Samples from hundreds of marine mammals show evolution of specialized ear tissues
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Seals, sea lions and walruses – collectively called pinnipeds – are the only mammals with excellent hearing above and below water. The evolution of this unique ability occurred almost 26.7 million years ago — about 100 million years after mammals were first heard in the air, researchers report July 15 Proceedings of the Royal Society B: Biological Sciences.
Their amphibious hearing is “a kind of superpower,” says James Rule, a vertebrate paleontologist at Monash University in Melbourne, Australia. “They are the only mammals capable of doing this.”
Human middle ears, sealed chambers between the eardrum and the inner ear, are permanently filled with air. They conduct sound well on land but cause problems underwater. “Water is more than 800 times denser than air,” Rule explains, so sound waves don’t reach our ears as effectively as they do in air. Less than 1% of underwater sounds are transmitted, he says.
Pinniped ears have spongy cavernous tissue that fills with blood when diving underwater to help equalize pressure. Because the density of blood is much closer to that of water, it “allows them to hear sounds clearly, filter out useful sound signals, and know where they’re coming from,” Rule says.
To find out how pinniped ears evolved, Rule and his colleagues micro-CT scanned 217 ear specimens from 119 mammal species from museums around the world, allowing them to obtain detailed 3D images of animals’ ears – “like the scanner you would get at the doctor’s office, but much more powerful and for very small objects,” Rule says. They analyzed all 34 modern species of pinnipeds, as well as closely related species including dogs, weasels, otters and raccoons, as well as extinct fossil lineages. Among these early ancestors were semi-aquatic freshwater carnivores called Pujilawho could only hear in the air, and Enaliarctoswhich is considered the first fully marine pinniped genus.
Creating a family tree of these ancient creatures’ ears to see when they developed this cavernous tissue allowed researchers to determine that pinnipeds evolved amphibious hearing around 26 million years ago.
“Among the things that surprise me is how early in the fossil record the shift to amphibious hearing is thought to have occurred,” says marine ecologist Steven Benjamins, who was not involved in the study.
The fact that underwater hearing developed very early in the ancestors of pinnipeds suggests that accurate hearing underwater is important for their survival, particularly as they evolved toward deeper ocean waterssays Benjamins, of the Scottish Association for Marine Science in Oban. The oldest species included in the study “are much more otter-like in their anatomy, likely having lived in shallow freshwater systems where underwater hearing may have been less important.”
The study suggests that the ability of the blood-filled cavernous sinus to conduct sound was an evolutionary accident. At first, this probably made the underwater sound too loud, but the average ears of pinnipeds again evolved to adjust for this amplification.
Thanks to their unique hearing, pinnipeds have exceptional rhythm and can imitate vocalizations, like a parrot, but are incredibly sensitive to sounds, such as noise pollution. “The evolution of amphibious hearing made pinnipeds the masters of sound,” Rule says.