Wouldn’t it be nice if we could do it too and close our ears
The daughter of one of my friend’s has recently learned to swim – and she loves it. But what she does not love, is having to shake out the water in her ear after a swim. So, she asked me why we can’t close our ears. Nice question, and I answered I don’t think dogs and cats can close their ears either when they swim and whales and harbour seals (so-called phocids) don’t have any ears at all -well, at least those parts of the ear that you can see, i.e. the outer part, the so-called “pinna”. But it made me think, because in Antarctica I was surrounded by fur seals (so-called otariid seals) and they all had small, but very prominent ears. Besides, even though whales and phocids lack the pinna, they can certainly hear quite well under water and possess all the inner ear structures (like ear canal, middle ear and cochlea ) that are similar to even our own ears.
All diving animals, whether they be whales, phocids and even walruses (none of which possessing external ears) or whether they be those with ears such as otariid fur seals and sea lions or otters and hippopotamuses or the aquatic insectivores known as desmans or egg-laying platypuses: they all can hear under as well as above water. However those with visible ears have means to prevent water from entering their ears and hippopotamuses, for example, angulate their small ears backward and close the ear canal by contraction when they dive; desmans achieve the same result by glandular swellings to seal up their ears temporarily and eared seals like fur seals are capable of controlling the state of their ear canals by muscles when diving or when in air. Rising pressure expels the air that happens to be trapped in the outer ear when the fur seal dives and improves underwater hearing.
In the aquatic mammals that lack an outer and visible ear like whales, harbour or hooded seals and walruses, adaptations to hearing under water are a little different. There is no need to close the outer ear, because there isn’t one. However, hollow structures like ear canals and the air-filled middle ear (connected via the Eustachian tube to the respiratory tract) need to be protected against a collapse during a dive owing to the increasing pressure. During dives the tissues around the external ear canal and the middle ear fill with blood to occupy any air spaces and as the air spaces get smaller and smaller with depth, hearing under water improves. In phocid seals the middle ear bones are less separated from the skull than those of the fur seals, and that enhances sound amplification, but fur seals are better in determining the direction of a sound under water. The underwater sounds of the latter range from about 1000 to 4000 Hz, but those of whales and phocids cover ranges from 40 to 8 kHz and 100 to 15 kHz, respectively. Some species of dolphins (with very long ear canals) can even hear frequencies across the enormous range of about 100 to 150 kHz.
Other adaptations include copious amounts of earwax (especially in the ears of walruses) and elastic fibres in the walls of the Eustachian tube. An interesting adaptation to strengthen the ear canal lateral to the eardrum (the well known tympanic membrane that causes the problem in our ears when it’s blocked and we are in an aeroplane or are diving) are the so-called exostoses and the latter are thought to facilitate deep dives into very cold water. Exostoses do sometimes also occur in humans, where they are often referred to as “surfer’s ear” and involve benign, non-tumorous, firm and sessile, often bilaterally symmetric, nodular bony growths within the ear canal. Their occurrence seems to be closely connected with the amount of time a person spends surfing or diving in cold seawater. Can this similarity to phocid seals and other diving animals be used by people who champion the idea that during the evolution of humans there was an aquatic period? It’s a long shot and I doubt it very much. But what this blog has shown once again is that responding to a child’s question is something not to dodge, because it makes you think. And that can only be something positive.
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