THE LANGUAGE OF ECHOES 



jects like insects or pebbles. It is simply too difficult to 

 keep small particles stationary in the air long enough to 

 make accurate tests of bats' ability to dodge them. When 

 wires are strung across a laboratory "flight" room, on 

 the other hand, as diagramed in Fig. 12, the animals 

 seem anxious to avoid collisions, although the brown 

 bats weigh so little that they do not seem to be injured 

 even in an occasional head-on crash against a taut wire. 



When the wires are spaced 30 centimeters apart, or 

 sUghtly more than the wingspread of the Uttle brown bat, 

 they make a difficult barrier that even the most skillful 

 animals brush against lightly from time to time. The 

 wires can be made smaller and smaller, without any 

 marked effect on the percentage of misses registered by a 

 really skillful animal, down to a wire diameter of a frac- 

 tion of a millimeter. To be sure, many bats wlQ be found 

 on first testing in such an obstacle course to be clumsy, 

 striking even the larger wires, but this is usually because 

 they are in poor condition or not completely awakened 

 from the deep sleep into which they lapse even on sum- 

 mer days. It is necessary to reduce the wire diameter to 

 0.07 millimeter (about the diameter of a human hair) 

 before the little brown bats strike them at random. Even 

 slightly larger wires, 0.12 millimeters in diameter, while 

 difficult to miss, are dodged by the really skillful "ath- 

 letes" among our experimental subjects far more often 

 than one would expect. 



Astonishing as it is that bats can detect wires as small 

 as 0.12 millimeters, these previous experiments do not 

 tell us at what distance this detection occurs. But motion 

 pictures of the bats will give some indication of the range 

 of detection when the translated orientation sounds are 

 put on a sound track of the movie. Careful study of such 

 movies, frame by frame, has enabled us to find the dis- 

 tance at which the rate of repetition of the bats' chirps 



93 



