Sunday, July 7, 2002		Nature

The earthworm hears without having ears
Nutan Shukla

ALMOST every movement that takes place on earth leads to some kind of noise. The ability to detect sounds and react correctly to the disturbances they produce is thus an asset of considerable value to living beings.

Response to the vibrations of sounds is found even in very primitive forms of life. Earthworms retreat into their holes when they feel the vibrations of footsteps through the earth. They are able to respond even though they have no ‘ears’ as such. In the same way, an amoeba can not be regarded as being able to ‘see’ light, although it will move away from bright light. But hearing is generally associated with special sense organs which have developed the ability to respond to vibration to a higher degree than the rest of the body’s tissues.

The human ear is not equally sensitive to vibrations of all intensities. In fact, we hear only in a comparatively narrow band of frequencies spanning about 10 octaves from 20 to 20,000 cycles per second. Below this level, if the vibration is strong enough, it is detected by feel, whereas above 20,000 cycles the sounds are outside our senses altogether.

In fact, human beings have a moderately developed sense of hearing as compared with many other animals.

We rely far more on our eyes than on other sense organs, consequently our hearing faculty is not as developed as in other living beings.

Every dog-owner will have noticed his pet pricking up its ears even when human beings in the vicinity have noticed nothing. This is because dogs have a larger range of hearing than man. This faculty is exploited in the so-called ‘ultrasonic’ whistles, which emits notes too high for the human ear to detect, but which are readily audible to dogs. The same ability to here sounds outside the human range is found in many insects.

One American research team set out to study the sounds made by crickets. They took tape-recorded imitations of the crickets’ chirps, but even when they played these in an area full of crickets, they were apparently unable to get any of the crickets to reply. The reason for this is that the crickets communicate with sounds that are of a far higher intensity than what the human ear can detect: the sound made by the crickets in scraping their wings is entirely incidental to the real communication.

The human ear is most sensitive to sound in the range of about 3,000 cycles per second, about four octaves above middle C. This sound is of about the same pitch as a women’s cream.

The ability to hear is the basic of language and this form of communication is widespread throughout the animal kingdom. Many bird songs and animal cries are familiar sounds but until recently the sounds made by fish and other sea creatures were almost unknown. In fact, water is a better medium than air for carrying sound.One startling demonstration of this fact was the picking up by a microphone off the American coast of the sound of the explosion of four pounds of TNT, off the coast of South Africa.

The sound-transmitting properties of water are used to the full by marine and fresh water animals.If a microphone is lowered into an aquarium, the fish are found to be making a cacophony of sounds. Some species of fish have specially developed muscles surrounding their swim-bladders which cause the taut surface of the sac to vibrate like a drum. Others rub their gill arches together to make sounds which are meaningful to others of the same species. Fishermen trying to sleep in their thin wooden ships off the coast of China have long found that the noise from the certain types of croaking fish keeps them awake at night. The same noises disrupted American anti-submarine defence during World War II. These defences made use of microphones under the water connected with loudspeakers above the ground. The sound made by large shoals of croakers was almost deafening, and the apparatus had to be redesigned.

Fish are not the only marine animals who communicate with one another through sound.Dolphins, the intelligent, friendly marine mammals, who follow ships and can be trained to perform tricks, are now known to have a well-developed language and seem to be able to express a wide range of feelings.

One question which puzzles many people about hearing is why it should be necessary to have two ears. The reason for this can be more readily appreciated by considering the ears not only as sound receiving devices but also as direction-finders.

When a sound is heard, the head turns slightly to find out where the sound is coming from. But even if the head were fixed in one position, it would still be possible to detect where the sound is. With only one ear functioning, however, and in the absence of other clues, it is hard to tell where a sound is coming from.

The brain can detect very small difference in the time of arrival of a sound at each ear. Differences of as little as 1/10000 of a second can be easily detected, an ability which enables humans to place the source of the sound. Blind people make the most use of sound and that is why the ability to respond to sound is highly developed in people without vision.

It is the presence of echoes in sound that enable them to locate objects and to avoid obstacles in their path.

This ability to navigate with the help of the ear is found in certain bats. These animals have small mouths which produce a note far outside the human range of hearing, with frequencies up to 1,30,000 cycles per second. Almost all smaller bats can use this very high-frequency note as a form of ‘radar’ to detect obstacles in their path. Their ears detect the reflections from the even quite small objects, and they are able to avoid them.