59Q 



BATS 



an octave in each pulse. The pulse repetition rate varies from less than 

 io/sec at rest to over ioo/sec when avoiding obstacles or hunting. The 

 pinna of these bats is very large and folded below to form an anti- 

 tragus (Figs. 367, 368). Vespertilionids commonly detect insects at 

 50 cm and may do so at 1 m. 



In the horseshoe bats the pulse is much longer (40-100 m. sec), 

 and of high and constant frequency (85-100 kc). It is emitted through 

 the nose (Dijkgraaf) and beamed by interference at the nostrils, which 



Fig. 369. Diagram showing interference pattern of the ultrasonic waves from the 



nares of a horseshoe bat. Peaks are indicated by a continuous line, valleys by 



dashes. The dotted lines limit the sector in which the waves are most intense, 



maximal in the sagittal plane. 



n. nares; s.p. sagittal plane. (After Mohres.) 



are set half a wavelength apart (Fig. 369). The pulse repetition rate is 

 low (<io/sec). This mechanism is even more effective than the other 

 and is said to detect insects even at 6 m (Mohres). 



It was first suggested by Hartridge by analogy with early audio- 

 location and radar devices that bats estimate distance by measuring 

 the echo delay. The middle ear muscles and intra-aural reflexes do 

 indeed allow a very rapid recovery of sensitivity after short loud 

 sounds, as would be necessary, forming a sort of transmit-receive 

 switch (Griffin, 1958). Yet it hardly seems possible that the reflex 

 can work fast enough to allow accuracy at short distances. An alter- 

 native hypothesis is that the bat measures the loudness of the echoes, 

 especially the horsehoe bats, with their long pulses (Mohres). The 

 beam movements might give direction and searching movements the 

 range by triangulation. This theory seems to require a very complete 

 acoustic separation of ear and nasopharynx and special cerebral 

 capacities for calculation. 



A third suggestion is that the ear receives both the outgoing and 



