THE LANGUAGE OF ECHOES 



in thickly wooded areas where competing echoes ob- 

 scure the important echo from the insect. 



The success of bats in catching one insect every few 

 seconds testifies to their ability not only to hear the in- 

 sect echoes but to sort them out of a welter of other, 

 competing echoes. This process has been studied in the 

 laboratory by modifying the circumstances to standard- 

 ize the conditions and permit measurements of the bats' 

 performance. Rather than studying bats as they hunt 

 insects in the woods, we generated artificial sounds in 

 our flight room so that these noises were added to the 

 echoes from wires, floor, and walls. In other words, we 

 tried to confuse or "jam" the bats. The result was a sur- 

 prising and revealing failure. The bats continued to 

 dodge wires of 1 to 2 millimeters even in the most in- 

 tense noise we could produce, a loud hissing that covered 

 the whole frequency range of their orientation sounds. 

 Skillful animals avoided wires of this size just as well 

 in the noise as in the quiet, even though the noise was 

 much louder than the echoes from the wires. These ex- 

 periments could theoretically be shown as a fifth graph 

 in Fig. 14, but an accurate representation of the noise 

 would obUterate any representation of echoes from the 

 wires. 



There are limits, however, to the discriminating ability 

 of even the most skillful bat. If the wires are made 

 smaller and smaller, a size is finally reached where the 

 echoes no longer can be detected. The smallest wire 

 which can be detected in noise is greater than the small- 

 est wire detectable in quiet. For one species of bat the 

 minimum in the quiet was about 0.25 millimeter, and 

 in the noise the minimum size increased to 0.5 to 0.7 

 millimeter, depending upon the individual animal and 

 its condition at the time of the particular experiment. 



103 



