HEARING 175 



better than any parrot — can imitate human speech in spite 

 of the differences in voice production and modulation be- 

 tween man and bird. It is therefore particularly puzzling to 

 find that the middle and the inner ear in mammals have 

 apparently a more complicated structure than birds ; on the 

 face of it the hearing of mammals should be much better 

 than that of birds, but observation gives no support to this 

 suggestion, and the functional difference between the two, 

 if any, is not obvious. 



The value of a distant-receptor sense such as hearing is 

 greatly increased if, in addition to receiving sounds, it is 

 able to locate their source. An animal with an ear on each 

 side of its head can readily locate the direction from which 

 a sound comes by unconsciously comparing the intensity of 

 sound received in the two ears ; it may then turn its head so 

 that the intensity is equal in both and it faces the source. 

 In addition practice and experience may enable it to pin- 

 point the source by the difference in intensities without turn- 

 ing the head. Head-turning depends upon the importance 

 the animal attaches to the sound; a familiar one may be 

 unconsciously noted but a strange one may rivet the atten- 

 tion. The movable ear pinna of many mammals, as already 

 mentioned, is also a great help in the location of sound 

 sources. 



The physics of a hearing system with two symmetrical 

 ears, one on each side of the head, is such that the location 

 of a sound source in the horizontal plane is easy, but it is 

 impossible to locate a source accurately in the vertical plane 

 without turning the head up or down and searching. This 

 arrangement seems to be adequate for most animals — 

 although we live in a three-dimensional world most land 

 animals live and move mainly in a two-dimensional one — 

 but it is not accurate enough for most of the owls. The owls 

 that hunt their prey at night locate it entirely by ear. Mice 

 in the grass, however, do not make a continuous noise and 

 the owl therefore has no chance to move the head and search 

 in order to pinpoint the source of a slight transient sound ; 

 it must be able to locate the source immediately on receiving 

 a brief signal. And the ears of owls are unlike those of all 

 other vertebrates : they are not symmetrical. 



