Owl monkey's have evolved big eyes to help them get around after 

 dark. Not as well adapted to the nighttime as many nocturnal 

 mammals, they are most active on bright moonlit nights. 



Tom McHugh; Photo Researchers, Inc.. Monkey Jungle. Miami 



Night Watch 

 on the Amazon 



When dusk falls in the Peruvian rainforest, the world's only nocturnal 

 monkey gears up for a noisy night of feeding in the canopy 



by Patricia Chappie Wright 



The full moon loomed above the Peru- 

 vian rain forest canopy, illuminating even 

 the forest floor where I sat with my field 

 notebook in hand. On this chilly and quiet 

 night, I strained my ears to catch every 

 sound. Suddenly, coming from some one 

 hundred feet up in the canopy, I heard 

 what I was waiting for: the low, mournful 

 hoot of an owl monkey, Aotus trivirgatus. 

 Three notes, a pause, and then a lower 

 note. Taking a compass direction, I wrote 

 down the time. The caU was repeated for 

 the next ten minutes, then stopped. From a 

 distance came the answering call: five 

 gruff hoots, a pause, and two lower hoots. 



I had been hstening to calls like this on 

 bright, moonht nights for almost a year 

 and had begun to piece together certain as- 

 pects of owl monkey life. I knew, for ex- 

 ample, that these owllike calls are given 

 by a monkey when it is alone, usually near 

 the borders of its family's territory. Calling 

 sessions are restricted to once or twice a 

 month and may be given by an adult male, 

 an adult female, or a subadult. The calls, 

 which can be heard 1,500 feet away, con- 

 sist of a series of ten to thirty short, low- 

 pitched hoots a minute. The session lasts 

 one to two hours, as the caller moves a few 

 hundred feet along its border The calls al- 

 most always evoke responses from neigh- 

 boring territories. At the end of this territo- 

 rial calling session, a calling monkey 

 usually returns to its family, which may be 

 resting in the center of the territory. When 

 a young monkey leaves its natal group, 

 however, it may travel long distances in 

 the forest, calling continually, perhaps ad- 

 vertising for a mate. 



As glad as 1 was to begin deciphering 



owl monkey calls, no call could tell me 

 what I had come to Peru to find out — why 

 this species is active at night. Found in 

 forested regions from Panama to northern 

 Argentina, it is the world's only nocturnal 

 monkey. All other nocturnal primates — 

 including mouse lemurs and aye-ayes in 

 Madagascar, tarsiers and Ions in Asia, and 

 bushbabies and pottos in Africa — are 

 prosimians, a more primitive group that 

 lacks the monkeys' relatively large brain, 

 enclosed eye sockets, dry rhinarium 

 (nose), and impressive manual dexterity. 

 And unlike the eyes of most nocturnal 

 mammals, the owl monkey's eyes have 

 cones for color vision and lack a reflective 

 shield on the retina (the tapetum lucidum), 

 which suggests that its ancestor was active 

 in the daytime only. A question that had 

 long intrigued scientists was why a day 

 monkey had evolved into a night monkey. 

 Since studying monkeys in captivity or 

 skins in a museum could not give satisfac- 

 tory answers, I decided to go to the Cocha 

 Cashu research station, situated in a pris- 

 tine rain forest in southeastern Peru's 

 Manu National Park, where I could ob- 

 serve owl monkeys in the wild. 



The owl monkey shares its rain forest 

 home with eleven other monkey species, 

 including Callicebus moloch, the dusky 

 titi. The diurnal titi and the owl monkey — 

 both about squirrel size — have similar so- 

 cial systems. I decided to compare the life 

 styles of the two species — their diet, sleep- 

 ing habits, movement patterns — in the 

 hope of gaining insight into the owl mon- 

 key's nocturnal life style. 



I first needed to survey the area for both 

 species of monkey and to select four 



groups (two of each species) to focus on. 1 

 chose one group of owl monkeys whose 

 territory bordered on the Manu River and 

 another whose territory bordered on Lake 

 Cocha Cashu. Then I identified titi territo- 

 ries that overlapped with the chosen Aotus 

 groups. With my study animals targeted, I 

 then began the lengthy process of getting 

 them used to my presence and learning 

 how to follow them through the forest. 



Owl monkeys are often habituated to 

 one sleeping tree. Charies Janson, a prima- 

 tologist studying capuchin monkeys at 

 Cocha Cashu, was the first to find an Aotus 

 sleeping free there. I began my real data 



Owl monkey's often sleep and seek shelter 

 in tree holes and vine tangles. 



Arthur W, Ambler; Photo Researchers. Inc. 



45 



