32 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



These facts would indicate that the vScreams were produced when 

 hostile motivation as a whole was very strong, when the escape tend- 

 ency was much stronger than the attack tendency (although probably 

 less preponderant than in the silent overt escape and alarm patterns). 

 The actual strength of the hostile motivation as a whole during 

 Screams was almost certainly greater than in any Gruff Grunt pat- 

 tern in which escape is predominant. 



9.000 

 8.000 



7.000- 

 t.OOO 

 5.000 

 4.000 

 3.000 

 2.000 

 1,000 



r^ T^H 



0.2 . 0.4 0.4 0.8 1-0 >^ '•♦ 1.4 I.B 2.0 



Fig. 6. — Two long Screams, uttered by an adult. 



Based upon a spectrogram by a Kay Electric Co. "Sona-graph." 

 Only the first harmonic is shown. There was at least one other with each note. 

 In this and all the following sketches which include more than one note, the 

 interval (s) shown between notes are the actual interval (s). 



Screams may be adapted to frighten or startle a predator, and thus 

 give a screaming animal a better chance to escape. 



LOW TRILLS 



Night Monkeys utter a variety of sounds that can be described as 

 Trills. 



The most common sounds of this type uttered by captive adults 

 on Barro Colorado Island were "Low Trills." A single Low Trill 

 was a "bubbling" series of short low-pitched notes, uttered very 

 rapidly one right after the other but still distinguishable by the human 

 ear. All or most of the individual notes in a series were slightly plain- 

 tive in tone, and sounded somewhat like abbreviated versions of simple 

 Moans (see page 36). The number of notes in a single series varied 



