Mystery solved: Scientists reveal how seals hear equally well in water and air

Scientists have finally unraveled the mystery behind the exceptional hearing abilities of seals, sea lions, and walruses, revealing how these marine mammals can hear equally well both on land and beneath the waves. This unique capability, unlike that of other mammals which typically hear poorly in water, has been detailed in a new study published in Proceedings of the Royal Society.

The research, led by paleontologist James Rule from the Natural History Museum in London, involved a comprehensive analysis of over 200 seal skulls from both extinct and extant species. Using micro-CT scanning technology, the scientists identified a specialized tissue within the seals' ear canals. This tissue fills with blood when the animals dive, serving a dual purpose: protecting the ear from pressure changes and facilitating sound transmission.

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It has long been unclear how pinnipeds – seals, sea lions and walruses – manage to hear equally well on land and below the water surface.

Unlike land mammals, which struggle to hear underwater because air in the ear canal reflects sound waves, seals benefit from this blood-filled tissue. Because blood has a density similar to water, sound travels through it directly to the cochlea without significant loss. This adaptation allows seals to effectively communicate and navigate using sound in their aquatic environment.

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Blood has roughly the same density as water, meaning that sound travels through the blood to the cochlea without being hindered by an air pocket.

The study suggests that this remarkable auditory adaptation likely emerged over 26.7 million years ago, during the Oligocene epoch. A genus of seals known as Enaliarctos is considered a potential early developer of this specialized ear structure. Researchers believe this finding is crucial for understanding how marine mammals might be affected by increasing ocean noise pollution from human activities, such as shipping and sonar.

This discovery sheds light on the evolutionary journey of pinnipeds (seals, sea lions, and walruses), which are thought to have descended from land-dwelling, weasel-like ancestors around 50 million years ago. Over time, these ancestors adapted to a marine lifestyle, eventually leading to the diverse array of 34 species present today. The ability to hear effectively underwater represents a significant evolutionary advantage that has shaped their survival and behavior.

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The ability to hear underwater appears to have emerged more than 26.7 million years ago, during the Oligocene epoch.