High-tech approaches reveal the evolution of whale hearing

Humans don’t hear that well underwater. But cetaceans – a group of marine mammals that includes whales, dolphins and porpoises – use sound in diverse ways. Baleen whales produce low-frequency calls that can carry messages to companions hundreds of kilometers away. Dolphins make high-frequency clicks that bounce off objects and help them navigate.

The ancestor of modern cetaceans, however, was a four-legged animal that lived on land about 50 million years ago. To investigate how cetaceans developed underwater hearing, scientists analyze sensory systems of fossil specimens spanning the evolutionary tree. These studies historically involved serial sectioning of samples. But scientists now use techniques like X-ray computed tomography (CT) to probe precious samples.

“You can actually make digital versions of them without ever having to damage the specimen,” said Rachel Racicot, a research assistant professor of biological sciences at Vanderbilt University. Researchers examine these “virtual fossils” to uncover the steps of cetacean evolution. Racicot described such nondestructive approaches in a recent review as part of a special issue of The Anatomical Record on marine mammal sensory systems.

Ear Evolution

The inner ear is an extremely well-studied region of the nervous system in cetaceans. It contains a hearing organ, the cochlea, that receives sound vibrations and transmits signals to the brain. It also contains an organ of balance, the vestibular complex.

The soft tissues that make up these organs aren’t preserved in fossils, but researchers can study the empty space that’s been left behind using X-ray CT.  For example, certain features of the cochlea, a spiral-shaped cavity resembling a snail shell, provide clues about what an animal was able to hear.

“In terms of baleen whales, it seems like having more turns is involved with lower-frequency hearing,” Racicot said. In contrast, toothed whales — which include dolphins, porpoises, and other whales with teeth — have cochleae with fewer turns, which seems to be correlated with their higher-frequency hearing.

By measuring the number of turns and other structural features, Racicot and colleague Eric Ekdale found that Zygorhiza kochii, an early whale that was fully aquatic, had a cochlea similar to modern baleen whales. This suggests that low-frequency hearing appeared early in cetacean evolutionary history and was maintained in baleen whales.

Other researchers went even further back in the past and inspected fossils from early amphibious whales known as protocetids. In addition to samples from modern whales, their analysis included cochleae from modern artiodactyls — even-toed hoofed animals, like hippopotamuses and pigs. Present-day whales might not have toes, but the ancestors of cetaceans did.

The researchers found that the protocetid cochleae were morphologically between those of land-dwelling artiodactyls and modern aquatic whales. Since these artiodactyls have mid-frequency hearing capabilities, the researchers propose that the divergent hearing abilities of modern whales derive from an ancestral mid-frequency ear.

In a more recent study, Racicot and colleagues suggested that high-frequency ultrasonic hearing evolved twice in the history of toothed whales, based on changes in cochlear structure in different species along the evolutionary tree.

Future Research

In addition to the cochlea, Racicot and others are using X-ray CT to investigate questions about the vestibular complex and the cetacean brain. Understanding how cetacean sensory systems evolved will benefit from additional fossils that provide increased coverage of the whale evolutionary tree.

“We don’t have the data yet to fully explore it,” Racicot says. “But I think we’re getting there.”