Temporal patterns of chimpanzee loud calls in the Issa Valley, Tanzania: Evidence of nocturnal acoustic behavior in wild chimpanzees.

Chimpanzees were acoustically active throughout the 24-hr cycle, although at low rates compared to diurnal activity, revealing night-time activity in an ape otherwise described as diurnal. Chimpanzee loud calls partially, and weakly, conformed to the acoustic adaptation hypothesis and likely responded to social, rather than environmental factors. Call rates accurately reflect grouping patterns and PAM is demonstrated to be an effective means of remotely assessing activity, especially at times and from places that are difficult to access for researchers.