238 APES AND MONKEYS 



tone, which rapidly increases in pitch and frequency, until 

 it becomes a terrific scream. The first sound of the series 



and each alternate sound are made by expiration ; the inter- 

 mediate ones appear to be by inspiration. How this is 

 accomplished it is dim cult to say. The sound as a whole 

 resembles the braying of an ass, except that the notes are 

 shorter, the climax is higher, and the sound is louder. A 

 gorilla does not yell in this manner every night, but when 

 he does so it is usually between two and five o'clock in the 

 morning. I have never heard the sound during the day 

 nor in the early part of the night. When screaming he 

 repeats the series from ten to twenty times, at intervals of 

 one or two minutes apart. I know of nothing in the wav 

 of vocal sounds that can inspire such terror as the voice of 

 the erorilla. It can be heard over a distance of three or 

 four miles. I can assign no definite meaning to it unless 

 it is intended to alarm some intruder. 



One morning, between three and four o'clock, I heard 

 two of them screaming at the same time. I do not mean 

 at the same instant, but at intervals during the same period 

 of time. One of them was within about a third of a mile 

 of me, and the other in another direction, perhaps a mile 

 away. The points we respectively occupied formed a 

 scalene triangle. The sounds made by the two apes did 

 not appear to have any reference to each other. Some- 

 times they would alternate, and at other times they would 

 interrupt each other. They were both made by giants of 

 their kind, and every leaf in the forest vibrated with the 

 sound. This was during the latter part of May. They 

 scream in this way from time to time throughout the 



