52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I46 



acteristic vocal patterns of adult Night Monkeys is a specialized 

 adaptation to such conditions. Apart from the Screams (and the 

 Squeaks, if they are a "normal" pattern of adults — see page 58), all 

 the adult calls and notes are comparatively low in pitch. The vocal 

 repertory of adult Night Monkeys is lower, on the average, than the 

 adult repertory of any other New World primate with which I am 

 familiar, except the howlers of the genus Alouatta. It is certainly 

 much lower than the repertory of any other New World monkey of 

 comparable size. All other factors being equal, a low-pitched sound 

 will carry farther than a high-pitched sound of equal energy value 

 at the starting point. In all circumstances, the energy of high-pitched 

 sounds will be used up more rapidly than that of low-pitched sounds. 

 Another property of low-pitched sounds may be particularly signifi- 

 cant in a forested environment. The relatively long sound waves of 

 low-frequency noises will be less affected, i.e., deflected or scattered, 

 by obstructions such as twigs and branches than the shorter waves 

 of high-pitched sounds. Low-pitched sounds may be particularly 

 useful to adult Night Monkeys because they are largely dependent 

 upon auditory signals for long-distance communication. 



It must be significant, in this connection, that the high-pitched 

 Screams seem to be the only purely hostile vocalizations of adults 

 that are always short-range signals. Similarly, the only Squeaks that 

 may be a regular part of adult behavior are uttered only when two 

 individuals are in actual physical contact with, or in very close prox- 

 imity to, one another (see below) . 



RUBBING 



The only individuals observed to perform this pattern were the 

 captive adults on Barro Colorado Island. 



In the most complete and vigorous Rubbing performances, an in- 

 dividual would flex its hind legs, press its perineal region against a 

 branch, and then rub the whole region against the branch with rapid 

 side-to-side movements. Generally there were two or three move- 

 ments in each direction. The tail was usually lifted at the same time, 

 and frequently twitched from side to side. The gland at the base of 

 the tail may have brushed against the branch during these movements, 

 but I was never able to make sure of this. 



Low-intensity Rubbing consisted of nothing more than one or two 

 sideways "swipes" at a branch, sometimes very poorly orientated. 



All Rubbing was usually silent. 



Some individuals had favorite Rubbing sites — particular spots 



