Passive acoustic monitoring for primate population assessments: A case study on the endangered eastern chimpanzee (Pan troglodytes schweinfurthii)

Understanding how habitat characteristics influence primate behavior and distribution is critical for effective conservation planning, particularly in regions undergoing rapid environmental change. Moreover, utilizing the most efficient and accurate monitoring methods are essential for effective conservation strategies. This study investigates the spatiotemporal vocalization patterns and density estimates of the endangered eastern chimpanzee (Pan troglodytes schweinfurthii) across two locations within the Greater Mahale Ecosystem (GME) of western Tanzania, using passive acoustic monitoring (PAM) as an alternative to traditional line transect surveys. The GME constitutes the eastern-most portion of the chimpanzee range and contains a mosaic of closed-canopy forests and open-canopy miombo woodland vegetation. Overall, this research aims to assess how vegetation type influences the spatiotemporal calling patterns and population densities of chimpanzees. We selected two locations within the GME with differing forest cover: the Issa valley (more open woodland) and the Ntakata forest (more forest). We deployed 16 acoustic recording units from May-August 2023, collecting over 8,000 hours of acoustic data. Using an automated acoustic classifier developed with 85% precision, we detected and manually verified over 1,000 chimpanzee calls. We converted these detections into call rates (calls/hour) and density (individuals/km²). Our results showed a relationship between forest vegetation and chimpanzee vocal behavior. Call rates were highest in areas with greater forest cover, with significant temporal differences in vocal activity across locations. Sites containing the highest percentage of forest cover in Ntakata exhibited dusk call rate peaks and greater overall acoustic activity, while Issa showed a dawn peak and overall lower call rates. Density estimates also positively correlated with forest cover. Call rate patterns likely reflect both chimpanzee density and environmental factors such as visibility and acoustic competition. Furthermore, Ntakata showed greater acoustic activity variation, indicating higher biodiversity. Overall, this study highlights the utility of PAM and soundscape analytics as a powerful, non-invasive approach for primate population assessments and provides new insights into the behavior and ecology of the endangered eastern chimpanzee.