NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 45 



The principal differences between these two types of platyrrhine 

 vocal repertories may be summarized as follows. In one type, the 

 information is coded very rapidly in an economical form but may be 

 difficult to read, or difficult to read rapidly. In the other type, the 

 information is coded less rapidly and efficiently but can be read more 

 easily and/or more rapidly. 



The relative importance of the advantages and disadvantages in- 

 herent in each type of repertory may be different depending on the 

 circumstances and environments. The type of repertory possessed 

 by adult Night Monkeys may be another (partly indirect) adaptation 

 to nocturnality. 



Adults of Saimiri, Cehus, and Ateles can avoid some of the dis- 

 advantages inherent in their type of vocal repertory by combining the 

 information received from auditory signals with information derived 

 by another method of perception. They are all thoroughly diurnal, 

 and must usually be able to see other animals and objects in their 

 environment quite clearly at considerable distances. As noted above, 

 both Cehus and Ateles species have a wide variety of facial expres- 

 sions which function as signals. They also have many other display 

 postures and movements involving other parts of the body ; and so 

 does Saimiri. Any adult individual of these species hearing calls or 

 notes from another individual of the same species usually will 

 perceive visual signals and/or receive visual clues from the physical 

 environment at the same time. These visual aids should enable the 

 individual to grasp the significance of any vocal pattern, even when 

 the latter is difficult to decipher or ambiguous in itself. 



As adult Night Monkeys usually receive less visual information 

 (because they are nocturnal and frequently become separated from 

 one another and have fewer visual displays), they are more dependent 

 upon auditory signals and more likely to misinterpret any possibly 

 ambiguous vocal patterns. It must be absolutely essential for them 

 to have a system of vocal patterns which are easily and immediately 

 comprehensible, whatever the inevitable disadvantages of such a sys- 

 tem. 



It is probably significant, in this connection, that infant and juvenile 

 Night Monkeys utter relatively more intermediate notes than do 

 adults (see below). The vocal signals of infants to their parents 

 may be supplemented by tactile stimuli, as the infants are always 

 carried by their parents. Even when no longer carried, juveniles may 

 be able to provide important supplemental visual information because 

 they tend to remain very close to their parents. 



To my knowledge, the only Old World monkey in which this aspect 



