Luiz Claudio Marigo 



Like the owl monkey, the dusky titi, facing page, is monogamous and lives 

 in small family groups. It forages in the daytime, however, and is forced 

 to compete — often unsuccessfully — with squirrel monkeys, below, and 

 other monkeys living in Peru's Amazonian rainforest. 



Luiz Claudio Marigo 



many ways. One of the dusky titi families, 

 for example, slept in a total of forty-three 

 different trees over the course of the year, 

 while owl monkey families used no more 

 than five. As they slept on open branches, 

 the titis were visible from below, whereas 

 the owl monkeys were always hidden 

 from sight as they slept in a tangle of vines 

 or in a tree hole. Callicebus was quiet, vig- 

 ilant, and cautious as it foraged low in the 

 trees during the day; Aotus was noisy and 

 careless as it went about its business high 

 in the canopy at night. 



1 began to suspect that the daytime pre- 

 sented some dangers that the nighttime did 

 not. Circumstantial evidence soon imph- 

 cated birds of prey as a probable daytime 

 threat. From studies conducted by or- 

 nithologist N. Rettig of remains under the 

 nest of a harpy eagle in Guyana, we knew 

 that monkeys were the main item in this 

 eagle's diet. Manu National Park is home 

 to six species of hawks and eagles, includ- 

 ing the harpy, that are big enough to eat 

 owl monkeys and dusky titis. During the 

 third month of my research, a harpy eagle 

 was sighted carrying a squirrel monkey in 

 its talons. A few weeks later, a crested 

 eagle attacked a group of capuchin mon- 

 keys. And one of the young titis bom the 

 year I started my study was last seen in 

 October of his second year in the talons of 

 a crested eagle. 



Also suggesting that the monkeys were 

 responding — although in different ways — 

 to the threat of predation were the times 

 monkeys entered and left their sleeping 

 trees. The titis were irregular. Between 

 October and May, when it was warm and 

 fruit was abundant, they would get up at 

 about dawn, but when the weather grew 

 colder, they would stay in their roost until 

 noon. This flexibility fit in with my theory 

 that while escaping predators was crucial 

 for these diurnal monkeys, it was how they 

 foraged — quietly, low down — that was 

 important; when they foraged was not. 



The owl monkeys couldn't have been 

 more different. They regularly left the 

 sleeping tree a few minutes after sunset 

 (after hawks and eagles would have gone 



to their roosts) and returned to it a few 

 minutes before the sun rose (and diurnal 

 birds of prey awoke). This precision, too, 

 fit my theory, with the monkeys behav- 

 ing — I fancied — as if they were afraid that 

 if they got up too early or stayed out too 

 long, they might wind up as a meal for 

 some hawk or eagle. 



But what about nocturnal predators? 

 Owls were of no concern, as my ornithol- 

 ogist colleagues explained to me, for large 

 species, such as the great homed owl, are 

 scarce in tropical forests of South Amer- 

 ica, and none of the other owls in the 

 Amazon rain forest were big enough to eat 

 a squirrel-sized monkey. Other noctumal 

 predators, such as cats and snakes, were 

 primarily terrestrial and no match for an 

 agile monkey in the trees. 



Foraging at night may do more for owl 

 monkeys than reduce the risk of being 

 killed by a predator. Different monkey 

 species compete strongly for fruit trees, 

 particularly in the season of fmit scarcity. 

 My data showed that spider monkeys, ca- 

 puchins, and even squirrel monkeys — all 

 species that are either bigger than the titis 

 or travel in larger groups — are able to 

 chase the titis away from large fig trees. 

 This forced the titis to subsist at this time 

 of year almost exclusively on leaves, 

 which are difficult to digest. The owl mon- 

 keys, in contrast, fed in the large fig trees 

 without harassment. Their only noctumal 

 competitors were opossums and kinka- 

 jous. I once observed an owl monkey ap- 

 proach an opossum feeding in a tree; to es- 

 cape, the small opossum dropped sixty 



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