March 11, 2018

Why the Long Nose on Proboscis Monkeys? It’s about Showing Off!

Why the Long Nose on Proboscis Monkeys? It’s about Showing Off!

Male proboscis monkeys have prominent schnozes for a good reason. Photo Credit Wikimedia Commons

Proboscis monkeys are most famous, just as their name suggests, for their bulbous noses. The critically endangered primates are endemic to Borneo, where locals have long marveled at their protuberant facial appendages, which can grow to as much as 14cm in some of the males and which make the monkey resemble endearing comic book characters.

But why their elongated noses? Scientists have posited that the monkeys’ sizeable protuberances serve to amplify their warning calls. Their swollen snouts contain a resonating chamber wherein sounds get amplified.

A proboscis monkey munches on leaves in a Malaysian mangrove forest. Photo Credit: Bernard Dupont via Flickr

Yet the monkeys’ noses, says a group of scientists, help the primates, or at least prominent males, in another way as well: by attracting females. After conducting research on proboscis monkeys in the wild in the Lower Kinabatangan Wildlife Sanctuary and at several zoos, scientists from Cardiff University, the Danau Girang Field Centre, Kyoto University and the Sabah Wildlife Department have reached the conclusion that the larger male monkeys’ snouts tend to be, the more females they tend to attract.

“Although the unique nature of the ‘odd-nosed’ proboscis monkey has long been admired as an extremely attractive visual feature by biologists, explanations for its evolution have so far been gleaned more from folklore than from science,” explains Dr. Sen Nathan, assistant director of the Sabah Wildlife Department. “We show evidence supporting both male-male competition and female choice as causal factors in the evolution of enlarged male noses.”

He elucidates: “We also observed that nasal enlargement modified the resonance properties of male vocalisations, which probably encode male quality. Our results therefore indicate that the audiovisual contributions of enlarged male noses serve as advertisements to females in their mate selection.”

Perhaps this should not come as a surprise. Certain trademark physical features in the males of myriad species, from the tails of peacocks to the manes of lions, have long been known to be prominent factors in sexual selection whereby females within a given species show marked preference for males with certain prominent visual characteristics.

Yet understanding the exact role that the snouts of proboscis monkeys (Nasalis larvatus) play isn’t entirely an academic pursuit. “Every piece of information that allows us to better understand the behaviour of these charismatic animals is important,” stresses Dr. Benoit Goossens, director of the Danau Girang Field Centre. “Now our tour guides will be able to tell their guests that size matters, and that males with larger noses attract more females in their harem.”

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