ECHOES OF BATS AND MEN 



ming or buzzing noises. But I have discovered in recent 

 years that the orientation sounds, the high-frequency 

 chirps of these bats, are repeated at remarkably high 

 rates as the bats locate and close in upon flying insects. 

 Furthermore, bats will often pursue imitation insects 

 such as pebbles or Uttle wads of wet absorbent cotton 

 tossed gently into the air as they fly by. They do not actu- 

 ally bite or swallow such decoys, but they swoop avidly 

 towards them with the same increase in the tempo of the 

 orientation sounds they employ when chasing real in- 

 sects under natural conditions. When one realizes how 

 silent are many of the small insects upon which bats 

 feed, it becomes rather Ukely, though not rigorously 

 proven, that the bats detect at least some of their insect 

 prey by hearing echoes of their own chirps bouncing oQ 

 the insects rather than relying solely on the sounds 

 emitted by the insects themselves. 



I shall return a Uttle later to the patterns in which 

 these orientation sounds are broadcast under various 

 conditions, including the pursuit of insect prey. But first 

 let us consider the effectiveness of the process of insect 

 himting. Just how many insects does a bat catch in a 

 given time? How big are the insects caught? At what 

 distances are they detected? Only very recently have we 

 been able to provide even partial and tentative answers 

 to such questions. Spallanzani and others who examined 

 the stomachs of bats just returned from a night's hunting 

 have marveled at the relatively large mass of finely 

 chewed insect remains present in the digestive tract of 

 every successful bat. One study showed that little brown 

 bats weighing 7 grams commonly catch 1 gram of insects 

 per hour of active hunting. Very recently we have been 

 able to persuade a few bats to hunt insects in a laboratory 

 flight room where the process could be studied and pho- 

 tographed. One smaller relative of the little brown bat, 



88 



