When it comes to attracting a mate, it seems size really does matter.
A new study of proboscis monkeys in Malaysia, published in the journal Science Advances, has found that males with the biggest noses—and other exaggerated male features—have the most number of females in their harems and were the most dominant animals in their groups.
Proboscis monkeys are notable for their unusually shaped noses, but the evolutionary importance of this strange body feature is only now becoming clear.
"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,” Sen Nathan, one of the scientists from the Sabah Wildlife Department and Ph.D. student at Cardiff University and Danau Girang Field Centre, said in a statement.
In the study, the researchers suggest that competition between males and females choice have influenced the evolution of larger noses in the male monkeys. Males with larger noses tend to be favored by females, meaning that the trait spreads as it is passed down through the generations.
The team collected data from free-ranging proboscis monkeys in the Lower Kinabatangan Wildlife Sanctuary, located in Sabah—a state occupying the northern part of the island of Borneo. The scientists made behavioral observations, took body measurements and also recorded the vocalizations of proboscis monkeys at three different zoos in Japan, Singapore and Sabah.
The researchers then examined the correlations between body mass, facial characteristics, testicular volume and the number of females a male had in their harem.
"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."
The proboscis monkey is one of the largest native Asian monkey species and is endemic to Borneo.