New research has identified the evolutionary origins of “amphibious hearing” in seals, confirming that this specialized ability to hear effectively in both air and water emerged more than 26 million years ago. The study, published in the journal Proceedings of the Royal Society B: Biological Sciences, provides new insight into how marine mammals adapted to survive in two distinct acoustic environments. The findings, led by researchers from Monash University and the Natural History Museum in London, offer a significant baseline for understanding how marine species may be impacted by increasing noise pollution in the world’s oceans.
The Mechanics of a Biological Superpower
Most mammals are restricted to hearing well in either air or water. Humans, for example, rely on ear canals that transmit sound through air to the middle ear. Underwater, this system functions poorly because sound travels through water at a different density than air, causing signals to be reflected or muffled. The research team, which analyzed 3D CT scans of over 200 living and fossil seal specimens, discovered that seals possess a specialized “cavernous tissue” within their ear canals and middle ears. This spongy tissue, rich in blood vessels, serves a dual purpose. When a seal dives, the tissue fills with blood and expands. Because blood has a density similar to seawater, it acts as a conductor, allowing underwater sound to pass efficiently to the middle ear. When the seal is on land, the tissue drains, restoring an air-filled space that allows for effective hearing in the atmosphere. According to the researchers, this mechanism allows underwater sounds to lose less than 1% of their signal, compared to a 99% loss if the middle ear remained filled with air.
Evolutionary Origins and Constraints
By comparing modern seals to terrestrial carnivorans—the group that includes dogs, bears, and cats—and examining extinct fossil relatives, the researchers mapped the development of this trait. They found that early freshwater relatives of seals, such as the extinct *Puijila*, were limited to hearing only in air. The earliest marine seals, including the genus *Enaliarctos*, evolved amphibious hearing approximately 26 million years ago. However, this early adaptation initially resulted in poorer hearing performance in both environments. Over time, the ancestors of modern seals developed modified middle ears that allowed for acute sensitivity in both air and water. Later, true seals, such as the 13-million-year-old *Devinophoca claytoni*, further refined these capabilities, achieving particularly acute underwater hearing.
Contextualizing Modern Ocean Noise
Beyond explaining the evolutionary history of pinnipeds—which include true seals, eared seals, and walruses—the study highlights the importance of understanding the natural baseline of marine hearing. As human-driven ocean noise increases due to commercial shipping, sonar, and offshore construction, researchers warn that it is difficult to assess the damage caused by these pollutants without knowing how seals naturally process sound. “If we don’t understand the baseline of how a seal’s unique hearing functions naturally, we can’t accurately measure the damage this anthropogenic noise is causing,” said Dr. Natalie Cooper of the Natural History Museum.
Seal Acoustic Adaptations
The research underscores that amphibious hearing is fundamental to the lives of seals, facilitating their complex vocalizations. Seals are known for a wide range of acoustic behaviors, including: * Vocal Diversity: The ability to communicate both on land and in the ocean. * Underwater Singing: True seals produce songs that can reach pitches beyond the range of human hearing. * Rhythmic Ability: Seals have demonstrated an ability to perform in tests of rhythm, a trait rarely seen in other animals. * Vocal Mimicry: Some seals have shown the capacity to learn and produce new sounds, including human speech. According to Professor Alistair Evans of Monash University, this sensory flexibility provides seals with an evolutionary edge that allows them to transition seamlessly between acoustic worlds while hunting, resting, or communicating.
Find more reporting in our Tech section.