22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



head. They lack the variety of facial expressions ( e.g., frowns, baring 

 of teeth, pursing of lips) which are important signal patterns in such 

 platyrrhines as Cehus and Ateles species. They also lack the special 

 elongated tufts and ruffs of hair on the head and around the face 

 which are typical of many marmosets and tamarins, and which can be 

 erected or depressed to convey signal information. The absence of 

 such characters may be correlated with nocturnality. Adult Night 

 Monkeys frequently become separated (by distances of at least several 

 yards) from their mates and subadult young while feeding and mov- 

 ing in the forest at night. In such circumstances, they probably can- 

 not rely upon perceiving visual signals, especially small and complex 

 signals, from their companions as frequently or as easily as can adults 

 of other platyrrhine species which are diurnal or crepuscular (see also 

 discussion on page 45) . 



Andrew {op cit.) says that Night Monkeys open the mouth in 

 threat. I never saw this except when sounds were uttered at the 

 same time. 



The complex black-and-white head pattern of Night Monkeys 

 (more complex than the corresponding patterns of any other New 

 World primate) may provide a partial substitute for a variety of 

 facial expressions, whenever Night Monkeys are close enough to- 

 gether to perceive the pattern clearly. Because the black and white 

 stripes and patches are convergent and divergent, and some of them 

 are curved, a slight alteration of the position of the head relative to 

 an observer will tend to alter the whole appearance or "gestalt" of the 

 pattern (see the accompanying sketches). This must help to empha- 

 size the signal effect of head movements. 



GRUFF GRUNTS 



Most of the vocal patterns of adult Panamanian Night Monkeys 

 can be divided into eight main categories : Gruff Grunts, Resonant 

 Grunts, Screams, Low Trills, Moans, Gulps, Sneeze-grunts, and 

 Hoots. All except the last seem to be purely or partly hostile. 



Gruff Grunts are among the most common of the vocal patterns. I 

 heard them uttered by both wild and captive individuals on Barro 

 Colorado Island and by all the other captive individuals studied. The 

 typical Gruff Grunts of almost all individuals were similar or identical 

 in sound (to the human ear), low-pitched, moderately long, and mod- 

 erately loud. Figure 4 is a sketch of a sound spectrogram of a single 

 note of this type. 



(All the sound spectrograms illustrated in this paper are derived 



