Researchers have found evidence of the origin behind the large, prominent nose of the proboscis monkey. Scientists looked into the size of the proboscis monkey's nose, along with other factors, to determine its effect on their mating habits.
The study concluded that proboscis monkeys do better when they have a larger nose.
Researchers from an international team published findings in Science Advances showing that sexual competition was the reason for the evolution of the large nose of the proboscis monkey. This type of sexual dimorphism -- difference in size or appearances between sexes of an animal -- is unique to the proboscis monkey.
In the 18-year study, scientists were able to determine that the larger nose came from the male monkey's call. A larger nose is able to modify the monkey's call to make it lower when attracting a female.
"In animals, males often have exaggerated physical traits as a consequence of sexual selection. But you can't get any odder than the long, drooping nose of the proboscis monkey," said Hiroki Koda, lead author of the study. "And although this unique feature has long been admired, explanations for its evolution and its ecological roles had remained unclear."
Size of the nose also determined other features common in those proboscis monkeys with big noses.
Other Attractive Characteristics
Besides just having a larger nose than the other monkeys, proboscis monkeys with larger noses also tended to have larger harems of females, more physical strength, and larger testis which indicated better reproductive ability.
"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, another researcher on the study. "This suggests that nose enlargement is a reliable predictor of social dominance and high sperm count."
Having the larger nose lets the monkeys be able to broadcast the fact that they have the other physical traits.
"We found significant correlations among nose, body, and testis sizes and a clear link between nose size and number of harem females," said the researchers in the abstract of the study. "Therefore, there is evidence supporting both male-male competition and female choice as causal factors in the evolution of enlarged male noses."
Researchers found that those monkeys with smaller noses would live together in bachelor group rather than the harems monkeys with large noses live with.