Phonoreceptiun 



491 



Mammals. In all mammals the cochlea is highly developed, and in all 

 mammals that are not completely aquatic the middle car contains three os- 

 sicles similar to those of man and an external ear is usually present (Fig. 168). 

 The cochlea is always coiled, but the number of turns has no apparent rela- 

 tion to the size or intelligence of the animal. The duck-billed platypus has 

 a quarter turn; the whale has 1.5 turns, the horse 2, man about 2.75, cat 3, pig 

 and guinea pig nearly 4, and the South American "paca" (^Coelogenys} 5. 



Many mammals are sensitive to frequencies above the range of human 

 hearing, i.e., in the so-called "ultrasonic" range. Pavlov'^'^ demonstrated that 

 dogs have a wide frequency range and a power of frequency discrimination 

 at least equal to that of man. We now know that the upper limit for dogs 



Fig. 168. Diagrammatic presentation of the ear of a mammal (rat) as seen from the 

 anterior. From Adams. ^ 



is about 35 kc, and "silent" whistles have a frequency well above 20 kc. The 

 upper limits for some other mammals are: 50 kc. for cats, 40 kc. for rats and 

 guinea pigs, 33 kc. for chimpanzees, and at least 98 kc. for bats. 



The hearing of bats is especially interesting in that not only does it serve 

 as a means of warning of the approach of other animals and as a means of 

 communication between individuals, but it also aids flying bats to locate 

 objects in the dark. The vision of bats is poor, but because of their normal 

 cave habitats and their nocturnal habits even good vision would be of litde 

 use in the avoidance of objects while flying. Caves which have sound-absorb- 

 ing wafls (e.g., lined with box crystals) are usually not inhabited by bats. 



It was originally proposed by Hartridge-' that bats might emit ultrasonic 

 cries which could be reflected from obstacles and, if heard by the bat, could 

 thereby serve as a sound echo-ranging device. Through binaural localization 



