36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



I think that the high-pitched Trills of all these animals were very 

 similar to, or even identical with, the High Trills of young Night 

 Monkeys in sound (see page 69). 



It is possible, therefore, that the apparently full-grown individuals 

 heard to utter such Trills were really immature, in spite of their size, 

 or had reverted to juvenile or infantile behavior for some reason (see 

 discussion, page 58). It is also conceivable that high-pitched Trills 

 are the only pattern of this type in the population from which the 

 National Zoological Park animals were obtained. 



As indicated in the accompanying figures, the notes of Low Trills 

 are usually or always slightly higher in pitch than simple Moans. This 

 might be evidence that Low Trills have been derived, in ontogeny 

 and/or phylogeny, from a combination of typical Moans and High 

 Trills. 



Some or all Low Trills seem to have a slightly "liquid" quality, 

 somewhat reminiscent of Gulp notes. There may be some sort of 

 special relationship between the two types of patterns, either direct or 

 indirect via the Gruff Grunt patterns (see both the accompanying 

 figures and the comments below). Gulps, Moans, and Low Trills are 

 all similar in some aspects of motivation. 



Many of the wild Night Monkeys on Barro Colorado Island and 

 almost all the captive individuals at the National Zoological Park and 

 at Iquitos as well as on Barro Colorado were heard to utter brief, 

 soft, plaintive-sounding Moans (see figures 8 and 10). 



Like the Low Trills, the simple Moans were uttered with the mouth 

 apparently closed and were not accompanied by conspicuous inflation 

 of the throat. 



All or almost all the animals uttered Moans more frequently than 

 Trills on the average. They uttered Moans in all the situations in 

 which Low Trills were characteristic, and in other circumstances as 

 well. Moans were relatively rare during the initial "greetings" be- 

 tween strangers and during the exploration of strange surroundings ; 

 but they were uttered frequently by individuals in familiar social and 

 physical environments. They were the most common notes uttered 

 by particularly tame (hand-raised) captive animals on Barro Colorado 

 Island when I came within a few feet of their cages. At such times, 

 they were uttered by animals approaching me in an apparently friendly 

 or "curious" manner, as well as by animals performing slight retreat 

 movements or intention movements. They were also uttered by mated 



