THE WORLD OF SCIENCE .sx ANIMALS' DEAF EARS .sx By MAURICE BURTON , D.Sc. .sx IT is some years ago since I first became interested in the possible effect of modern noises on animals .sx I started with the assumption that if animals had more sensitive ears than mine , or were as allergic , as I am , to the sounds of traffic on the roads , there should be a noticeable tendency for them to shun the borders of roads .sx It soon became apparent that this was not so , and this conclusion is reinforced by the abundance of hares on London Airport .sx There , people put their hands over their ears as the jet-planes go out , but the hares are to all appearances unmoved , which is contrary to what might have been expected .sx During the course of my study of this problem several striking points emerged .sx The first is that although the ears of animals are often more acute than ours , and their powers of discrimination seem to be higher , they also appear to be less bothered than we are by a cacophony .sx There is constantly passing through the human brain a stream of impulses we call thoughts .sx These are closely linked to everyday life , are built upon experience , and our experiences are based largely on sensations received through the senses , one of which is hearing .sx These experiences are continually being added to because everything that impinges on our senses is meaningful .sx For example , while writing these last three sentences I have heard a number of sounds , each of which has set up a train of thought in my mind .sx The church clock striking the hour reminds me that I must hurry if this is to be ready on time for the printer .sx It reminds me also , once again , that yet another hour has gone on the inexorable road to eternity .sx These are two ideas that could never enter an animal's head on hearing the sound of a clock .sx Within the space of these few seconds , also , there has been the sound of a telephone bell , of a distant motor-bicycle and of a dog barking .sx Each has been a minor distraction .sx The telephone made me wonder whether I need drop this task to answer the call and with it came a tangle of thoughts that at 11.30 I must not fail to telephone so-and-so , that the telephone is a nuisance but what could we do without it , and others of like nature .sx The distant motor-cycle caused me to give a momentary reflection on the calamity of road accidents .sx The barking dog made me pause to find out if it was one of my own dogs barking , and if so for what reason .sx By contrast with our continual alertness to noises and their meaning it is possible at times so to lose oneself in preoccupation as to be oblivious to outside sounds .sx Then , a sudden noise may recall us with a mild or even a violent shock .sx So throughout our waking hours we tend to alternate between an awareness of every small sound and the danger of shock , mild or otherwise , through not having been aware of them .sx Whatever views we may hold about how far the higher animals are able to think or to reason , there can hardly be any doubt that they are not affected by sounds in the same way as we are .sx They are not distracted by trivial sounds and are unlikely to be off-guard as a result of being lost in their thoughts .sx The best way to test this is by direct observation .sx In this we can employ indicators such as the way the ears are used as well as the animal's moments of alertness , usually with a tensing of the muscles .sx It then soon becomes apparent that an animal normally pays little attention to sounds that are not a cause for alarm , an indication of a source of food or made by a member of its own species .sx Where the air is free of sounds made by machinery it may be filled with those made by birds , insects , rustling leaves and other natural sounds .sx It can be alive with them , yet so far as we can tell an animal ignores them all unless one or other of them has a special significance .sx It will , however , immediately react to any alarm note or a note of aggression .sx To put it another way round , it seems to be able to shut its ears to noise in general yet remain on the alert for particular sounds which by tradition or experience compel its reaction .sx We also possess this faculty , although some have it more than others , but it seems likely that animals can , and habitually do , exploit it more than men , largely because their world of experience makes fewer demands on their senses .sx Some animals have a pronounced ability to turn a deaf ear .sx This is difficult to test in a wild animal because the mere presence of the human observer , however well hidden , tends to threaten its security and put it on the alert .sx Domesticated animals , whose security is assured , often provide outstanding examples of it .sx Dogs and donkeys can appear to be stone-deaf , ignoring all words of command or entreaty , all persuasive or cajoling sounds , but responding instantly to even a slight noise suggestive of something pleasurable .sx A dog may lie as if in a trance , apparently unhearing , yet spring to action at the slight metallic sound of its lead being taken from a hook or the faintly whispered word " walk .sx " There is a category of sounds , however , to which all the higher animals at least react violently .sx These are the explosive sounds .sx A car backfiring will send the city pigeons flying .sx One theory has it that because they are descended from rock doves there is a survival value in this innate reaction because it would have made them fly up at the sound of a fall of cliff that might otherwise engulf them .sx The theory has many weaknesses .sx One is that many kinds of birds will react in the same way .sx In fact , it seems reasonable to say that the explosive sound creates alarm among most animals with