Luiz Claudio Marigo 



'l^i 



A 



Hai-py eagles, below, regularly prey on 

 small monkeys of the Peruvian rainforest. 

 The heftier red howler, right, weighing 

 several times as much as a titi or squirrel 

 monkey, rarely winds up as a meal for 

 one of the forest's diurnal birds of prey. 



Ken Lucas; Planet Earth Pictures 



feet out of the tree, landing at my feet. 

 Kinkajous, at five pounds nearly twice the 

 size of an owl monkey, are not so easily 

 dominated. However, kinkajous are soli- 

 tary and thus would be no match for a 

 group of four to five owl monkeys; when 

 these two species meet, they usually move 

 apart to feed in different parts of the tree. 



To test my theories about the owl mon- 

 key's nocturnal life style, I decided to ob- 

 serve the species in a different sort of habi- 

 tat. After my year in the ram forests of 

 Peru, I visited the dry, open forests of the 

 Paraguayan Chaco. Few diurnal monkey 

 species live in the Chaco, and none of the 

 species that had attacked Callicebus in 

 Peru. Diurnal raptors are also rare, but 

 great homed owls are common. One pair 

 raised two young in a nest near my camp- 

 site during my time there. 



Interestingly, I found that owl monkeys 

 in the Chaco had reverted partly to day- 

 time activity. I watched in amazement as 

 the monkeys browsed on flowers and fruits 

 at the top of the canopy in bright sunhght. 

 They foraged at night as well, but now 

 they moved quietly and avoided the upper 

 canopy, where they would be exposed to 

 the owls. On average, the owl monkeys 

 traveled and foraged one to three hours in 

 daylight and some nine hours at night. In 



50 Natural History 5/94 



the cold Chaco winter, during the times of 

 the month when there was no moonhght, 

 the monkeys increased their daytime ac- 

 tivity, traveling nearly as far in the daytime 

 (850 feet) as in the night (about 1,000 

 feet). The monkeys' sleeping patterns 

 changed in the Chaco, too. They slept on 

 open branches, not in hidden vine tangles, 

 and used many different sites; one group 

 slept in forty-two different trees in five 

 months. Moreover, they were never 

 chased from a fruit tree, day or night, with 

 their only possible food competitor being 

 Alouatta, the howler monkey. Overall, the 

 behavior of the Chaco owl monkeys 

 seemed to support the idea that avoidance 

 of predators and food competitors may 

 have played a role in the evolution of a 

 nocturnal life style in the Peruvian rain 

 forest. 



If being active at night can, under the 

 right circumstances, confer so many ad- 

 vantages, why haven't more monkeys 

 adopted it? Most nocturnal mammals, in- 

 cluding the nochrmal primates in Asia and 

 Africa, have the tapetum lucidum, which 

 allows them to see in the dark. Monkeys, 

 apes, and humans have lost the tapetum 

 and thus are relatively helpless at very low 

 hght levels. A short walk at night without 

 the aid of a flashlight wiU show just how 

 serious a loss this is. 



How, then, does Actus manage? Over 

 the course of its evolution, the aptly 

 named owl monkey evolved very large 

 eyes, which assist it greatly as it searches 

 for food in the dark and jumps from 

 branch to branch high up in the canopy. 

 Some of my findings, however, indicated 

 that the monkeys' movements were re- 

 stricted by low light levels. On totally dark 

 nights, the owl monkeys I followed in Peru 

 traveled nearly a thousand feet less than 

 on clear moonlit nights; they also tended 

 to stick to the most famiUar paths. Certain 

 activities — such as playing, territorial 

 fighting, and calling — are engaged in only 

 when the moon is bright. I gradually real- 

 ized that I was not alone in my nightly 

 stumbles through the rain forest; even for 

 the successful owl monkeys, night life had 

 its disadvantages. □ 



- AY J 



t 



