NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN I3 



in whatever posture it was in when it first perceived a disturbing 

 stimulus. Such freezes tend to be performed as reactions to distant 

 stimuli or familiar near stimuli. They are not usually associated very 

 closely with vigorous movements or displays in such circumstances. 



Sometimes an animal will close its eyes, briefly, immediately before, 

 during, or immediately after pausing in a simple freeze. 



The most conspicuous escape behavior of Night Monkeys in the 

 wild or in large cages is simply rapid, running or leaping, retreat. 

 This tends to be performed as a reaction to some strong, near and/or 

 unfamiliar stimulus, such as actual attack or other overt aggressive 

 behavior by another individual of the same species or the sudden 

 approach of a "potential predator" such as a strange human being. 

 Active retreats frequently are preceded and/or followed by elaborate 

 displays. 



In similar social circumstances, a Night Monkey in a small cage 

 may either run or leap around its cage very rapidly or, alternatively, 

 perform a pattern which is reminiscent of the simple freeze described 

 above but somewhat more complex. In this pattern, the animal sits 

 motionless with its head lowered (such lowering is not characteristic 

 of ordinary freezes). The head is sometimes lowered almost to the 

 level of the hands or the branch on which the animal is sitting. The 

 animal may look straight down, or keep its head turned sideways in 

 order to fixate the disturbing stimulus. At the same time, the limbs 

 may be drawn in under the body and strongly flexed. This is obvi- 

 ously a preparation for (or an "intention movement" of) leaping. As 

 a result of the lowering of the head, the back is more or less curved 

 (but not raised). Some typical variations of this "head-down posture" 

 are shown in figure 2. 



It is possible that Night Monkeys also assume head-down postures 

 in the wild under completely natural conditions. I never actually saw 

 such reactions in the forest on Barro Colorado Island (they would be 

 extremely difficult to distinguish in trees at night) ; but some of the 

 animals kept in large cages, which approximated natural conditions, 

 did assume head-down postures from time to time. 



The captive individuals in large cages did not, however, assume 

 head-down postures as frequently as individuals of many other spe- 

 cies of platyrrhine monkeys in similar situations. And their head- 

 down patterns were not as exaggerated in form as the homologous pat- 

 terns of some related species. 



All these overt escape or alarm patterns are usually silent. The 

 fact that Night Monkeys customarily are silent during rapid retreats 

 from potential predators is another distinctive feature of the species, 



