Abstract
All primate species produce vocalizations for complex communication, conveying a wide range of information, regulating social interactions, coordinating activities, and serving as antipredator strategies. Together, these vocalizations form a species' vocal repertoire. However, despite their widespread presence and ecological significance, the vocal repertoires of less than 15% of primate taxa have been documented to date. The purpose of this study was to quantitatively characterize the vocal repertoire and behavior of a wild population of Ucayali bald or red uakaris ( Cacajao ucayalii ), a rare Amazonian platyrrhine primate. We analyzed and categorized 1,155 red uakari vocalizations using a combination of machine-learning-based random forest analyses, bioacoustic analysis, and field observations. We identified and described 12 acoustically distinct call types, two of which were not previously reported ( High-chick and Shriek ). The Hic was the most common vocalization, both in context and frequency of use. Some calls were specific to age, sex, or context, while others occurred across a range of situations. The vocal repertoire exhibited properties of both graded and discrete calls, depending on the call type and its apparent function. The uakaris’ tail plays a key role in the species’ acoustic-visual multimodal communication, as evidenced by the frequent combination of different call types and tail wagging, probably to draw attention from other group members and emphasize the transmission of information. The bioacoustic characterization of the Ucayali bald uakari's vocal repertoire provides a groundwork for potential acoustic monitoring of this species and the potential use of uakari monkeys as models for studying multimodal communication in primates.