Though we assigned, and this is true, the sense of smell to be prior to all other senses nevertheless one should not forget that our pets have perfect hearing. Every cat-holder can confirm this watching his four-footed friend rushes to the front door only if a member of the family is knocking and doesn't pay attention if there's a stranger. At the same time your family will confirm how far your pet discerns your own steps. Or consider a situation when a kitten peacefully plays with a ball and then suddenly rushes in the corner and suspiciously gazes into a chink waiting for desired mouse. Of cause, one may suppose cats and mice communicate in a special language, incomprehensible for our ears. But to regard this assumption seriously let's find out how does a cat organ of hearing is constructed and what kind of sounds it can percept.
First of all we'd like to mention that it is constructed in a very complicated manner and is divided into three sections - external ear, intermediate and internal; and besides this there's a central section situated inside the brain. The external ear is most noticeable and well-known part, because permanently attracts our attention, and cat often uses it, at least the auricle, that we just call ear. The auricle is a high skin fold, in a form of triangle in all breeds but few in which it is different. More that ten muscles control for movements of the auricle and that's why it can change its' shape, and above all, its' position relative to head: can bend, clasp to head and turn almost 180 degrees. At the base of the auricle half the way long there's a small opening leading into a narrow channel - acoustic duct that has a dead end closed with very thin tympanic membrane. Here the intermediate ear begins, but we'll return to this section later, and now let's talk directly about the auricle.
It's essential that here many blood vessels pass over and it is filled with long and thin hairs from the inner surface and short and thick hairs from the outer surface. Auricles are afoot all the time, at that right and left ones may move independently. These peculiarities of auricle are the evidence of its' functions. It's not true if one thinks they deal with hearing solely. Nature is never wasteful and everywhere, where it is possible, loads one the same organ with several tasks at once. For example, cat ears release superfluous heat and, hence take part in heat regulation. Remember how your cat suffers in summer, because covered with thick and warm fur. Auricle is one of few places on its' body that may bring redundant heat out.
Auricles fulfill some signal functions, displaying other animals and master cat emotions and intentions. Thus, clasped and turned back ears mean declaration of war; clasped - alertness; turned back - warning; directed forward and pulled - watchfulness; turned forward - curiosity. In one word, ears play the same role in communication ad, for instance, tail.
And what about hairs inside auricle? They are also not useless, since protect acoustic duct and tympanic membrane from hit of rubbish and insects. Cats suffer from them in the same way as people do. Remember troubles caused by mosquito flied into acoustic duct and your visit to a doctor on this account. Let's feel sorry for cats that are unable to visit a doctor in similar case, though they more often face insects trying to use their ears as asylum. Therefore nature cared for them and created a lattice of rigid hairs that protect acoustic duct.
In that way, hairs deal with hearing. But the direct use of auricle is detection of faint but very important sounds. First of all, rustle of mice and other rodents, which cat eats. Then, steps of other cats both friends and enemies. And ultimately, steps of its' master, steps of dogs, and steps of foxes or wolfs in nature. If a cat in time percepts a signal from it's prey and reacts in time then it catches it and eats. If a cat doesn't percepts a signal from its' enemy in time and doesn't prepared or run away, then it is caught and ate. Tuning in essential sounds underlies the whole work of hearing organ. But this work starts at the point of auricle that already acts in such way as to give green light to most important sounds and keep less important as an alternative.
Some of these sounds are very faint and we've talked about them already.
But there are quite strong and loud but also vital sounds that are perceived without special efforts since they are not at the breaking point of audibility. These are voices of other cats, especially males, kittens, dogs, master and members of family, etc. But it's important that they can be masked by other, also strong everyday sounds that are not essential for a cat. In this case a hearing organ has to solve a problem of picking vital sounds out from the sound environment and discern them. Central sections are mainly responsible for the task, though the external ear assists this due to resonance of its' air cavities, besides the latter may change during auricle movements.
But in solution of another - acoustic task - the auricle also plays a very important role. This task includes determination of source of sound, or, as they say, sound location. Indeed, what the opportune determination of mouse rustles is, if a cat doesn't know where the mouse is now. Or what the voice identification of neighbor cat is if it's impossible to determine where from it screams. For this purpose the heariong organ uses both auricles simultaneously. Due to an ability to control each of them separately the brain gets the information about the differences of sounds that come from the left and right. These differences, first of all, concern time of incoming and this is natural, because less impaired sound will reach closer the ear closer to the source because of the "sound shadow" effect. Other characteristics will also differ comparing them a cat will accurately determine where the source of sound is and will undertake certain actions: attack it, if it is a mouse, or run away if an enemy sneaks towards.
