*The following blog post is an excerpt from the book Twenty-Two Fantastical Facts about Dolphins*
Whales and dolphins evolved from furry mammals with hoof-like feet that roamed the Earth 50 million years ago. Called Pakicetus, these animals looked like a cross between a dog, a cat, and a tiny, angry hippopotamus. They walked on four legs, had a snout with eyes on the front of their heads, and sported cute little gerbil-like ears.
As natural selection began the process of transforming Pakicetus into the modern-day dolphin, the ancestors of dolphins lost many anatomical features that weren’t helpful to them in their new aquatic environment. They traded in their body hair for a thick layer of blubber to help keep them warm. Their gangly limbs were transformed into sleek flippers and flukes – far more useful for propelling them through the water. And those cute little gerbil-ears – which would have slowed them down in the water – disappeared altogether.
The visible part of Pakicetus’ external ear – that floppy little pancake that most extinct and living mammal species have on the sides or tops of their heads (including humans) – is called an auricle or pinna. It acts almost like a satellite dish that amplifies sound and directs it toward the ear hole. If you look closely at the side of a dolphin’s head, you can still see a tiny pin-prick where their ear hole is, but the pinna itself is completely gone.
But it wasn’t just the pinna that underwent a radical change as Pakicetus evolved. The strangest transformation took place under the hood. The tiny ear hole seen in modern dolphins, which originally transported sound waves to the middle or inner ear, no longer serves any purpose at all. Dolphins’ ear canals are completely blocked up with fibrous tissue, ear wax, and other fleshy debris. Sound is no longer able to make its way from a dolphin’s ear hole to its inner ear.
How then do dolphins hear at all? This was a question that stumped scientists back in the 1940s when they first began examining dolphins’ hearing anatomy. Dolphins clearly have very sensitive hearing, and are able to detect sounds at extremely high frequencies, well beyond what a human or even a dog could hear. But how were dolphins transporting sounds from the outside world into their inner ears if they had blocked-up ear canals?
It turns out that all the work that used to be done by the pinna – amplifying sound and directing it to a dolphin’s inner ears – is now done by a dolphin’s lower jaw. A dolphin’s jaw is filled with a kind of fatty substance that leads directly up into their middle ear. As sound waves travel through the water, they are absorbed by the dolphin’s jaw and are directed up along this fatty canal. With a jaw bone on each side of its head, a dolphin is able to use its jaws much like we would use pinna on the sides of our head – allowing them to pinpoint where a sound is coming from. And since the fat in their jaws is similar in density to water, this allows sound waves to travel easily to their inner ears.
This fancy new lower-jaw hearing system is made extra effective with the help of dolphins’ teeth. The more-or-less evenly spaced rows of 22 teeth that dolphins have in each jaw actually help them to amplify sound. Their teeth act a bit like an antenna, with the teeth resonating at frequencies that dolphins use for their echolocation. This hearing system likely evolved in tandem with dolphins’ echolocation ability.
Although dolphins might have lost their cute gerbil-ears, they appear to have traded them in for some rather sophisticated auditory technology. It’s yet another bizarre feature of an animal that has taken a rather unorthodox evolutionary path.