NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 23 



from recordings of captive individuals on Barro Colorado Island. The 

 recordings were made by an Ampex 601, using an Electrovoice 666 

 Variable D Cardioid microphone.) 



Gruff Grunts usually are uttered singly or in short series of two 

 to five notes. Each note of a short series seems to be essentially the 

 same as a single note, and the successive notes may be uttered at 

 slightly irregular intervals. Series are frequently repeated, but suc- 

 cessive series generally are separated by relatively long pauses (much 

 longer than the longest intervals between notes of a single series) . 



6,000 



4.800H 



3,600 



2,400- 



1.200- 



» ■■ T 



0.2 0.4 



Fig. 4. — A typical Gruff Grunt, uttered by an adult. 



Based upon a sound spectrogram made by a Kay Electric Co. "Vibralyzer." 

 In this and all the following sketches of vocal patterns, the figures along the 

 vertical axis indicate cycles per second and the figures on the horizontal axis 

 indicate elapsed time in seconds. 



All Gruff Grunts seem to be uttered with the mouth almost or com- 

 pletely closed ; but they are usually, or perhaps always, accompanied 

 by conspicuous inflation of the throat region, the throat swelling up 

 just as a note begins, and then subsiding immediately as soon as the 

 note ends (even when the note is part of a series). 



Both single Gruff Grunts and short series of such notes are uttered 

 frequently during all sorts of hostile encounters, apart from partly 

 sexual situations, during both brief and prolonged intraspecific dis- 

 putes in the wild as well as in captivity, and as reactions to the 

 appearance of a predator or potential predator. During intraspecific 

 disputes, they are uttered by aggressive individuals before and after 



