Seals have a unique superpower in their ears, and we finally know how it evolved

Seals have a unique superpower in their ears, and we finally know how it evolved

Most mammals are restricted to hearing well in either air, like humans and dogs, or water, like whales. Seals, however, possess a rare "superpower" known as amphibious hearing. This ability allows true seals, eared seals, and walruses to balance their acoustic needs across both land and sea.

Research published in Proceedings of the Royal Society B: Biological Sciences has detailed the biological mechanisms and evolutionary timeline of this adaptation. To uncover these secrets, researchers 3D scanned seal ears from global museum collections and compared them with modern land-dwelling carnivorans and extinct fossil relatives.

The Mechanics of Amphibious Hearing

For land mammals, sound travels through air in the ear canals to the middle ear and then to the inner ear. In water, sound travels four times faster and moves through muscles and bones directly to the inner ear, bypassing the middle ear. This shortcut creates a muffled signal for humans because the middle ear cannot filter out unnecessary sounds.

Seals overcome this through specialized cavernous tissue located in the middle ears and ear canals. This spongy tissue is rich in blood vessels and expands as a seal dives to equalize air pressure. Because blood has a similar density to water, it conducts underwater sounds to the middle ear. This system is highly efficient; underwater sounds traveling through cavernous tissue lose less than 1% of the signal, whereas an air-filled ear would lose 99%.

True seals, such as leopard, elephant, and harbour seals, further optimize their hearing through physical adaptations:

• Hydrodynamic Design: True seals have lost external ear flaps, or pinnae, to reduce drag and create a fusiform body shape. This prevents turbulence, or vortex shedding, allowing for more energy-efficient deep-sea diving.
• Bone Conduction: Along with cavernous tissue, seals use acoustic fat pads near the jaw and ear area. These lipids create a low-resistance path, allowing sound to move through fatty tissue and the skull bones directly to the cochlea.

These adaptations allow seals to detect frequencies as high as 60 kHz underwater, which is nearly triple the maximum frequency audible to humans. While they remain sensitive to low-frequency sounds on land to detect threats like polar bears, their hearing is most acute in the ocean.

An Evolutionary Timeline

Seals are part of the carnivoran group, which includes cats, bears, and dogs. The research indicates that the ability to hear amphibiously was not always present. The earliest relatives of seals, such as the extinct freshwater ancestor Puijila, had ears similar to terrestrial carnivorans and could only hear in the air.

Amphibious hearing first evolved 26 million years ago in the earliest marine seals, including Enaliarctos. This transition initially came with a trade-off, as hearing was poorer in both environments. Eventually, the earliest relatives of modern seals evolved modified middle ears that provided acute hearing in both air and water.

Acoustic Abilities and Threats

The unique auditory hardware of seals supports a variety of complex behaviors. True seals sing underwater at pitches that can exceed human hearing ranges. They also exhibit vocal mimicry—exemplified by a US seal named Hoover who mimicked a Maine fisherman—and outperform humans in tests of rhythm.

However, these sensitive systems are vulnerable to human-made noise pollution. Sound from sonar, construction, and ships can muffle communication between marine mammals or even cause physical harm. Researchers note that the experience is similar to trying to hear a friend in a noisy pub or the temporary hearing loss following a loud concert.

To protect these animals, experts suggest lowering the human acoustic footprint. Simple measures, such as slowing down ships around marine wildlife, have proven remarkably effective in preserving the auditory capabilities of seals.