XXII. 2 



HEARING IN BATS 



59i 



reflected notes and constructs difference or summation tones by the 

 introduction of a specific non-linear device (Pye, i960). Since the 

 sound is used only for location its absolute qualities are not important 

 for a bat as they are for man. With this method any object within 

 range will be located by the variation in the beat notes that are pro- 

 duced as its position changes. 



The distortion required to produce the beat notes might perhaps 

 be a function of the middle ear muscles, but is more probably a 

 property of the cohlea. Distortion of 

 cochlear microphonic potentials is 

 known to occur in other animals at 

 sound intensities lower than those 

 emitted by bats. Such a mechanism 

 could thus readily have been evolved 

 from a more conventional hearing 

 system without any fundamental 

 change either in the ear or brain. 

 Only the cochlear microphonics need 

 to follow the high notes of the bat's 

 voice. The auditory nerve carries in- 

 formation only about the difference 

 notes of a few kc, which could be 

 readily recognized by the brain. 



Horseshoe bats often seem to search 

 around when they are stationary and 

 there is some evidence that under 

 these conditions they introduce an 

 artificial velocity factor, and hence a 

 Doppler shift, by movements of the 

 ears, which may occur at as much as 

 50/sec. However, it is not clear exactly how this is achieved or how it 

 is related to a second opening of the meatus that lies at the non- 

 moving base of the pinna and leads by a groove to the nose-leaf, 

 with its lance and shield (Fig. 367). 



Some bats (Hipposideridae) may use both methods, the later part 

 of each pulse being modulated. All bats also produce sounds of lower 

 frequency, perhaps as specific recognition signals. 



The placenta is of a discoidal form and haemochorial, at least in 

 some types. A peculiar feature is that copulation occurs in the autumn 

 and the sperms remain alive (but presumably not active) within the 

 female until fertilization takes place in the spring. The young are well 



Fig. 370. Diagrammatic view from 

 above the head of a horseshoe bat, 

 showing a possible means of estimating 

 distances using the principle of differ- 

 ences in intensities. 



The left ear does not receive an 

 echo, but the right ear because it is 

 turned inwards, receives the reflected 

 waves. The thick black line that limits 

 the pinna anteriorly is the antitragus, 

 which is used as the echo receptor. 



m.l. the median line is the zone of maxi- 

 mum intensity of the ultrasonic waves ; 

 g. is an object in the wave field. 

 (After Mohres.) 



