NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 35 



either actual fighting or copulatory reactions. The only prolonged 

 and vigorous dispute during which I heard such notes uttered was an 

 encounter between a male and a female that had fought with one 

 another previously, been separated for several days, and then were 

 brought together again. 



These facts would suggest that many or all of the Low Trills were 

 partly hostile reactions, produced when the escape tendency was 

 stronger than the attack tendency, but effectively counteracted by 

 some other nonhostile tendency, such as sex or feeding, or a non- 

 hostile tendency plus attack. The nonhostile tendencies involved prob- 

 ably were not direct causes of the Low Trills in the same way as the 

 escape tendency, as different nonhostile tendencies were involved in 

 different situations while the sounds themselves remained essentially 

 the same in all situations. 



Some of the circumstances in which Low Trills were uttered were 

 similar to those in which many other species perform appeasement 

 displays. The Low Trills may have had a similar function ; but I 

 could not distinguish their exact signal effects from those of associated 

 patterns such as social sniffing. 



Two full-grown Night IMonkeys at the National Zoological Park 

 in Washington, D. C, and some apparently full-grown individuals in 

 the forest on Barro Colorado Island were heard to utter Trills remi- 

 niscent of typical Low Trills in sound but higher pitched, more 

 mechanical or metallic in tone, and without any plaintive or moaning 

 quality. 



The significance of these patterns was difficult to assess, as the 

 captive individuals in the National Zoological Park were observed 

 briefly and the wild individuals on Barro Colorado uttered their notes 

 in obscure and/or complex social situations. 



The National Zoological Park animals were a male and a female. 

 I was told that they had been placed together in the same cage only 

 a few days before my observations. They uttered their high-pitched 

 Trills whenever they came face to face. Many of their Trills were 

 followed by social sniffing, without any (other) overt indications of 

 hostility. I did not hear them utter anything more closely similar to 

 the Low Trills of the captive adults on Barro Colorado. This might 

 suggest that their high-pitched Trills were equivalent to the latter, 

 produced by the same or very similar motivation and serving the 

 same function (s). 



Unlike the Zoo animals, the wild Night Monkeys that uttered high- 

 pitched Trills were also heard to utter typical Low Trills during the 

 same social encounters. 



