NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 5I 



own species in their cages, at the times that they uttered Hoots. Any 

 adult that had been Hooting regularly every night always (again with 

 the same single exception) stopped Hooting as soon as another Night 

 Monkey was put in the same cage. The Hoots stopped irrespective 

 of whether the animals fought with one another and/or performed 

 copulatory reactions or did neither. (I might add that Hoots were 

 not uttered during disputes between individuals in different cages, 

 even "long-range" hostile reactions between individuals in cages quite 

 far apart from one another.) 



As far as I could tell, all the wild individuals heard to utter Hoots 

 were also alone at the time. 



These facts would suggest that the Hoots of adult individuals 

 are produced when a gregarious and/or a pairing tendency is thwarted 

 by the absence of a suitable companion. 



Although I was never able to observe responses to Hoots in the 

 forest on Barro Colorado Island, it seems likely that their primary 

 function, when uttered by unmated adults under natural conditions, 

 is to attract potential mates. H so, they are essentially similar to cer- 

 tain types of bird vocalizations which have been called "song." (An- 

 other resemblance to most "song" is that an individual uttering Hoots, 

 i.e., "wanting" a companion, apparently always remains stationary, 

 instead of going in active search of a companion. It has already been 

 mentioned that the captive individuals sat during their Hoots. Wild 

 individuals frequently moved from tree to tree between series of 

 Hoots, but they were never heard to move during the utterance itself.) 



The single case of Hooting by an adult in the same cage with an- 

 other individual of the same species, cited above, occurred immediately 

 after a male and a female had been brought together again after a 

 long separation. They were probably mated before, but may have 

 become incompatible during the period of separation. The female 

 uttered many Hoots during the first few nights after the animals 

 were brought together again ; but all her Hoots were unusually soft 

 and probably low intensity. 



The Hoots of Night Monkeys sound very much like some notes 

 of owls. There is, in fact, one species of owl on Barro Colorado 

 Island which utters hooting notes that are almost identical (to human 

 ears) with those of Night Monkeys (except for the fact that they are 

 always or almost always uttered singly). It is possible that notes of 

 this type have acoustic properties which are particularly advantageous 

 in tropical forests at night. 



It seems probable that the pitch, at least, of almost all the char- 



