Ecological and Behavioural Factors Influencing Territorial Call Rates for the Bolivian Titi Monkeys, Plecturocebus modestus and Plecturocebus olallae.

Primate territorial calls have been used to locate groups in censuses. Daily variations in call rates are a potential source of error in these studies. To obtain more accurate estimations of population density it is necessary to determine how much variation there is in group call rates and to identify the factors that influence them. We present data on the emission of territorial calls by two threatened and endemic titi monkeys in Bolivia: Plecturocebus olallae and P. modestus. We found interspecific differences in daily call rates (52% P. modestus and 33% P. olallae). Groups inhabiting more continuous forests vocalized more frequently than groups in fragmented forests, which might be linked to the higher abundance of groups in less fragmented forests. We found seasonal differences in call rates between species, with more frequent calling in P. modestus during the dry season, while P. olallae called more frequently in the rainy season. The study groups emitted territorial calls mostly from the central zones of their territories, suggesting they do not face intense spatial competition with neighbouring groups. Our results improve the general ecological knowledge on P. olallae and P. modestus, and can also be used to improve population abundance studies and ongoing conservation efforts.