Fossils show that ancient whales heard like hippos and camels, which is similar to the hearing of Hippos and Camels.

A new study that examined the structures of fossilized ancient whale ear bones has shown that ancient whales could hear just like their terrestrial ancestors.

To navigate, communicate and forage, whales depend on their keen senses of hearing. It may be surprising to find out that modern whales, including the baleen and toothed groups (Mysticetes), have hearing loss at opposite ends. Humpback whales for example, as well as other whales, can hear ultrasonic frequencies too high to be heard by humans. Sperm whales, for instance, can hear infrasonic frequencies too low to be heard by humans.

Modern whales are also affected by the auditory frequencies they perceive. This affects almost every aspect of their lives, including what they eat and where they live. This has sparked a lot discussion: When did the hearing differences begin to evolve?

There are currently two competing theories about which end of hearing frequency spectrum developed first. Did early whales (Protocetidae), hear low-frequency sounds first (i.e. ref), or did they hear high-frequency sounds first ( refer_)?


Two ancient protocetid whales describe the structure of the cochlea as the headquarters for hearing.

Mickael Mourlam is a Paleobiologist and a graduate student at the Universite Montpellier in France. His dissertation advisor, Maeva Orliac (a researcher at The French National Center for Scientific Research), discovered the fossilized earbones of early whales that were buried in deposits that had accumulated between 46 and 43 million years ago. These deposits were found in Kpogame in Togo, an African mining site.

This study examined fossil petrosal bones from early protocetid whales. These bones revealed that they were transitional forms, meaning they spent some time in water and part on land. Dr. Orliac and Mr. Mourlam were able to travel back in time by studying the structure of their petrosal bone structures. They compared the anatomy and petrosal bones of early whales with their relatives, the even toed ungulates. This group includes both terrestrial and semi-aquatic animals like hippopotamuses and pigs. They were able to decode the hearing abilities of protocetids based on these analyses.

Dr. Orliac and Mr. Mourlam used Micro xray Computed Tomography to examine the internal structure of the fossil petrosal bones. The x-rays used in micro-CT scanning to probe fossils are much more intense than those used by medical micro-CTs to examine the inner workings of living organisms. The powerful micro-CTs are able to examine the three-dimensional structures of fossil specimens without needing to remove them by cutting them into thin slices.

“This was a difficult process because this cavity was filled by sediments and partially recrystallized, and because the petrosal bones in cetaceans are particularly dense and thick, which lowers quality images and sometimes hinders analysis,” Dr. Orliac, the study’s coauthor.

Dr. Orliac explained in an email that “[T]he high bone density in protocete cetaceans made scanned images difficult to work with.” The density bone made the contrast very low in the latter. Based on scans, we spent many days reconstructing the skull fragment’s internal structures.

A sample is placed under an xray generator to create an individual image. The xrays travel through the sample and are recorded by the xray detector on the other side. Once the image is complete, the specimen is rotated to a new location and then the process continues until the entire specimen is x-rayed. Figure 1: The micro-CT “thin slices”, which resulted in the image, are assembled using computer software to create three-dimensional images.

Dr. Orliac said, “Based on scans taken by the scanner we were able to extract a virtual mould of the hollow cavity that once contained the hearing organ when it was alive.”

An endocast is a three-dimensional representation of the interior structure within the petrosal bones. Endocasts were used by researchers to examine the interior cavities of the petrosal bone. This bone contains the cochlea (a spiral bone that is part of the inner ear’s auditory portion). This bone looks like a coiling snail shell.


The hearing abilities of early amphibious whales were similar to their terrestrial kin

Dr. Orliac and Mr. Mourlam compared nine parameters in the cochlea for terrestrial and semi-aquatic even toed animals (orange region, Figure 2) with those of early whales, (red area of Figure 2), and to modern whales, (Mysticeti, Odontoceti, pink area, figure 2)

These structural similarities also suggest that early whales were unable to echolocate or communicate underwater with infrasound.

Dr. Orliac said that they were “most likely able to communicate underwater, like seals,” however.

Modern whales were able to hear the echolocation and ultrasonic hearing specializations later. This was only after they had returned to sea.

Dr. Orliac stated in an email that “[S]ensitivity [to high frequency sounds (ultrasounds] and echolocation offer advantage for hunting furtively, or with reduced visibility of tooth whales which are piscivores.” “At the other end, sensitivity to ultrasonic noise offers advantages for long-distance communication and was selected by baleen whales that travel across the ocean.


Whales have extreme hearing abilities due to a high-frequency ancestral ear

Based on their research, Dr. Orliac and Mr. Mourlam concluded that modern whales have an extreme hearing ability because of a low-frequency ancestral ear.

The authors write in their paper that “our findings show that infrasonic or ultrasonic hearing evolved after the emergence fully aquatic whales.” Neoceti encompasses the major whale groups (Odontoceti, Mvsticeti) and is therefore a taxonomic category.

Dr. Orliac said that she and Mr. Mourlam will be returning to Togo in December to look for more protocetid-whale specimens. They have only examined two of three species of whales in Togo so far.

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