ears .sx There may be exceptions , as among fishes or frogs , but it seems to be a rule among birds and mammals .sx It probably created alarm among human beings also before ever gunpowder or TNT were invented- the word " explode , " in fact , antedates their invention , and in modern but pre-nuclear warfare the wear on the nerves from explosives was probably more telling than the casualties inflicted by the exploding missiles .sx It is not possible to deal in more than the broadest generalities about animals' reactions to sounds because hearing varies widely from one species to another , as does the structure of the ear .sx So far as the explosive sound is concerned there are some animals that use it themselves .sx A dog may use a particularly explosive bark to another dog under certain circumstances , and the effect of this can be almost as devastating as the bursting of a modern projectile on the human ear or the report of a rifle on a flock of pigeons .sx It is necessary , to avoid confusing the issue , to ignore some of the extreme examples of deleterious sounds , those that make telephone operators faint or the jingling of a bunch of keys that sends a mouse into something approaching hysterics .sx What is at least as interesting is the way inventors seem to have chosen , probably intuitively , a combination of explosive and aggressive sounds as warning signals to be used on automobiles .sx Apart from the purely explosive sounds , those that stir most animals to rapid action are the snarls , growls , barks or long drawn-out roars of predators or rivals .sx A representative series of sounds made by motor-horns would approximate fairly closely to the aggressive or warning sounds made by wild beasts .sx One important factor in the toleration of noise is familiarity .sx Our Victorian ancestors probably found the noises from horse traffic insufferable at times and at an earlier age it may be that the cry of the night-watchman was held to be a necessary but excruciating nuisance .sx Each generation seems to be able to bear the noises it grows up with and to abominate the additional noises that appear later .sx Generations of hares succeed each other with far greater rapidity than generations of humans , and the hares of London Airport have probably by now accepted the noise of jet-planes as part of their environment .sx They have , moreover , one great advantage over us , and this is probably one of the reasons why mammals in general can put up with the noise of traffic on the roads .sx Those that have movable ears can not only turn them in the right direction to pick up slight or distant sounds , they can also turn them away from disagreeable sounds- and I have seen them do so .sx THE WORLD OF SCIENCE .sx COYPU AND PEST-CONTROL .sx By MAURICE BURTON , D.Sc. .sx THE coypu is one of the animals introduced into this country whose residence here we are beginning to regret .sx It is a large South American rodent , rat-like although its nearest relatives are the porcupines , measuring over a yard long to the tip of the tail and weighing up to 20 lb .sx Originally brought here about 1930 to be farmed for their fur , which is known as nutria , the coypu began to escape and are now well established in the countryside , notably in East Anglia and especially on the Norfolk Broads .sx At first it was believed they did not constitute a nuisance but opinion has now turned against them .sx Last week it was reported that the suggestion had been put forward to use the coypu to combat another nuisance .sx The Kariba Lake , formed when the Kariba dam was completed , has become infested with a water plant , one that grows at an alarming rate and threatens to damage the special intakes at the dam .sx The menace from the plant is serious enough to merit almost any suggestion aimed at controlling it , and this one , put forward by Mr. George Atkinson of Lowestoft , is brilliant in its simplicity .sx It is that some of the coypu in East Anglia , estimated at a quarter of a million , should be trapped and exported to Kariba Lake to feed on the menacing weed .sx Were such a plan to be shown to be successful it would contain the perfect form of biological control , using one nuisance to combat another .sx Throughout the world animals and plants have been transported , either accidentally or deliberately , from one continent to another .sx In some the results have been beneficial , in a few they have been harmless but in far too many they have been disastrous , so that to-day one looks at any further plan to introduce animals into an alien environment with caution if not deep suspicion .sx The first question one needs to ask is whether the coypu would eat this particular weed and in sufficient quantity to counterbalance its own remarkable powers of multiplication .sx The most obvious comment to make is that there are remarkably few animals , outside the insects , that feed exclusively on one item of diet .sx The koala feeds on nothing but eucalyptus leaves and is always quoted as a striking and exceptional example of an animal with a restricted diet .sx Most animals like variety in their food , and this is especially true of rodents .sx It is highly important , therefore to know something of the diet of the coypu .sx There are , on my shelves , a score of authoritative works on mammals , and it is noteworthy that although they all contain at least one reference to the coypu most of them make no mention at all of its diet .sx A few state that its food is green vegetation , or just " vegetation , " or say that it feeds on water plants .sx For our present purpose none of these is satisfactory .sx Water plants range from the wholly aquatic , like water lilies , and such plants are usually soft , to waterside plants which are usually tough and fibrous .sx