A group of French scientists have looked into the ear cavities of 45 million year old whale ancestors, and found striking similarities with modern-day camels. More so, even, than with modern day whales.
Whales rely on a keen sense of hearing for their underwater existence. But whales show surprisingly vast differences in hearing ability. Baleen whales tune into infrasonic sounds—at frequencies too low for humans to hear—to communicate over long distances. Toothed whales do just the opposite, relying on ultrasonic frequencies too high for humans to hear.
Now researchers have fossil evidence from extinct early whale species to suggest that those differences in hearing arose only after whales evolved into the fully aquatic animals we know today. That's based on their findings that whales known as protocetes, which spent time both in water and on land, appear to have hearing more like their terrestrial, even-toed ungulate relatives, including pigs, hippos, 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," says Maeva Orliac of CNRS andUniversité de Montpellier in France.
Protocetes' lack of hearing specialisation suggests that the early whales were unable to echolocate and communicate through long-distance calls in the way that modern-day cetaceans, the group including whales and dolphins, do.
The researchers came to those conclusions based on studies of 45-million-year-old protocetid whale remains found in marine deposits from Togo in West Africa. The researchers studied the bony labyrinth, a hollow cavity that would have housed the hearing organ, in two species of early whales.
Orliac and her colleague Mickaël Mourlam used micro-CT scanning to peer inside the internal structures of rocks and fossils, in much the same way that an X-ray scanner makes it possible to see bones inside a person’s body. Those images allowed them to analyse the internal cavities of the petrosal bone, which shelters the organs of hearing and balance.
"Based on the scans provided by the scanner, we could extract a virtual mould of the hollow cavity that used to contain the hearing organ when the animal was alive," Orliac says. "This process was long and difficult because this cavity was filled with sediments and partly recrystallised and because the petrosal bone in cetaceans is particularly thick and dense, which lowers the quality of the images and sometimes impedes analysing them."
Nevertheless, the scans suggest that early cetaceans had hearing closer to that of their terrestrial relatives. Specialisation to infrasonic or ultrasonic hearing as seen in modern whales came only later, in whales that had already found their way back to the sea.
The findings highlight the importance of studying these early cetaceans to get an accurate picture of whales' evolutionary history. It also suggests that whale'’ evolutionary past is more complicated than had previously been described, the researchers say.
Orliac says they'll be back in field in Togo in December to search for additional protocetid whale specimens. They've so far described two of three species identified in Togo based on dental remains. They hope to find a specimen that will allow them to explore the ear of the third.
Should we be worried? Of course not, why should we? What a ridiculous question.
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