Acoustic Fats, Ear Trumpets, and How Whales Hear

Up until a few days ago, I had never heard a blue whale. I wasn’t even aware they made sounds that my primate ears could pick up. But, thanks to Australian Antarctic Division researchers, I was able to listen to the massive mammal for the first time:

It’s an incredibly soothing sound. Certainly moreso than Jeff Bridges and his singing bowls. But I’m probably not hearing the blue whale’s “song” as it’s meant to be heard. My ears are all wrong.

I take in sounds through my ear canal. But modern whales are weird. As their four-legged ancestors slipped into the sea and took up permanent residence there, evolution granted whales an entirely different way of hearing that relies on “acoustic fats” that help transmit sounds to a specialized “ear trumpet” on the skull.

But when did whales evolve this alternative auditory apparatus? To answer that question, National Museum of Natural History researchers Maya Yamato and Nicholas Pyenson looked to fetal whales and fossils to determine when whales started listening through acoustic funnels.

Yamato and Pyenson scrutinized 56 fetal whales collected during the heyday of 20th century commercial whaling, as well as specimens saved by the Smithsonian from bycatch and strandings. This sample encompassed ten different living whale lineages, which was critical because of differences in the way the two main groups of modern whales listen to the sea.

Toothed whales – odontocetes – have acoustic funnels that are oriented forward and connect to acoustic fat in the jaw. This is probably because a forward-facing ear is essential to accurately reading returning pings from echolocation. But some baleen whales – mysticetes – have acoustic funnels oriented more toward the side. By studying the early development of the ear trumpets in each of these lineages, Yamato and Pyenson were able to outline how this common structure diverged amongst the whales.

In both toothed and baleen whales, Yamato and Pyenson found, the acoustic funnel starts out as a forward-facing, V-shaped structure made by the malleus and goniale bones of the ear. As toothed whales grow, the acoustic funnel extends forward from the bones to the acoustic soft tissues, but, among some baleen whales, the acoustic funnel shifts to the side. Why this happened isn’t entirely clear, but it may have something to do with baleen whales communicating with low-frequency sounds over long distances and no longer needing the forward-directed hearing of their echolocating cousins.

Since the acoustic funnel starts off the same way in both toothed and baleen whales, Yamato and Pyenson hypothesize, the feature must have been present in the last common ancestor of both, around 34 million years ago. And the fossil record bears this out. Adult specimens of fossil baleen whales such as Aetiocetus and Albertocetus have forward-oriented ear funnels. This means that head-on hearing was probably the default state for both baleen and toothed whales, only later modified by humpbacks, minkes, and their relatives. Sadly for me, though, this means that even if I could learn to speak whale, I wouldn’t be able to properly understand a whale without some pretty drastic modifications to my head.

Reference:

Yamato, M., Pyenson, N. 2015. Early development and orientation of the acoustic funnel provides insight into the evolution of sound reception pathways in cetaceans. PLOS ONE. doi: 10.1371/journal.pone.0118582

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