26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



certainly were not aware that their performances were being observed 

 by a human being (from a bHnd). A very tame hand-reared male 

 kept in a cage in the animal house, with a female to whom he was 

 well adjusted, frequently uttered similar series at night and (more 

 frequently) in the morning well after dawn, usually just as or just 

 after an animal-keeper passed by his cage and/or some howler mon- 

 keys (Alouatta palliata) in an adjacent cage uttered "barks" and/or 

 "roars" (see below). 



The simple series of all these individuals except the solitary female 

 were composed of essentially monosyllabic notes. At least some of the 

 notes of the solitary female were definitely bisyllabic. Unfortunately, 

 it was not possible to record any of these performances. The series 

 of monosyllabic notes might be transcribed by something like 



"Roo roo roo roo . . ." 

 rr rr rr rr 



while the series of bisyllabic notes were something like 



"Rooa rooa rooa rooa. . . ." 

 rrr rrr rrr rrr 



In all these performances, the individual uttering the Resonant 

 Grunts assumed a moderate or extreme Arch Posture as soon as it 

 began a series and went out of the posture as soon as the series was 

 ended. Most of these postures were identical with typical silent 

 Arches, with both hands and feet grasping the perch. The male en- 

 gaged in disputes with his neighbors, however, tended to stand up on 

 his hind limbs when he assumed an Arch. During some or all of 

 these standing postures, his arms hung downward as usual but his 

 elbows went in and out in rhythm with the accompanying notes. I 

 think that the elbows always went in as a note was uttered and out 

 between notes. Night Monkeys have a large ventral air-sac communi- 

 cating with the larynx (Hill, 1960). It is possible that the inward 

 movements of the elbows helped to expel air from the sac and that 

 the outward movements helped to draw air into the sac. This may 

 have facihtated sound production. (The other individuals were able 

 to utter similar and probably equally loud notes without similar move- 

 ments ; but they did not utter as many series of notes in a short 

 period of time.) 



All these series of Resonant Grunts were immediately preceded by 

 more or less prolonged periods of intense activity, the performing 

 animal running and leaping madly about its cage and uttering many 

 Gruff Grunts, Gulps, and Sneeze-grunts (and sometimes Moans and/ 

 or Low Trills). 



