Proboscis monkey

The male proboscis monkey of Borneo grows a huge, drooping nose, and it works as an advertisement. A 2018 study of these monkeys found that males with bigger noses tended to be heavier, had larger testes and kept more females in their harems. The nose also changes his voice. It enhances his calls through resonance, and larger nasal passages seem to produce deeper calls, so females and rivals can hear the size of the nose as well as see it. Females have much smaller noses, which makes a mixed group look almost like two different species.