6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



from one another to approximately the same extent. At other times, 

 all five fingers of the hand may be kept pressed closely together. 

 When some appreciable opposition does occur, it is perhaps most 

 common for the thumb to be opposed to the other four fingers as a 

 group ; but this is by no means always the case. I have, in fact, seen 

 Night Monkeys arrange their fingers in every possible way when 

 grasping objects of different sizes and shapes. The first two fingers 

 may be partly opposed to the last three ; or the first three fingers may 

 be partly opposed to the last two ; or the first four fingers may be 

 opposed to the fifth. 



It is quite probable that this type of hand and use of fingers is 

 primitive among Platyrrhini. (The actual structure of the Night 

 Monkey hand is described in detail by Biegert, 1961.) 



The most important general feature of the social behavior of Night 

 Monkeys is their slight degree of gregariousness. On Barro Colorado 

 they are seldom found in groups larger than a single pair or a single 

 family of two parents with one young. 



ORDINARY GROOMING AND CLEANING ACTIVITIES 



New World monkeys perform two types of grooming: Self-groom- 

 ing and mutual or social grooming, i.e., the grooming of one animal 

 by another. The former type may be called "autogrooming" and the 

 latter "Allogrooming." (These terms have been suggested by the terms 

 "autopreening" and "allopreening" applied to birds by Cullen, 1963.) 



Most monkeys devote considerable time and effort to autogroom- 

 ing ; but different species tend to do this in slightly different ways. 



The autogrooming of Night Monkeys consists almost entirely of 

 scratching with the hands and/or the feet. Only very rarely does a 

 Night Monkey use its teeth during autogrooming to "nibble" at its own 

 fur or skin. The relative frequency of different types of autogrooming 

 movements performed by some adult and subadult individuals in 

 captivity on Barro Colorado Island is shown in the accompanying 

 tabulation. 



The actual frequency of autogrooming movements may vary widely, 

 depending upon the situation. Most of this variation seems to be a 

 function of the amount of dirt or ectoparasites in the fur, or of its 

 disarrangement. In captivity, however, it is evident that most adult 

 individuals tend to perform fewer autogrooming movements, on the 

 average, than young individuals. This may be because adults tend to 

 be less tame than young individuals in captivity. Night Monkeys 

 seldom, or perhaps never, perform autogrooming movements when 



