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One distinctive feature of a cat is his ears. Proud and erect, they sit up there atop his head, swivelling to and fro like little radar antennae. An earless cat looks strange, no longer a cat. Those cute furry things on his head do more than make him look like a cat. They are just the start of an amazing system of sound reproduction that can hear a watch ticking from a hundred metres away, whose fidelity and acuity rivals the most feature-filled optoelectronic home stereo system.

If a cat's vision is not as clear as ours, his hearing is far more so. He can hear insects scuttling underground, and possibly the subtle vibrations that precede an earthquake. He can hear frequencies - musical notes - twice as high as we can. And he can pinpoint the direction of a sound with the accuracy of a naval missile-targeting system. So, for a cat, sound is largely a matter of "hear, here"!

The ears we see on his head are called pinnae. Those of small cats (like the domestic ones) are triangular - or, pedantically, intersected conical. Those of big cats like lions, tigers, leopards, etc., are round and stubby. Whatever the shape, it does its job well: powered by the auricularis muscles under the scalp, it can swivel to the front, side, or back, up and down, and, to protect it in case of a fight, lie flat against the head. Its horn-like shape collects sounds and funnels them down into the ear canal, giving the faint vibrations a little extra air pressure as it narrows, until by the time they've reached the circular, dish-shaped eardrum deep inside the head, they already have some amplification. The eardrum vibrates with the sound, and on the other side of it, a tiny match-head-sized bone called the malleus, vibrates also. The malleus conects to a second tiny bone called the incus in a lever arrangement that mechanically doubles the strength of the vibrations, and the incus connects likewise to a third bone called the stapes. By the end of this section, called the middle ear, the faint vibrations have been given a pretty good kick in the strength department. Now for the inner ear.

The entrance to this is the ovum fenestra, the "oval window", against which the stapes presses. The oval window - actually a membrane - is like the eardrum earlier on, but much smaller and tighter. Beyond the now heavily vibrating oval window is the secret of hearing - the cochlea.

Inside the cochlea is liquid under pressure. The sound vibrations, amplified by the bone levers of the middle ear, now ripple through the liquid inside the cochlea where they make two ribbon-like membranes vibrate. The vibrations slide the two membranes back and forth against each other. Microscopic hairs, hundreds of thousands of them, are bent to and fro by this sliding action - slowly for low-pitched sounds, fast for medium-pitched sounds, faster for high-pitched sounds. And the hairs are attached to nerve cells.

Each time a microscopic hair is bent, the attached nerve cell fires, sending a coded signal to the brain via the 8th cranial nerve. Now, this is how all mammals hear, from mice to men. What makes cat ears better than ours is in the pinnae - the outer ears we can see - and the cochlea. By swivelling his pinnae, a cat can target a sound source so well he can (and it's been proven) pounce on a scuttling mouse without using sight at all. Humans who cannot move their outer ears like this have to rely on a feature called spatial phase-differentiation to deduce where a sound is around us - sound coming from one side is slightly delayed when it arrives at the opposite ear, so by "listening" to the near-imperceptible delay, our brains can figure out which ear is nearest the sound. This sole feature is what gives us our glorious sense of stereo when listening to our beloved CDs. Cats can combine phase-differentiation with
knowing where their ears are pointed to give them what I suppose must be a kind of "super-stereo" far in advance of ours. To them, every ordinary noise must be a wonderful surround-sound experience.

The shaped pinnae collect very faint sound, of course, in a way ours cannot, but it is in the cochlea that the real little miracles of hearing do their thing. I mentioned the nerve cells before. They vibrate and fire electrical pulses to the brain when their hairs are vibrated by the fluid in the cochlea. In fact, only one cell in four actually does the sending-to-the-brain thing - the other three are amplifiers! Each tiny cell twitches - physically - when its hair is stimulated, and a BB King concert makes it wriggle and dance and vibrate just like the concert-goers themselves. It's like a muscle, sensing a small input and delivering a much-amplified output, which can give the true nerve cell surrounded by these amplifiers a good kick in the pants.

