Individual vocal identity is enhanced by the enlarged external nose in male proboscis monkeys (Nasalis larvatus)

Adult male proboscis monkeys, Nasalis larvatus, develop an enlarged external nose. Males often produce loud, long-distance calls filtered through the nasal passage. The enlarged nose likely functions as a visual badge of social status and a visual key representing the owner’s physical and sexual quality and thus is useful for females in selecting mates. In addition to such visual signaling, a larger external nose enhances the lower frequencies in calls, possibly exaggerating acoustic signals related to body size. Here, we used computational simulations with three-dimensional models of the nasal passage to show how the external nose modifies the acoustic property, indicating that the external nose develops to enhance lower frequencies in adults but varies in a specific formant position among adult males. This finding suggests that the external nose generates acoustic signals about physical–sexual maturity in adult males and individual identity among them. The unusual features of the social organisation in this species, a patrilineality of a multilevel community consisting of one-male-multi-female units, may reinforce the functional importance of individual male recognition for males and females to monitor the location of both their own units and those of other males.