Pinnipeds—including seals, sea lions, and walruses—developed their unique ability to hear exceptionally well both in air and underwater approximately 26.7 million years ago, according to research published July 15 in Proceedings of the Royal Society B: Biological Sciences.
Evolutionary Origins of Amphibious Hearing in Seals and Sea Lions
Pinnipeds are the only mammals capable of exceptional hearing in both terrestrial and aquatic environments, a trait vertebrate paleontologist James Rule of Monash University in Melbourne, Australia, describes as “something of a superpower.” While human middle ears are permanently air-filled chambers that conduct sound efficiently on land, they falter underwater. Because water is over 800 times denser than air, sound waves fail to reach human ears effectively, letting less than 1 percent of underwater sound through, according to Rule.
To bypass this physical barrier, pinniped ears contain spongy cavernous tissue that fills with blood when diving underwater to equalize pressure. Because blood density closely matches water density, this anatomical feature enables the animals to hear clearly, filter out useful sound signals, and pinpoint sound direction underwater, Rule explains.
To trace when this trait emerged, Rule and his colleagues performed micro-CT scans on 217 ear specimens representing 119 mammal species from global museum collections. The scans yielded high-resolution 3-D images of modern pinniped species, their closest living relatives like dogs, weasels, otters, and raccoons, and extinct fossil lineages. Among these ancestral fossils were Puijila, a semi-aquatic freshwater carnivore restricted to hearing in air, and Enaliarctos, widely considered the first fully marine pinniped genus.
Mapping the Pinniped Ear Family Tree
Constructing an evolutionary family tree using these ear scans allowed researchers to pinpoint when cavernous middle-ear tissue first developed. Marine ecologist Steven Benjamins of the Scottish Association for Marine Science in Oban, who was not involved in the study, notes that the early emergence of this shift in the fossil record is particularly surprising.
The early development of underwater hearing indicates that sound perception was critical for survival as these lineages ventured into deeper ocean waters, according to Benjamins. The oldest species examined in the study possess otter-like anatomies suited for shallow freshwater systems where underwater hearing carried less importance.
Data from the study indicate that the sound-conducting capacity of the blood-filled cavernous sinus originated as an evolutionary accident. Initial adaptations likely rendered underwater sounds excessively loud, prompting subsequent evolutionary adjustments in pinniped middle ears to regulate sound amplification.
Sensitivity to Ocean Noise Pollution
While these specialized ears grant pinnipeds acute vocal mimicry skills and exceptional rhythm, they also render the animals vulnerable to environmental stressors like noise pollution. As Rule points out, the evolutionary path toward amphibious hearing has established pinnipeds as masters of sound, but also heightened their sensitivity to noise pollution.
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