the world of science . animals &apos; deaf ears . by Maurice Burton , D.Sc . it is some years ago since I first became interested in the possible effect of modern noises on animals . 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 . it soon became apparent that this was not so , and this conclusion is reinforced by the abundance of hares on London Airport . 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 . during the course of my study of this problem several striking points emerged . 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 . there is constantly passing through the human brain a stream of impulses we call thoughts . 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 . these experiences are continually being added to because everything that impinges on our senses is meaningful . 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 . the church clock striking the hour reminds me that I must hurry if this is to be ready on time for the printer . it reminds me also , once again , that yet another hour has gone on the inexorable road to eternity . these are two ideas that could never enter an animal &apos;s head on hearing the sound of a clock . 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 . each has been a minor distraction . 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 . the distant motor-cycle caused me to give a momentary reflection on the calamity of road accidents . the barking dog made me pause to find out if it was one of my own dogs barking , and if so for what reason . 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 . then , a sudden noise may recall us with a mild or even a violent shock . 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 . 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 . they are not distracted by trivial sounds and are unlikely to be off-guard as a result of being lost in their thoughts . the best way to test this is by direct observation . in this we can employ indicators such as the way the ears are used as well as the animal &apos;s moments of alertness , usually with a tensing of the muscles . 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 . 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 . 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 . it will , however , immediately react to any alarm note or a note of aggression . 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 . 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 . some animals have a pronounced ability to turn a deaf ear . 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 . domesticated animals , whose security is assured , often provide outstanding examples of it . 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 . 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 . there is a category of sounds , however , to which all the higher animals at least react violently . these are the explosive sounds . a car backfiring will send the city pigeons flying . 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 . the theory has many weaknesses . one is that many kinds of birds will react in the same way . in fact , it seems reasonable to say that the explosive sound creates alarm among most animals with ears . there may be exceptions , as among fishes or frogs , but it seems to be a rule among birds and mammals . 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 . it is not possible to deal in more than the broadest generalities about animals &apos; reactions to sounds because hearing varies widely from one species to another , as does the structure of the ear . so far as the explosive sound is concerned there are some animals that use it themselves . 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 . 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 . 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 . 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 . a representative series of sounds made by motor-horns would approximate fairly closely to the aggressive or warning sounds made by wild beasts . one important factor in the toleration of noise is familiarity . 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 . each generation seems to be able to bear the noises it grows up with and to abominate the additional noises that appear later . 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 . 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 . 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 . the world of science . coypu and pest-control . by Maurice Burton , D.Sc . the coypu is one of the animals introduced into this country whose residence here we are beginning to regret . 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 . 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 . at first it was believed they did not constitute a nuisance but opinion has now turned against them . last week it was reported that the suggestion had been put forward to use the coypu to combat another nuisance . 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 . 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 . 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 . 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 . throughout the world animals and plants have been transported , either accidentally or deliberately , from one continent to another . 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 . 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 . the most obvious comment to make is that there are remarkably few animals , outside the insects , that feed exclusively on one item of diet . 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 . most animals like variety in their food , and this is especially true of rodents . it is highly important , therefore to know something of the diet of the coypu . 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 . a few state that its food is green vegetation , or just vegetation , or say that it feeds on water plants . for our present purpose none of these is satisfactory . 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 . 