Figure 1. Orang-utan call spectrograms.

Spectrograms of orang-utan calls: a) ‘nest smacks’; b) ‘raspberries’; c) ‘harmonic uuhs’; d) ‘throat scrapes’. The nest smack and raspberry are produced by orangutans during nest building. The harmonic uuh and throatscrape are produced by mothers towards infants that are separated from them and functions as a ‘come-hither’ call because infants return to the mother after these calls.