The next section of hearing organ - intermediate ear - is tuned in vital sounds. The intermediate ear is represented by a cavity, three acoustical boned and two muscles. Vibrations of tympanic membrane are passed of bones - malleus, incus, and stapes, which sets against the membrane of oval opening where the internal ears begins. Thus the intermediate ear reminds a drum with two membranes: a big one and a small one. Vibrations through the bones are passed from one membrane to another, changing their characteristics and thereby regulating sound transmission. Due to tuning in vital sounds some characteristics are amplified, others are weakened. Resonance of intermediate ear that is determined by the size and partly by shape of its' cavities contribute to this; the ratio of membranes is also important. Since bones make zigzag they form several levers along with muscles and weaken and even block some very loud sounds.
Such protection from loud sounds is very important for a cat hearing, after all, otherwise perceptive cells (receptors) inside the internal ear will not be able to work. Their main and the hardest task is mainly the perception of faint sounds of certain range that are essential for a cat. Perceptive cells are the very delicate instrument, and nature did it best to protect them from overloading. First of all, it plunged them into liquid and put them into a soft case, that is protected with membraneous tissue from above and from below, and covered it with an osseous container stranded in a from of helix (from here is the second name of intermediate ear - cochlea). A human cochlea-helix is stranded in 2.75 turns, cat's - in 3 turns. The place where the perceptive cells and multiple auxiliary structures are situated is called a Corti's organ. There are about 13000 perceptive cells in cat ear that is a little less than in human ear. At its' top each perceptive cell is covered with perceptive projections-"filaments" (don't confuse them with hairs inside an auricle!). From below the endings of acoustic nerve go up to a perceptive cell. Nerve endings get information about the sound previously processed by perceptive cells and pass it further along the fibers of acoustic nerve, that is much larger in cat than in human (52000 against 31000).
Through the medulla and intermediate brain and interbrain the acoustic information at last gets into the higher authority - the acoustic cortex of cerebral hemispheres. Along this way brain analyses sounds comprehensively and totally. Finally brain takes a decision "what to do" and gives certain commands to motor centers. The circle is closed and this allows a cat to act expediently in current situation: run away in some cases, approach in another, or hide etc.
What is the range that an acoustic system works with, and if this range is comparable with human abilities? Here some things are still obscure, however one may assert that in a whole the range of cat hearing goes into ultrasound area reaching 65 kHz according to some returns and 100 according to another (remember that 1 kHz corresponds to 1000 of sound oscillations a second).
Voices of small rodents, especially their young are inside the cat range of hearing. Our ear is unable to percept this range. Hence, the assumption that cats and mice communicate their own "language" inaccessible for our perseption is true.
From this point of view the fact that cat hearing, unlike ours, is tuned in high-pitched sounds is quite interesting. The tuning is already observed on the level of external hearing that amplifies sounds better than our hearing in a range of 1,9-3,8 kHz, and quite considerably in a range of 4-6 kHz and higher.
If we want to compare the total threshold sensibility then human and cat hearing are about equal - up to 0.7 kHz, and then the threshold for cat reduces dramatically, testifying for better perception of higher frequencies. Within the limits of 10 kHz a cat hearing is much better (the difference in thresholds of sensibility is up to 10 dB and higher) to percept faint sounds than human.
A cat ability to localize, find the source of sound in space is high and exceeds human abilities much. A cat is able to catch a mice running by with its' eyes closed and orienting by rustle and squeak only, while our ears can't determine the source of sound location to such accuracy.
And in the end, cat hearing possesses one more peculiarity: it is able to pick up sounds that come in substratum, pass them through the limbs further to the receptors and brain ("seismic hearing"). A cat, sitting on a thick brunch, Is able to precept vibrations of wood, and walking on the ground - vibrations of soil. This allows cats to make forecasts beforehand, feeling infrasound front foregoing changes. Owing to this cats are regarded as foretellers of weather. But we will talk about this some other time.
Valeriy ILYICHEV, Doctor of Biology, professor
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).