The sac fills with air from the airway and is used in two distinct call types: a low, hollow boom produced with the mouth closed, and a sharper bark produced with it open. Pairs combine these into a structured duet, with the female's contribution built around a long call and the male's around a series of shorter elements, assembled in a repeated sequence.
Duets are usually given in the morning and serve territorial advertisement. The sound carries over two kilometres in hill forest, which for an animal weighing ten to fourteen kilograms is a large acoustic footprint. Neighbouring groups often answer, producing a chain of replies across a valley.
What the duet signals beyond territory is debated. Pair-bond strength, mate defence and joint resource defence have all been proposed, and the hypotheses are difficult to separate because the same song serves several audiences at once.