Seals long ago evolved to hear above the water — and below

On land, a mother seal can pick out her pup’s cry on a noisy beach. Underwater, seals listen for songs of potential mates or calls warning of predators. This ability to hear well both in and out of the water is unique among mammals. And some marine mammals evolved it nearly 26.7 million years ago, a new study finds.

Seals, sea lions and walruses are known as pinnipeds. They have amphibious hearing — or ears adapted to work well on land and in water. This is “something of a superpower,” says paleontologist James Rule. “They are the only mammals which can do this.” Rule works at Monash University in Melbourne, Australia.

The human ear has an inner chamber filled with air. It conducts sound well when we’re on land but not when we’re underwater. “Water is over 800 times more dense than air,” Rule points out. So sound waves don’t travel to our ears as effectively as they would in air. In fact, less than one percent of underwater sound gets through, he says.

Pinnipeds have spongy tissue inside their ears. It fills with blood when they dive into water. That helps to keep the pressure the same inside and outside their ears. Blood’s density is much closer to that of water. So sound waves moving through water can travel well through that blood, too.

This allows the creatures to hear sounds clearly underwater. They also can filter out background noise. It allows them to better hear useful sounds, Rule says, and to tell what direction they come from.

Swimmers’ ears

It wasn’t clear how pinnipeds developed that trait. So Rule’s team used X-rays to scan 217 ear specimens from museums around the world. The samples included 119 mammal species.

Known as micro-CTs, these scans provided detailed 3-D images of the animals’ ears. It’s “like the scan you would get at the doctor, but much more powerful,” Rule explains — “and for very small objects.”

The scans included specimens from all 34 modern pinniped species. They also scanned ears of other mammals. These included dogs, weasels, otters, raccoons and fossils of their extinct relatives. Among those early ancestors was a creature called Puijila. It spent a lot of time in water but could hear only in air. They also studied animals known as Enaliarctos. Scientists think these were the first fully sea-dwelling pinnipeds.

The team created a family tree of these ancient creatures. Then they looked at how their ears changed over time.

Sea otters, another type of marine mammal, live in shallow waters. Making only make short dives, they don’t need excellent underwater hearing. That may be why they never developed the acute below-water hearing found in pinnipeds.

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That revealed when animals developed this special tissue that can be filled with blood. And that allowed the researchers to work out when pinnipeds evolved amphibious hearing. The researchers shared their findings July 15 in Proceedings of the Royal Society B: Biological Sciences.

Steven Benjamins was surprised at how early underwater hearing emerged. Though not involved in the new study, he is a marine ecologist. He works at the Scottish Association for Marine Science in Oban.

That underwater hearing developed early — in pinniped ancestors — suggests this trait likely aids the animals’ survival. Some of the oldest fossils scanned had ears more like those of otters, Benjamins says. These creatures had lived in shallow waters where underwater hearing wasn’t essential.

Accurate underwater hearing likely became key, he says, as pinnipeds moved into deeper seas.

The ability of the blood-filled tissue to conduct sound began as an accident, the new data suggest. At first, it likely made underwater sounds too loud. But pinniped ears evolved again to adjust to that sound level.

This means they are also incredibly sensitive to sounds, including noise pollution, notes Rule. These animals rely on sound to communicate. Loud sounds, such as from boats, can alter pinniped behaviors, including how they care for their young.

This unique hearing has also has endowed many pinnipeds with great rhythm. Like parrots, they can mimic speaking and singing. Says Rule, “The evolution of amphibious hearing has made pinnipeds the masters of sound.”