2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I46 



adults ; and their eyes are relatively stnaller, in proportion to the rest 

 of the head (see figure 22). 



This account is based primarily upon studies of captive animals, 

 checked (whenever possible) by observations of wild animals living 

 under natural conditions. 



Fifteen Night Monkeys were kept in cages at the Smithsonian In- 

 stitution laboratory on Barro Colorado Island between August 1958 

 and July 1963, for periods ranging from several months to more than 



3 years. Some individuals were kept in small cages, 8' x 4' x 4', inside 

 a house ; but the majority' were in larger cages outdoors in the forest. 

 The outside cages varied in size from 6' x 6' x 8' to 30' x 12' x 8'. Gen- 

 erally each cage contained a single individual, a mated pair, or a 

 family group; sometimes, however, larger groups (up to five in- 

 dividuals) were kept together in a large cage for a few days. All 

 the Night }»Ionkeys kept in captivity on Earro Colorado Island were 

 wild-caught animals from mainland areas in Panama (probably, in 

 most cases, from regions east of the Canal Zone) or were the 

 offspring of such animals. 



Some other captive Night Monkeys were observed briefly in the 

 National Zoological Park, in Washington, D.C., in October 1959 

 and July 1960, and in Iquitos, Peru, in December 1958. 



All observations of wild Night Monkeys were made on Barro 

 Colorado Island, in both apparently mature and old second-growth 

 forest (see Bennett, 1963). 



The captive animals on Barro Colorado Island were sometimes 

 observed from blinds ; but such concealment usually uas not neces- 

 sary. Blinds were not used for observations of other captive in- 

 dividuals or the wild animals in the forest. Some of the captive in- 

 dividuals on Barro Colorado Island were accustomed to continuous 

 but dim artificial light at night. This did not appear to affect their 

 behavior after the first few nights. All other individuals were observed 

 without using artificial light whenever possible. Sometimes moonlight 

 was sufficient to reveal at least the most conspicuous and vigorous 

 movements. Even in nearly complete darkness, it was sometimes 

 possible to follow the behavior of the animals by the sounds of their 

 movements and vocalizations. When artificial light was necessary, 

 flashlights were used (either intermittently or continuously). This 

 certainly affected the behavior of the animals, but did not always 

 alarm them greatly or cause them to escape immediately. 



According to Hershkovitz (1949), there is only one species of 

 Night Monkey: Aotus trivirgatiis; and the Panamanian population 



