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collection sitting under this tree with my 

 binoculars and notebook. Just as dusk fell, 

 the first owl monkey — a male — emerged 

 from his secluded den in the center of the 

 tree and began to scratch himself. Seconds 

 later, three other owl monkeys appeared. 

 From their size, I presumed these three 

 were his mate, an adolescent, and a half- 

 sized juvenile. They spotted me immedi- 

 ately and began to give an alarm call, but 

 they didn't flee. After ten minutes, they 

 began to move on through the canopy. I 

 followed, but by this time it was dark. 

 They were moving quickly and soon dis- 

 appeared from my view. 



Dusk after dusk, I returned to the tree 

 and followed the group as far as I could. 

 Each night, I went a little farther I cut nar- 

 row trails under their arboreal pathways. I 

 listened carefully as group members ex- 



ff%.^**^. 



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changed contact calls. I was grateful that 

 they dashed carelessly through the trees, 

 making abundant noise as tihey jumped 

 from branch to branch. Still, several 

 months passed before I could follow them 

 all night long. 



During the day, I began tracking the 

 dusky titis. I had a different problem with 

 them. Although they moved much lower 

 in the trees than the owl monkeys (an av- 

 erage of thirty feet above the ground), they 

 were dark and blended into the foUage. 

 They were also cautious in their move- 

 ments, nearly impossible to hear as they 

 jumped from branch to branch, and they 

 often rested, hidden in tangles of vines. I 

 had hoped that the titis would be the easy 

 part of my fieldwork, but I was often frus- 

 trated during the first two months of my 

 effort to keep track of them. 



■x^-r^ 



.^ 



Eventually, however, I could follow 

 both day- and night-monkey groups. I 

 couldn't, of course, keep going twenty- 

 four hours a day, so I developed a routine. 

 First, I would spend five days with the owl 

 monkeys in Group One, following them 

 from dusk to dawn. Then I would switch 

 to five days with dusky titi Group One, 

 this time from dawn to dusk. After iJiat, I'd 

 move on to owl monkey Group Two for 

 five days and finish up with a round of five 

 days with titi Group Two. With such con- 

 stant disruption to my circadian rhythm, I 

 felt as if I had jet lag for the entire year. 



The work proceeded well, but since I 

 was not using radio collars, I was continu- 

 ally plagued with the problem of losing 

 track of the owl monkeys. One night, for 

 instance, they quietly left a large fig tree 

 without my detecting them. When I real- 



46 Natural History 5/94 



