Orangutan Night-Time Long Call Behavior: Sleep Quality Costs Associated with Vocalizations in Captive Pongo

Figure 2

The occurrence of adult male long calls was recorded at all hours from 17:00–09:00. The circadian distribution of calls revealed a bimodal pattern. Bimodal long call frequency is characteristic of orangutan male vocalizations in the wild but is in this instance of captivity expressed in a temporally unique manner.