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.