The mystery solved – this is why seals have unique hearing
Seals, sea lions and walruses have unique hearing abilities – they are the only mammals that can hear well both in water and in air. How this works has been clarified in a study that also reveals when this ability first emerged.
The study, published in Proceedings of the Royal Society, was carried out by researchers from several countries led by palaeontologist James Rule at the Natural History Museum in London.
It has long been unclear how pinnipeds – seals, sea lions and walruses – manage to hear just as well on land as they do underwater. Whales, dolphins and porpoises, for example, hear very well in water but poorly in air.
Blood-filled tissue
We humans and other land-dwelling mammals, on the other hand, hear very poorly in water. Normally, we transmit sound waves from the eardrum to the cochlea, but this does not work in water because the air pocket behind the eardrum reflects almost all sound away.
Seals, however, have solved this problem. Using micro-CT scanning, the researchers scanned over 200 seal skulls from both modern and extinct species. They found that seals have a special tissue in the ear that fills with blood when the animals dive, to protect the ear from damage as the pressure changes.
This gave them a further advantage – blood has roughly the same density as water, which means that sound travels through the blood to the cochlea without being obstructed by an air pocket. This is why seals can hear and communicate with one another beneath the water’s surface.
Eerie sounds
This means they have added yet another skill to their repertoire. They are already masters of everything to do with sound.
Several of the species produce eerie sounds that resemble spooky songs; they can imitate human speech; and sea lions can keep in time with music.
The ability to hear underwater appears to have emerged over 26.7 million years ago, during the Oligocene epoch.
A genus comprising five species, Enaliarctos, may have been the first to develop this specialised tissue. Earlier ancestors probably did not have the ability to hear well underwater.
The researchers point out that the study may be important for understanding how seals are affected by engine noise and other forms of noise pollution in the oceans.
Seals, sea lions and walruses make up three families within the caniform order, Caniformia, which also comprises dogs, bears and martens.
They probably evolved from weasel-like forms that, over time, abandoned the land for the sea. This may have happened around 50 million years ago, but it was not until much later – just over 26 million years ago – that modern seals first appeared.
Today, there are 34 species, comprising 15 species of eared seals (sea lions and fur seals), 18 species of true seals – which also include our own species, the harbour seal, the ringed seal and the grey seal – as well as the walrus.
Source: Natural History Museum
