10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



able variety of hostile patterns toward one another in at least some 

 circumstances (see below). 



This aggressiveness must be one of the factors keeping pairs and 

 family groups apart in the wild. Captive adults of more gregarious 

 species of the genera Saimiri, Cebus, Ateles, and Alouatta usually do 

 not fight with one another as frequently or as vigorously as do Night 

 Monkeys kept in similar conditions. 



It is perhaps surprising, therefore, that overt fights between pairs 

 and family groups of Night Monkeys seem to be relatively rare in 

 the wild, at least on Barro Colorado Island. This seems to be due to 

 the fact that different pairs or family groups simply do not come into 

 contact with one another often. Each pair or family group seems to 

 have its own territory or home range, in which it remains all or most 

 of the time. (These territories or home ranges may be largely "tradi- 

 tional." Their boundaries may be established, originally, by means of 

 disputes. Once established, the boundaries may be respected simply 

 because the animals remember the results of the original encounters. 

 If so, the territories of Night Monkeys on Barro Colorado Island are 

 maintained in the same way as those of many birds occurring in the 

 same area — see Moynihan, 1962a.) 



(I have seen relatively large groups of wild Night Monkeys [includ- 

 ing four or five individuals] only in the immediate vicinity of major 

 food sources, i.e., trees in fruit. In such cases, one pair or family 

 group may have been so strongly attracted to the food that it crossed 

 over the boundary of its territory or home range. Some of the indi- 

 viduals in these groups showed obvious hostility toward one another ; 

 and the groups always broke up within a few minutes.) 



An aggressive Night Monkey may perform a variety of displays 

 (see below) immediately before and/or after attacking; but the overt 

 attack behavior itself is usually relatively simple. 



An attacking individual usually begins by advancing, walking, slowly 

 and cautiously toward its opponent. Then, when it has come close 

 enough, it suddenly leaps forward and tries to bite the opponent and/ 

 or hit the opponent with its hands. This hitting is reminiscent of the 

 jabbing of a human boxer. Usually two or three jabs are delivered 

 with great rapidity one right after the other. 



It is my impression that Night Monkeys strike with their hands 

 during fights relatively more frequently than all or most other platyr- 

 rhines. This may be correlated with the fact that they do not have 

 very long canine teeth. 



A Night Monkey that has been attacked usually retreats as soon 

 as possible; but its attacker seldom follows immediately. Even when 



