Seals’ amphibious hearing developed a long, long time ago

Seals, sea lions and walruses — collectively known as pinnipeds — are the only mammals with excellent hearing both above and below the water. The evolution of this unique ability occurred nearly 26.7 million years ago — about 100 million years after mammals first heard in air, researchers report July 15 in Proceedings of the Royal Society B: Biological Sciences.

Their amphibious hearing is “something of a superpower,” says James Rule, a vertebrate paleontologist at Monash University in Melbourne, Australia. “They are the only mammals which can do this.”

Human middle ears, chambers sealed between the eardrum and inner ear, are permanently filled with air. They conduct sound well on land but cause issues underwater. “Water is over 800 times more dense than air,” Rule says, so sound waves don’t travel to our ears as effectively as they would in the air. Less than 1 percent of underwater sounds get through, he says.

Pinniped ears have spongy cavernous tissue that fills with blood when diving underwater to help equalize pressure. Because blood’s density is much closer to that of water, this “enables them to hear sounds clearly, filter out useful sound signals and tell what direction they come from,” Rule says.

To find out how pinniped ears evolved, Rule and colleagues micro-CT scanned 217 ear specimens from 119 mammal species from museums around the world, allowing them to gain detailed 3-D images of the animals’ ears — “like the scan you would get at the doctor, but much more powerful and for very small objects,” Rule says. They scanned all 34 modern pinniped species, as well as closely related species including dogs, weasels, otters and raccoons and extinct fossil lineages. Among these early ancestors were semi-aquatic freshwater carnivores called Puijila, which could hear only in air, and Enaliarctos, which is believed to be the first fully marine pinniped genus.

Creating a family tree of these ancient creatures’ ears to see when they developed this cavernous tissue enabled the 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 towards amphibious hearing is meant to have occurred,” says marine ecologist Steven Benjamins, who wasn’t involved in the study.

The fact that underwater hearing developed early among pinniped ancestors suggests that accurate hearing underwater is significant to their survival, particularly as they evolved into deeper ocean waters, says Benjamins, of the Scottish Association for Marine Science in Oban. The oldest species included in the study “are much more otterlike 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 underwater sound too loud, but pinniped middle ears evolved again to adjust this amplification.

Thanks to their unique hearing, pinnipeds have great rhythm and can mimic vocalizations, like a parrot, but are incredibly sensitive to sound, such as noise pollution. “The evolution of amphibious hearing has made pinnipeds the masters of sound,” Rule says.