function: conducting underwater sounds to the seal's middle ear.
As a result, underwater sounds travelling through the cavernous tissue lose less than 1% of sound signal, as opposed to 99% if the ear was still filled with air.
How did seals evolve amphibious hearing?
Next, we wanted to know how and when this remarkable adaptation evolved. This required some comparisons.
Seals are carnivorans - they're part of the major group of mammals that includes dogs, bears and cats. We measured the ears of seals and of some modern land-dwelling (terrestrial) carnivorans, as well as some extinct fossil relatives of seals.
We then used several statistical analyses to estimate whether these extinct seals could hear amphibiously or not.
What we found was that the earliest relatives of seals, including the extinct freshwater ancestor Puijila , could only hear in the air, like us. Their ears are most similar to their terrestrial relatives.
By contrast, the earliest marine seals, including Enaliarctos, evolved amphibious hearing 26 million years ago. But this came at a cost for these early seals; their hearing was initially poorer in both environments.
This issue was eventually solved in the earliest relatives of modern seals, who evolved modified middle ears capable of acute hearing in both environments.
Crucially, the true seals (such as harbour, leopard and elephant seals) became very sensitive to underwater sounds.
Remarkable seal acoustics
Seals have other remarkable acoustic abilities too. True seals sing underwater, sometimes at pitches beyond the range of human hearing . Seals can also outperform humans in tests of rhythm , a rare ability in the animal kingdom .
They can also learn to produce new sounds via vocal mimicry. A famous example is Hoover the talking seal in the US, who mimicked some phrases from a Maine fisherman.
Seal acoustics are indeed special, and they are at the forefront of research into the evolution of speech .
The problem of noise pollution
The ocean is becoming increasingly noisy , and human-made noise pollution is an emerging threat to marine mammals.
Just like how we have difficulty hearing a mate in a noisy pub, marine mammals struggle to hear each other over noise from ships, construction, and sonar.
Ever been to a concert and your hearing is temporarily worse? That happens to marine animals too , except it's from the din of our ocean activities. Loud underwater sounds even have the potential to cause harm .
Marine life, including seals, need to hear each other. How do we keep the racket down? Simple solutions, such as slowing down ships around marine wildlife , are proving remarkably effective. By lowering our acoustic footprint, we can better protect the superpowered hearing of our aquatic neighbours.
James Patrick Rule previously received funding from a UK Research and Innovation Fellowship from the Engineering and Physical Sciences Research Council. James is currently funded from an Australian Research Council Discovery Project.
Ian G. Brennan received funding as a Marie Sklodowska Curie Actions fellow under the Horizons 2020 scheme from the European Research Council. Ian currently receives funding from an Australian Reesearch Council DECRA fellowship.
Travis Park receives funding from an Australian Research Council DECRA Fellowship.
