Male monkeys with large noses have more females in their harems - proving that size does matter, according to a team of scientists.
Researchers studying proboscis monkeys in Malaysia found that males with exaggerated masculine traits, such as big noses, "serve as advertisements to females in mate selection".
Those involved in the study from Cardiff University, Kyoto University and Sabah Wildlife Department and the Danau Girang Field Centre in Malaysia, say the new evidence points to both male-male competition and female choice as contributing to the evolution of enlarged male noses.
The primate, which is native to Borneo, sports a sizeable snout, which Dr Sen Nathan, assistant director of Sabah Wildlife Department and PhD student at Cardiff University and Danau Girang Field Centre, said had long been admired as an "extremely attractive" visual feature by biologists.
But he said explanations for its evolution had so far been gleaned more from folklore than from science.
Dr Ikki Matsuda, from Chubu University and Kyoto University in Japan, said the team tested the correlations between body mass, facial characteristics, testicular volume, vocalizations, and number of harem females in captive and free-ranging proboscis monkeys.
She said: "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.
"This suggests that nose enlargement is a reliable predictor of social dominance and high sperm count."
The researchers observed and took morphological measurements from free-ranging proboscis monkeys in the Lower Kinabatangan Wildlife Sanctuary and recorded the vocalisations of male and female proboscis monkeys at three different zoos; Yokohama Zoo in Japan, Singapore Zoo and Low Kawi Zoo in Borneo.
Dr Benoit Goossens, director of Danau Girang Field Centre and reader at Cardiff University, added: "The proboscis monkey is endemic to Borneo and is a totally protected species in Sabah.
"Every piece of information that allows us to better understand the behaviour of these charismatic animals is important.
"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."
Sabah Wildlife Department and Danau Girang Field Centre are currently preparing a 10-year State Action Plan aiming at helping conservation efforts for the proboscis monkey.
- The study Nasalization by Nasalis larvatus: larger noses audiovisually advertise conspecifics in proboscis monkeys, which was partially supported by the Sime Darby Foundation, is published in Science Advances.