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Evolution of the Cochlea and Hearing in Odontocetes

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Toothed whales’ sight of sound revealed

For toothed whales, a special sixth sense called echolocation allows them to locate food and navigate when diving into deep dark oceans. EU-funded research focuses on the cochlea to shed light on this evolutionary success.

Fundamental Research

Toothed whales are an extraordinary group of marine mammals that have evolved over millions of years. Most remarkable is their ability to ‘see’ in the deep dark ocean. Not able to rely on their sight, whales, dolphins and porpoises, Cetaceans, have learned to listen very carefully and are able to echolocate. This incredible sixth sense of echolocation is a necessary adaptation for toothed whales (Odontoceti), which are the most species-rich marine mammal lineage. In fact, it’s the catalyst for their evolutionary success. The whales use this acute sense of hearing to ‘see’ their surroundings, spot predators, find food and communicate. While research into odontocete hearing has so far been biased towards physiological experiments and auditory pathway identification, the ECHO project focused on the cochlea.

3D cochlea models

Echolocation is a form of biological sonar that uses high-frequency sound produced in the forehead and ultimately detected by the cochlea. Using cutting-edge scanning and visualisation and quantitative analytical techniques like 3D geometric morphometrics, researchers identified the differences in morphology of Odontocete cochlea. The main aim of the ECHO project was to explore the diversity of toothed whale cochleae. “This is something we have certainly delivered on as we now have 3D cochlea models of 95 % of living toothed whale genera, as well as representatives of about 20 extinct families of toothed whales. This represents a massive leap forward in our sample size which was several times smaller prior to the beginning of this project,” explains Travis Park, project coordinator and a postdoctoral research assistant at the Natural History Museum in London. The project also set out to explore the convergent evolution in the echolocation abilities of toothed whales. “This was initially only meant to be in river dolphins, but we ended up extending this study to looking at all living groups of toothed whales, resulting in a broader-scale, better study,” notes Park. From the convergent evolution study, the ECHO team was able to piece together critical data to make some remarkable findings. For instance, the data showed that the extreme acoustic environment of the deep ocean likely constrains cochlear shape, causing the cochlear morphology of sperm whales and beaked whales to converge. “More broadly speaking, the key finding has been that echolocation ability is tied to the habitat/ecological niche the animal inhabits and this is interesting as it fits with what we’ve been finding with other aspects of toothed whale anatomy and hearing, e.g. the skull, mandible. It’s nice to see that the results of ECHO are fitting in with the bigger picture,” clarifies Park.

Six papers, one book chapter and counting

With so much data collected during the project, Park is still making use of the research. So far, six papers have been published and three are on the way. There is also a book chapter. “So, it’s been a very productive period overall. Moving forward, I plan to add to the data set where possible and use the skills and methodological approaches to explore marine mammal evolution more broadly, rather than just focusing on hearing specifically. Lastly, there is one more paper that I want to publish that is directly related to ECHO, and hope to do that this year.”

Keywords

ECHO, toothed whale, cochlea, echolocation, hearing, marine mammal, odontocete

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