Proboscis monkey
Proboscis monkey in a sanctuary in Malaysia's Sabah state, Borneo Alex Ogle/AFP/Getty Images

Scientists have been intrigued by the unusually large and floppy nose of a male proboscis monkey for years, but have now finally understood the exact purpose of the weird facial feature – to attract mates.

In a recent study, published in Science Advances, researchers took a close look at the behaviour and body measurements of free-ranging proboscis monkeys found in the Lower Kinabatangan Wildlife Sanctuary, Sabah, and recorded vocalisations of another group from three different Asian zoos.

They analysed the correlation between body mass, facial features, testicular volume, and the number of females in each proboscis male monkey's harem, according to

The findings revealed that monkeys with the biggest noses, going over and below their mouths in some cases, and other exaggerated physical features had most females in their harem.

The research team also found that an enlarged nose also affected vocalisation of male proboscis monkeys, ultimately helping females to seek out their mating calls.

"In addition to finding that enlarged male noses serve as advertisements to females in mate selection, we also found that males with larger noses also tended to have larger body mass and testis," said Ikki Matsuda, from Chubu University and Kyoto University in Japan. "This suggests that nose enlargement is a reliable predictor of social dominance and high sperm count."

This is the first study to suggest that male-to-male competition and female mating choices may have influenced the evolution of proboscis monkeys' nose sometime in the distant past. Until now, most of the explanations for the unique feature revolved more around folklore than science.