Fossils hold hidden clues to the evolution of whales' incredible hearing

Scientists find that a 27-million-year-old whale's ear likely could pick up remarkably high-frequency sounds.

Courtesy of A. Gennari 2016
Echovenator produces sound that bounces off prey, creating echoes. The whale's inner ear receives the sound waves.

In the dark depths or the murky shallows of the ocean, it can be difficult to see obstacles, predators, or tasty prey. But orcas, porpoises, dolphins, and other toothed whales famously have a way to "see."

The toothed whales, scientifically called odontocetes, use echolocation, emitting sounds and listening to how they echo back in order to map their surroundings. And those sounds they emit are at a particularly high frequency, a quality which adds finer details to their auditory map.

It turns out, these toothy marine mammals have had this special hearing for a remarkably long time. A team of researchers found that the inner ear of Echovenator sandersi, an odontocete that lived 27 million years ago, looked a lot like today's toothed whales

"What this shows is that you get fairly advanced echolocation and ultrasonic hearing abilities right at the base of the evolution of all these toothed whales," study lead author Morgan Churchill of the New York Institute of Technology tells the Monitor in a phone interview. 

This study doesn't come out of nowhere, says Erich Fitzgerald, senior curator of vertebrate paleontology at the Museum Victoria in Melbourne, Australia, who was not part of the study.

In a 2014 study, researchers found evidence in a 28-million-year-old fossil that toothed whales could produce the high-frequency noise used in echolocation. Then, published in April of this year, Dr. Fitzgerald's own research found that a 26-million-year-old whale could likely hear that high pitched sound when it echoed back.

Still, Fitzgerald says of the new study, "This really nice paper, analysing superb fossils from South Carolina, provides yet more compelling support for the idea that the echolocation, or sonar, of living dolphins, killer whales, and porpoises can be traced right back to their earliest days, more than 25 million years ago." 

When E. sandersi lived, it probably looked a lot like today's dolphins or porpoises, study co-author Jonathan Geisler says. The animal would have probably used echolocation to track down small fish to snack on, although there are hints that it might not have been as good at it as modern toothed whales.

"Fossils supported the idea that high frequency hearing was important as a behavioral breakthrough early in the evolution of the group," Ewan Fordyce, a vertebrate paleobiologist at the University of Otago in New Zealand who was not part of the study, tells the Monitor in an email. "But direct compelling evidence had to be found. This paper provides that evidence – CT scans of the hearing organ, cochlear."

'Hidden intrigue'

This new study, published Thursday in the journal Current Biology, doesn't just add detail to the story of the incredible hearing of toothed whales. It also holds "hidden intrigue," Fitzgerald says.

That's because when the researchers studied CT scans of E. sandersi's inner ear bone, they compared it to the structures found in many living and extinct whales. When they looked at all the fossils in their study, the researchers found "some suggestion that there's a tendency to hear at higher frequencies before the evolution of echolocation," Dr. Churchill says.

And that could go as far back as the common ancestor to all whales, even those that make particularly low frequency, or infrasonic, sounds instead.

"We were pretty surprised that we saw these hints of high frequency hearing even earlier in the tree before you have the split of the modern groups," Dr. Geisler tells the Monitor. And "it raises some really interesting questions like why: Why was that starting to evolve?"

The scientists still don't have an answer to that question. And the hint that high frequency hearing might have evolved in the common ancestor of all whales is the exact opposite from what some other researchers have suggested.

Highly diverged hearing

Today there are two main lineages of whales: Toothed whales (odontocetes) and baleen whales (mysticetes). While the toothed whales can emit and hear high-frequency sounds to echolocate, the baleen whales (which include blue whales, humpback whales, and gray whales) are sensitive to low-frequency noises and aren't thought to echolocate.

"A prevailing question surrounding whale evolution is the origin of those two very different hearing regimes," Eric Ekdale, a research scientist at San Diego State University and San Diego Natural History Museum, tells the Monitor in an email. And Dr. Ekdale and the authors of the new paper agree that the answer lies with the common ancestors of odontocetes and mysticetes: the archaeocetes.

Ekdale and Rachel Racicot, a researcher at the National History Museum of Los Angeles County, found evidence in their own study of fossils of the archaeocete Zygorhiza that the animal's hearing was more like that of modern baleen whales, and was sensitive to infrasonic frequencies.

This "sounds at the surface like contradictory results," Ekdale says. "However, we sampled a larger number of baleen whales, but only a small number of toothed whales. In contrast, the current study by Churchill samples a larger number of toothed whales, but a smaller number of baleen whales. Both studies sample a small number of archaeocetes."

Dr. Racicot agrees. "The different result we obtained (lower frequency hearing being ancestral based on our one example) may imply that there was a lot of variation in hearing abilities of early whales that needs to be investigated further so we can understand more about ancient (Eocene) whale ecologies," she writes in an email to the Monitor.

Geiser agrees that this question is far from settled. "I wouldn't say we've changed the story," he says. "I think we've suggested some very interesting avenues for additional research that maybe people haven't thought of."

So what difference could it make?

"Animals hear for one of three reasons," Hans Thewissen, a professor of anatomy at Northeast Ohio Medical University, tells the Monitor in a phone interview. They're listening for predators or prey, or they're listening to each other, he says.

If these sounds are being used for communication, Dr. Thewissen suggests, it's possible that this high-frequency hearing could have evolved even earlier than the first whales.

Whales evolved from land-dwelling mammals that thrived near the water. In fact, hippos are thought to be the closest living relative of modern whales. 

"So it's possible that they were actually making high-frequency sounds to talk to each other on land," Thewissen says. "But," he admits, "I'm speculating. We don't know that."

Thewissen's own research on the middle ear of archaic whales supports the idea that the common ancestor of baleen whales and toothed whales could hear high frequency sounds, so "I also agree with them that high-frequency and ultrasonic sound reception originated well before Echovenator," he tells the Monitor in a follow-up email. But "I think the judges are still out on when that was."

There's a lot more research to be done to determine how the ears of baleen and toothed whales became so different, Dr. Fitzgerald says. But "the answer lies out there in the rocks ... or maybe in a museum collection somewhere, waiting to be uncovered."

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