24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



attacking, by obviously frightened or defensive individuals before 

 and after performing overt escape movements, and by individuals 

 that perform both overt attack and escape movements as well as 

 other displays. They are also uttered during some encounters between 

 mates or potential mates (see below), but they are relatively (if not 

 always actually) uncommon in such circumstances. These facts would 

 suggest that all or most Gruff Grunts are produced by simultaneous 

 activation of the attack and escape tendencies, and these tendencies 

 alone. The range of motivation is probably quite broad. Some Gruff 

 Grunts seem to be produced when the attack tendency is appreciably 

 stronger than the escape tendency ; others when the escape tendency 

 is appreciably stronger than the attack tendency ; and still others when 

 the two tendencies are effectively equal. The attack tendency probably 

 is never as preponderant during any Gruff Grunt performance as 

 during silent overt attack movements ; and the escape tendency is 

 probably never as preponderant during any Gruff Grunt performance 

 as during silent overt escape movements or simple freeze or head- 

 down patterns. The actual strength of the two tendencies involved 

 may be quite different in different circumstances. Some of the single 

 Gruff Grunts and short series of such notes during brief disputes 

 probably are produced when both tendencies are actually weak. Some 

 of the similar-sounding series of notes during prolonged and vigor- 

 ous disputes may be produced when both tendencies are actually 

 quite strong. The escape tendency may be relatively weaker during 

 the least aggressive typical Gruff Grunt performances than during 

 silent Swaying; or, if the relative strength of the two hostile tenden- 

 cies is the same in the two types of performance, their actual strength 

 may be greater during the Gruff Grunts than during silent Swaying. 

 Similarly, the most aggressive Gruff Grunt performances may be less 

 aggressive than typical silent Arch Postures or, perhaps less probably, 

 produced when the total actual strength of the hostile motivation (the 

 attack and escape tendencies together) is less than in the silent Arches. 



Gruff Grunts certainly function as threat during intraspecific dis- 

 putes. They probably do not have a regular signal function when pro- 

 voked by the appearance of a potential predator. 



Some of the captive individuals on Barro Colorado Island were 

 heard to utter single but frequently repeated soft notes which sounded 

 as if they might be nothing more than weak or muffled versions of 

 typical Gruff Grunt notes. Some of these were so soft that they were 

 little more than audible expirations of breath. They were uttered by 

 untamed individuals watching me approach, and were followed (when 

 I came closer) by moderately vigorous escape movements, sometimes 



