. The Ecology of arboreal folivores : a symposium held at the Conservation and Research Center, National Zoological Park, Smithsonian Institution, May 29-31, 1975 . LEGEND Male orang utans Female orang utans Gibbons Afa-Afd Gl - GIO Presbytis aygula Macaca fasicularis PI -P7 M Macaca nemestrina N Ridge top ^ Figure 4. Estimated centers of group ranges. The centers of ranges of individual adult orang utans are given. It is likely that the 2 centers of ranges of M. nemestrina belong to the same group, which circled the southern end of the study area. The text describes the method of estima
. The Ecology of arboreal folivores : a symposium held at the Conservation and Research Center, National Zoological Park, Smithsonian Institution, May 29-31, 1975 . LEGEND Male orang utans Female orang utans Gibbons Afa-Afd Gl - GIO Presbytis aygula Macaca fasicularis PI -P7 M Macaca nemestrina N Ridge top ^ Figure 4. Estimated centers of group ranges. The centers of ranges of individual adult orang utans are given. It is likely that the 2 centers of ranges of M. nemestrina belong to the same group, which circled the southern end of the study area. The text describes the method of estimation of centers for gibbons and P. aygula. or entirely within the study area. The centers of ranges of all primate groups (in- cluding orang utans) are plotted in Figure 4. There were 25 groups of primates in the study area at the time of this study. Group size. The frequencies of group counts in first contacts with each species are presented in Fig- ure 5, but the relationship between actual group size and numbers counted at first contact is not simple. Factors such as individual size, coloration, and gen- eral activity level of a species influence "average" visibility of members of the species, and visibility must influence the relationship between numbers counted and numbers actually present. It is probably safe to say that in the forest the discrepancy between number seen and actual number present will increase with increasing group size, and that the mean number of animals counted at first contact will be lower than the actual mean size of groups observed when there are several groups of a species present. The situation is somewhat simpler if only a single group is present since, if counts are conservative and careful, the group size must be very close to the highest number of animals ever counted at 1 contact. Keeping these indefinite "criteria" in mind, it is possible to estimate the group size or mean group size of each species from data presented in
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