Hearing in Marine Mammals

Today was the annual Marine Naturalists' Gear-Down, an end-of-season lecture series hosted by The Whale Museum. While there was a lot of great information shared, I thought I would focus this post on one interesting fact I learned and have since been pondering (and reading about further in a marine mammal biology textbook).

Our first speaker was talking about the adaptations required by marine animals to deal with the conditions of oceanic life. He made an interesting point when talking about hearing underwater. Evolutionarily speaking, animals originally had ears designed for hearing in the water, but as animals moved to land there had to be changes in the structure of the ear to deal with hearing in air. Humans, like all terrestrial mammals, have air in the middle ear, which makes us great at hearing sounds transmitted through the air. If, however, we have our heads underwater, our hearing is impaired due to impedance mismatch, which basically happens when sound switches from one medium to another. When sound going through the water encounters the air in our middle ear, it impairs our ability to hear it clearly or determine which direction its coming from.

Marine mammals evolved from terrestrial mammals, so their ears still have the basic structure designed for hearing in the air. How, then, have they adapted to hear so well underwater? They've had to find a way to overcome the impedance mismatch caused by the air/fluid barrier.

Toothed whales spend all their time underwater and have "solved" this problem by receiving sounds directly to their inner ear not through their external ear canal but through the fatty tissue of their lower jaw, which conducts sound in a similar manner to water. But what about pinnipeds, which need to hear both in the air and underwater?

This harbor seal pup is adept at hearing in the air and underwater, due to a very interesting adapation

It's theorized that most pinnipeds primarily receive underwater sound through bone conduction, which means that sound reaches the inner ear by resonating through the bones in the skull. Some human hearing aids actually make use of bone conduction. This process isn't very well understood in pinnipeds, since orienting the direction of the sound must still be difficult. These pinnipeds then hear sound in air via the typical pathway through their external ear canal.

Different pinniped species have different hearing abilities in air/water depending on their life history. Elephant seals have sharper underwater hearing, whereas sea lions have better hearing in the air. Harbor seals, however, get the best of both worlds. As our speaker shared today, harbor seals actually have a mechanism to fill part of their middle ear with blood (a fluid which transmits sound more like water than air) when they are underwater to better receive sound. When they are at the surface, this blood drains, restoring the air to the middle ear and allowing them to hear better in the air. Since harbor seals use the external ear canal to hear underwater instead of bone conduction, they have better underwater directional sensitivity than other pinnipeds.

The biologist in me is fascinated by this kind of thing....

2018 Update: I continue to get inquiries about this post years later! Unfortunately I don't recall who the speaker was, but did find this reference in the book Sensory Ecology (pages 271-272) that describes this theorized mechanism a bit further:

https://books.google.com/books?id=6lTxBwAAQBAJ&printsec=frontcover&dq=sensory+ecology&hl=en&sa=X&ved=0ahUKEwjC9ai_wI3aAhUUwGMKHc9XBWgQ6AEILzAB#v=onepage&q=harbor%20seal%20blood&f=false