Fish (and whale) ears 4

Gray whale cranium showing the deep groves where the maxilla and pre-maxilla hinge into the rostrum, just below that is where the long-channel “vomer” terminates into the cranium below a porous process directly between the fatty lipids held by the vomer and the brain.

In our previous newsletter we explored how fishes have likely evolved to sense their environment using complex acoustical cues that are, like the fishes, part of that environment. While many terrestrial animals – including humans, have some subtle acoustical senses that provide them with premonitions of things to come – like earthquakes, weather changes, or approaching helicopters before they can be heard and start thumping our immediate surroundings. 

We know that migrating birds sense the barometric fluctuations of the weather – which informs them of when and where to fly. And like the fishes, the pressure-gradient (acoustical) cues are well outside of their voluntary auditory thresholds.

An important distinction between us critters that live in an air environment and critters that live under water is the density difference between our environment and our bodies. A cubic centimeter meter of fresh water is one gram – defining density as 1gm/cm³. Average ocean water density is 1.02gm/cm³. Human bodies have an average 0.985gm/cm³ from flesh, to fats, to bone (or 0.945gm/cm³ on a full inhale). This is why we float in water. Average ocean fish density is about the same as seawater, so they don’t have to struggle with buoyancy issues. (I suspect halibuts and flounders are a bit more dense.)

But here is the setup for the following argument: the average density of air (at sea level) is 1.2kg/m³ – about 1/800^th the density of water. So the disparity in critter/water density, and critter/air density makes for the difference in “acoustical compliance” between critters and the medium in which they reside.

As most of the acoustical energy we all perceive propagates through our respective residential  mediums, the density difference directly translates to a difference in acoustical compliance – meaning that most of the sound propagated through the air bounces off our bodies because of the 800/1 density difference. But sound passes through marine animal’s bodies because the close density equivalence. 

So most terrestrial vertebrates have evolved complex appendages to collect, convert, and amplify sound into something we can hear – the most pronounced pathway being our ears. On the other hand (or the other fin), marine animals – including vertebrates, have a lot more more possibilities. 

So while marine vertebrates have complex inner ears, there are a number of other evolutionary adaptations that transmit sound to their brains. The anatomies of these adaptations are likely as varied as the various species taxa. For example, flounders and halibut lay flat on the sea floor, often covered with the sands or mud around them. While they have a lot of cilia on their upper side to sense acoustical activity above them, it would not surprise me if they sensed vibration from below in the benthos that they are laying on or in.

Cetaceans, having evolved from what were once terrestrial animals, have retained the complex inner ears of terrestrial animals, but due to migrating back into aquatic habitats over the last 46 million years, the outer ears of terrestrial animals have been abandoned. Getting sound to those inner ears underwater has taken at least two directions.

Lower mandible of a common dolphin. The pocket that extends in to the jaw can be seen. this is filled with fatty lipids which extend back into the skull to engulf the optic capsules – which, unlike the mysticetes, are not fused to the cranium. (Maritime Museum of Tasmania)

Dolphins and porpoises have fats in their lower jaws filled with what’s has been termed “acoustical lipids” – a fatty substance that runs from their234 lower jaws back to their lower cranium to enclose the “otic capsule” – an envelope that contains the cochlea and workings of the inner ear. In these animals the otic capsule is not fused to the cranium, so vibrations propagated through the lipids can freely oscillate the otic capsule and excite the inner ear.

Baleen whales conversion of habitat acoustics to sound is a bit more ambiguous. While they have what appears to be vestigial outer ear channels, these are filled with earwax that may or may not transmit vibration into their inner ear. Unlike the dolphins and porpoises, their otic capsules are fused to (although somewhat cantilevered off) the cranium.

Ventral view of a Gray whale cranium. The two kidney-shaped bulbs are the otic capsules, that while firmly attached to the skull, are somewhat cantilevered, with loads of space surrounding them to allow for acoustical lipids to fill. The vomer extends away from the cranium and becomes quite thin at the distal end – as evidenced by it having been torn easily.

But baleen whales also have an ambiguous rostrum configuration. Nestled within the maxilla and premaxilla bones is the “vomer,” which is a common mammalian nose feature, but in the case of the mysticetes, this is a channel filled with the same acoustical lipid found in the acoustical channels of dolphins and porpoises (melon for projection, jaws for reception).

In the gray whale, this is about the size of a human leg. This is minimally vascularized, contains no apparent nerves, and terminates on the posterior end into a highly porous process between the cranium and the vomer.

Does this have some role in converting acoustical energy into the brain? Like much of how aquatic animals receive and process sound, this will likely remain a mystery.