Sexual selection and adult male orangutan long calls

Long calls given by adult male orangutans (Pongo pygmaeus) have been hypothesized to mediate interindividual spacing among male orangutans and to attract female orangutans, to high-ranking mates over long distances. To test these two hypotheses I conducted observations of calling behaviour and a series of experimental playbacks of recorded long calls. The findings were consistent with the male-spacing hypothesis but did not support the mate-attraction hypothesis. Experimental playbacks demonstrate that long calls regulate spacing between males through an approach-avoidance system based on dominance relationships: the highest-ranking adult male approaches calls, while lower-ranking, males avoid calls. The results of playbacks also reveal that sexually active females do not move toward long calls, and therefore do not indicate that long calls function in attracting mates. This latter finding suggests that female mate choice may not have been an important selective factor during the evolution of long calls.