Early Whales Lacked Specialized Hearing, Study Finds

An illustration of one of two protocetid species from Togo in West Africa. (Drawing © M.J. Orliac, based on the reconstruction of the skull drawn by Róisín Mourlam.

(CN) – Differences in modern whales’ sense of hearing trace back to habitat-based genetic variations among their extinct relatives, a new study finds.

To communicate underwater, baleen whales tune into infrasonic sounds – at frequencies too low for humans to hear – to interact over great distances. Toothed whales, on the other hand, rely on ultrasonic frequencies too high for humans to hear.

In a paper published Thursday in the journal Current Biology, French researchers present fossil evidence that suggests these variations developed only after whales evolved into the fully aquatic animals they are today.

Protocetes – early whales that spent time both on land and in water – had a sense of hearing similar to that of their land-based, even-toed ungulate relatives, including hippos, pigs and camels.

“We found that the cochlea of protocetes was distinct from that of extant whales and dolphins and that they had hearing capacities close to those of their terrestrial relatives,” said study co-author Maeva Orliac, a researcher at the French National Center for Scientific Research.

The protocetes’ lack of hearing specialization suggests that the early whales could not echolocate, which is the use of sound waves and echoes to determine where objects are in space, and communicate through long-distance calls in the manner that modern cetaceans, a group that includes whales and dolphins, do, according to the study.

The team came to those conclusions after analyzing 45-million-year-old protocetid whales’ remains discovered in marine deposits from Togo in West Africa. The researchers examined the bony labyrinth, a hollow cavity that would have supported the hearing organ, in two species of early whales.

Micro-CT scanning allowed the scientists to capture images of the internal structures of rocks and fossils in the same way an X-ray scanner makes it possible to see bones inside a person’s body. Those images helped the team analyze the internal cavities of the petrosal bone, which shelters the organs for balance and hearing.

This is a 3-D model of the virtual reconstruction of the bony labyrinth. (Credit: Mourlam and Orliac)

“Based on the scans provided by the scanner, we could extract a virtual mold of the hollow cavity that used to contain the hearing organ when the animal was alive,” Orliac said. “This process was long and difficult because this cavity was filled with sediments and partly recrystallized and because the petrosal bone in cetaceans is particularly thick and dense, which lowers the quality of the images and sometimes impedes analyzing them.”

The scans suggest that early cetaceans had a sense of hearing similar to that of their land-based relatives, while the specialization to ultrasonic or infrasonic hearing seen in modern whales came later in species that had already returned to living in the sea.

The findings also indicate the evolutionary past of whales is more complicated than previously known, which highlights the value of studying early cetaceans to establish an accurate picture of whales’ unique history, the team said.

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