Association between olive colobus (Procolobus verus), Diana guenons (Cercopithecus diana), and other forest monkeys in Sierra Leone

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Abstract
Close association between olive colobus (Procolous verus) and other monkeys (especially Cercopithecus species) has been observed throughout the range of P. verus in the forest zone of West Africa. To investigate the basis of this association, we made new observations in Sierra Leone, concentrated at Tiwai Island. We obtained data on the association patterns of monkeys over a large area of the island from line-transect samples, and studied association behavior via long-term observational sampling of habituated groups of olive colobus and Diana monkeys (C. diana). During transect sampling, olive colobus always were seen less than 50 m from monkeys of other species, especially Cercopithecus. In studies of habituated groups, we found that one groups of olive colobus associated closely with a larger groups of Diana monkeys for more than 3 years. Members of the two groups were within 50 m of each other on over 80% of scan samples; the two groups shared the same range, but foraged in different parts of the canopy and had little dietary overlap; association was maintained by the behavior of the colobus. The olive colobus was the only Tiwai monkey species seen less than 50 m from members of a secon Diana study group more frequently than expected, although in this case the two species were associated during less than 12% of samples. In both cases, we detected month-to-month variation in association frequency. We suggest that olive colobus has a strong tendency to associate with other monkeys as part of an evolved strategy; that observed association patterns depend on the ranging habits and group dispersion patterns of the other species in the area; and that this strategy evolved because it reduces predation risk for a small-bodied monkey that forages in small, dispersed groups.