Marine researchers have identified the biological origins of “amphibious hearing” in seals, a specialized evolutionary trait dating back 26 million years. By analyzing CT scans of fossil and modern specimens, scientists discovered cavernous tissue in seal ears that allows for seamless acoustic perception both underwater and on land.
Evolutionary Origins of the “Amphibious Hearing” Superpower
For most mammals, hearing is a terrestrial affair. Humans and dogs rely on air-filled ear canals to conduct sound, a system that becomes largely ineffective when submerged. Seals, however, have evolved a unique sensory flexibility that allows them to transition between air and water without losing auditory performance. According to research published in Proceedings of the Royal Society B: Biological Sciences, this capability is not a recent development but an ancient adaptation that emerged approximately 26 million years ago.
The study, which assembled the largest dataset of pinniped ears to date, indicates that early ancestors of seals—such as the freshwater-dwelling Puijila—lacked this ability and relied solely on air-based hearing. It was only with the emergence of the earliest marine seals, including Enaliarctos, that the specialized anatomy required for underwater sound conduction began to evolve.
Cavernous Tissue and the Mechanics of Underwater Sound
The secret to the seal’s acoustic range lies in a specific type of spongy, blood-vessel-rich tissue known as cavernous tissue. Located within the ear canals and middle ears, this tissue expands with blood when a seal dives, effectively equalizing internal pressure. Because this blood-filled tissue possesses a density similar to water, it acts as a highly efficient conduit for underwater sound, allowing vibrations to reach the middle ear with minimal signal loss.
“Seals are the only creatures in their class to have evolved this sensory flexibility. It allows them to transition seamlessly between two entirely different acoustic worlds without a single drop in performance, giving them an evolutionary edge whether they are resting, hunting or communicating.”
Professor Alistair Evans, Head of the EvoMorph Research Group
This anatomical shortcut ensures that seals do not experience the muffled sound quality humans encounter when submerged. By bypassing the limitations of air-conducted sound, the tissue allows seals to maintain their complex communication and hunting behaviors across both mediums. Dr. James Rule, the study’s lead author from the Monash School of Biological Sciences, noted that this adaptation is as fundamental to the survival of pinnipeds as their flippers. This evolutionary innovation facilitated the remarkable vocal diversity of pinnipeds, which allows them to communicate both on land and in the ocean,Rule explained to Xinhua.
Museum Collections as Tools for Modern Conservation
The research relied on 3D CT-imaging of specimens held in museum vaults, highlighting the importance of long-term biological preservation in understanding current ecological crises. Dr. Natalie Cooper, a Merit Researcher at the Natural History Museum, emphasized that these historical archives provide the necessary baseline data to gauge how human activity—such as sonar, offshore construction, and commercial shipping—affects marine life.
Photo: The Conversation
As the underwater world becomes increasingly noisy, scientists are concerned that anthropogenic sound could mask the natural communication channels seals have perfected over millions of years. By defining the natural parameters of how these mammals hear, researchers hope to better assess the physiological and behavioral impacts of noise pollution. 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, Cooper said.
The international research collaboration involved experts from institutions across the globe, including Monash University, the Natural History Museum in London, and the Max Planck Institute, among others. The study concludes that the specialized ear tissue remains a defining characteristic of true seals, eared seals, and walruses, serving as a critical component of their success in diverse marine habitats.
