46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



of behavior has been studied is the Rhesus, Macaca mulatta. Accord- 

 ing to Rowell and Hinde (1962), the vocal patterns of this species also 

 intergrade through frequent intermediates. Rhesus Monkeys are 

 diurnal, and have many visual displays, including many different facial 

 expressions (Hinde and Rowell, 1962). 



These facts would suggest that there may be a general rule among 

 monkeys that species or classes of individuals largely dependent upon 

 auditory signals for the regulation of their social behavior tend to have 

 discrete, sharply delimited vocal patterns, while species or classes of 

 individuals less dependent upon auditory signals tend to have inter- 

 grading vocal patterns. 



The major vocal patterns of adult Night Monkeys not only are 

 discrete but also sound quite different from one another to human 

 ears. This is true of patterns that look somewhat similar in sound 

 spectrograms, e.g., Gulps and Sneeze-grunts, as well as of patterns 

 that look very different. There is every reason to suppose that the 

 contrasts between different types of patterns are equally obvious to 

 the Night Monkeys themselves. It is quite evident from their behavior 

 that the hearing of Night Monkeys is at least as good as that of 

 human beings. 



Contrasts in sound may facilitate rapidity of comprehension when 

 messages are composed of many notes of different types. 



The probable importance of this feature is emphasized by the fact 

 that all the associations between different types of notes that are 

 particularly common and/or seem to be ritualized per se are associa- 

 tions between notes that contrast with one another in a particularly 

 striking manner. In some cases, e.g., Gulps and Sneeze-grunts, the 

 associated notes seem to serve similar functions. In other cases, e.g. 

 Sneeze-grunts and Moans, they seem to serve quite different func- 

 tions. Associations between notes that are somewhat less conspicuous- 

 ly contrasting in sound, e.g., Moans and Low Trills, seem to be less 

 frequent and/or less close. 



It is conceivable that information conveyed by contrasting stimuli 

 may tend to produce stronger reactions {i.e., may appear to be more 

 "emphatic" to the receiver) than the same information conveyed by 

 stimuli with less contrast; but this would be very difficult to prove. 



In spite of the obvious differences between the hostile vocal patterns 

 of adult Night Monkeys, there are some morphological resemblances 

 between many or most of them. This is certainly true of Gruff Grunts, 

 Resonant Grunts, and Sneeze-grunts. A few similarities between 

 Gulps and both Gruff Grunts and Sneeze-grunts have already been 

 mentioned. It may be convenient to refer to all these patterns, together, 



