The male proboscis monkey’s large nose may help it attract a mate
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The bizarre enlarged nose of male proboscis monkeys acts like a trumpet, enabling them to make extremely loud sounds that attract females and intimidate rivals.
Until now, researchers have speculated that it must play a role in mating and dominance, but evidence for the exact purpose and how it functions has been elusive.
“No other monkeys have a big nose like that,” says Katharine Balolia from the Australian National University in Canberra. “That nose has become a signal of health and dominance. Females find it attractive and males threatening.”
To find out more, Balolia and her colleagues conducted a detailed analysis of the cranial anatomy of proboscis monkeys (Nasalis larvatus) and compared their nasal structure with three other old-world species: blue monkeys, king colobus monkeys and crab-eating macaques.
They found that the nasal aperture in male proboscis monkeys is 29 per cent larger than in their female counterparts. For the other three species the difference between sexes was between 7 and 15 per cent.
Male proboscis monkeys also had a 26 per cent larger nasal cavity than females, compared with a 7 to 17 per cent difference in the other three primates. Bigger differences in certain bodily features between the sexes can indicate that sexual selection has been at work.
Crucially, this nasal anatomy only reaches its full size once males become sexually mature. Before this, young males are in bachelor groups, and only once their noses are fully grown do they establish groups with multiple females, providing evidence that this body part is crucial to successful mating.
Balolia says the monkeys are also famous for their extremely loud calls, which they make with their nose rather than their mouth. “The shape of the nasal cavity allows it to work like an echo chamber,” she says. “They use the nasal cavity to build up the sound and resonance and then straighten the soft tissue and use their nose like a trumpet.”
The team suspects such distinctive noses may have evolved because of the dense forest the monkeys live in, which meant being able to make loud calls was important.
Females might have started to select mates with large noses because it was an indication of their health and dominance, says Balolia, and this would have driven the evolution of even larger noses.
“Maybe it’s like a peacock’s tail – it got bigger and bigger and bigger until it’s a ridiculously big size and eventually it gets so big it can’t get any bigger.”
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