Animal Senses #9: Hearing

The sense of hearing has some advantages over every other sense, explains Ed Yong in Immense World. Unlike touch, hearing can operate over distances. Unlike vision, it can operate in darkness and through objects. Unlike surface vibrations, it can operate in air and water. Unlike smell, which needs time for the molecules to disperse, sound is very quick. No wonder then that some animals make so much use of hearing.

 

But sound has a major disadvantage – interference by other sounds. The noise of the bird’s own wings flapping, for example. The owl has solved this problem by making its own flapping almost impossibly quiet or more accurately, the sound its wings make are at a frequency that neither the owl nor its prey can hear.

 

Some sense organs seem symmetrically placed in all animals. Like ears. Not always. A drawback of symmetrical ears (like ours) is that if a sound hits both ears simultaneously, then the animal cannot make out if it came from above or below. That’s probably why the owl’s ears are asymmetrically placed – it allows for differentiating above v below sounds. A useful capability for a creature that hunts at night, often in pitch darkness.

 

Across creatures, the placement of ears isn’t fixed – it can be on the head, or knees (crickets), abdomen (locusts), mouths (hawkmoths), antennae (mosquitoes)…

 

As you might have guessed, the range of frequencies different animals can hear is different. But like we saw with sight, even the “speed” at which animals can hear can vary. Let Yong elaborate on what that means. Make a series of rapid clicks, and keep increasing the speed of the clicks. At different speeds, you’ll find different animals can or cannot keep up. When birds sing, what we hear and what the birds themselves hear can be different things because of this “speed” of hearing difference, even though the range of frequencies for both is the same.

 

There is an important tradeoff that ears have to make. Say, you played two musical notes at 1,000 Hz and 1,050 Hz frequencies, each for only 10 milliseconds. They’d be indistinguishable. Why? Because in 10 ms, both notes would oscillate 10 times each. Increase the time to 100 ms, and they would oscillate 100 and 105 times, and now you could make out the difference. That is the tradeoff – to differentiate among lower frequencies, ears need to gather data over longer periods. But in doing so, they become less sensitive to fast changes within those periods. You can have great temporal resolution or great pitch sensitivity, but you can’t have both.

 

Except, there are some birds which have two different hearing systems, one for each side of the tradeoff. They flip between the two, as the situation demands.

 

Birds can also hear differently based on their sex and the season! The sex difference one can und/erstand – it may be some genetic difference between males and females. But seasonal difference? It happens for many North American birds – in spring, their hearing sense is slightly worse than in autumn. Why? Because, in autumn, the trees shed their leaves, leaving birds exposed to predators. So the bird in autumn needs to hear better to detect the danger earlier.

“An animal’s Umwelt cannot be static, because an animal’s world isn’t static.”

 

As mammals and vertebrates, and after having read this blog, you’d assume every living thing can hear. Not so. Most insects seem to be deaf. Hearing is an optional feature in nature.