The Proboscis Monkey has its distinctively large bulbous nose, which is thought to be an adaptation to sexual selection and to help the males compete for mates. The large nose, which can become red and swollen when the monkey is aroused, serves as an indication of its health and size. In addition, the long and wide nose of the Proboscis Monkey helps to amplify its calls, helping it to attract mates from far distances.
Ultimately, the Proboscis Monkey developed its long nose to attract mates and make itself more attractive.