So the sound a lion or tiger or cat hears is boosted by a sophisticated multi-amplification process until even a pin dropping can be a clear, loud "CLANKKKKKK . . . "

But wait, there's more. Lions, tigers, and domestic cats can also hear higher than we can. Look at it this way: I'm a pianist, and I know that the top note of a piano is getting towards the limit of human hearing - a piano with higher notes on it would realy be pretty useless to us, because we wouldn't be able to hear them, any more than we can hear those nifty ultrasound machines they use to cure backache and see inside pregnant women. But a cat's hearing would make excellent use of a piano with an extra octave on the top. In other words, cats can hear twice the upper frequency we can.

Inside the cochlea inside the skull, where all those nerve cells are, there are not just a few nerve cells - there are literally hundreds of thousands. Now, your average nerve cell takes several fractions of a second to recover when stimulated. Sound frequencies, even at human limits, occur in ten-thousandths of a second. To get over this problem we have cascades of nerve cells in the cochlea taking turns to respond to the high-pitched sounds, like the several pistons of a car taking turns to make the crankshaft spin. And just as more pistons gives better performance, so more nerve cells, firing in sequence, improves the high-frequency abilities of the ear. Inside the cochlea - the hearing mechanism - of a cat's ear, there are so many nerve cells firing one after the other that the cat can easily pick up sounds much too high-pitched for us to hear.

Imagine, then, what it's like for a cat in his natural environment - say, a lion in the Serengeti. He can hear so clearly that even agama lizards scurrying across a rock 100 metres away make a definite noise, the squeaking of mice and the ultrasonic calls of termites are a fact of life, and he can keep track of his pridemates from ten kilometres away when they roar. A domestic cat at home is bombarded with a clutter of sounds we can't hear: beetles in the walls, roaches in the kitchen two doors down the hall, the high-pitched scream of the Tesla coil in your TV set, the hiss of water in the main plumbing across the road, the collective tickings of all the family members' watches. Even things we can hear take on a new character in a cat's ears: CDs and computer-generated sounds often include harmonics (parts of sounds) inaudible to us, so if you think the kids' new Playstation is driving you nuts, just think of your poor cat!

But with all this remarkable hearing of tabbies and tigers alike, there is one strange deficit. We mere humans can hear sounds down below the bottom of a piano's bass end, and still hear them as musical notes; only extremely low sounds lose their pitch to us and appear as individual vibrations or throbbings.
Cats, apparently, lose the ability to hear a musical pitch in a note a little below the piano's middle "C". To them, anything below that, such as Barry White or Darth Vader, is still heard - but only as a series of pitchless vibrations.

Which brings the final comment to bear: how do cats hear human music? Clearly, differently to what we hear. I imagine a fine Mozart concerto being played roughly four octaves too high, with the low notes from the 'cellos' being felt rather than heard. I had a cat once who loved to lie on top of the piano and purr like mad when I played the low keys, I assume because they sounded to her like a cat purring and so she responded in kind. As a classical organist, I've also had the comical experience of a dog (not a cat, however, but with similar hearing abilities) howling and barking outside the church only when I played certain high-pitched notes on the 4' diapason stop. I have it on personal authority from friends that most cats who respond at all to music (most don't) unanimously prefer classical and light pop music to jazz, and rock is generally an outright turnoff. I've experimented with lions, tigers, and a Teac studio cassette player in a zoo: one male tiger jerked his head up at David Bowie's "China Girl" and the male lion roared non-stop at any brass band music, especially German polkas, but apart from that I got no response. Joy Adamson, in her book "Living Free", noted that her lioness Elsa sometimes took interest at the radio, or when Joy played the piano, and that she seemed to prefer a low-pitched sound to a high-pitched one. It's been said that one note, the bottom "E" on the piano, is a potent aphrodisiac to male cats.

With all this super-hifi sensitivity, it's a wonder that house cats aren't renedered deaf soon after kittenhood - loud stereos and TVs, clattering pots in the kitchen, raucous lawnmowers outside. In fact, all mammals can limit their hearing in loud sounds. Just as the iris of the eye involuntarily constricts to prevent too much light, so the three bones of the middle ear - malleus, incus, and stapes - can be drawn back by tiny muscles to limit the sound hitting the oval window of the cochlea.
 
 

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