New research suggests that the fleshy, pendulous noses of male proboscis monkeys – the biggest of any primate – are distinguishing features in more ways than one.
These pot-bellied, cud-chewing monkeys are found only on the island of Borneo, where they inhabit riverside rainforest.
The remarkable nose is particularly pronounced in mature males, and previous research has shown that it serves as a visual signal of dominance that attracts females and deters rival males without the need for physical aggression. It also amplifies the long-distance ‘bray’ calls that males emit through their nose while keeping their mouth closed.
But biologists in Japan have now found evidence of another function for this formidable facial feature. As described in Journal of the Royal Society Interface, they used a medical CAT scanner to map the internal structure of the nasal cavities of different males and then printed detailed 3-D models to analyse how sounds are shaped as they pass through them. This showed that, by amplifying and dampening different frequencies of sound, each nose emits a distinct vocal signature.
“The idea the proboscis itself serves to signal or amplify individual signatures is interesting. Normally such morphological adaptations are explained through their role in mate attraction or signalling body size rather than ID, so this is an interesting development,” says Simon Townsend, who works on vocal communication in animals at the University of Zürich, Switzerland, but was not involved in this study.
The team behind the research suggest that individualised calls make sense in terms of the species’ natural history.
Proboscis monkeys form core groups consisting of a single male and multiple females, but several groups can come together in looser associations, called bands, which can also include bachelor males. The result is intense competition between males for access to females, and band-members might be expected to benefit from keeping track of who’s who.
Townsend adds that vocal signatures might be especially helpful in the monkeys’ rainforest habitat, where the dense foliage is likely to obscure visual cues.
That said, how other members of the group use these individual calls when they hear them is not yet known. Takeshi Nishimura of the University of Osaka, told BBC Wildlife that his team is now working out how to investigate this by playing back recordings of individual calls to other monkeys. “However, we have not yet come up with a good way to test it. What behaviour would indicate that they use such calls? This is a difficult task.”
Top image: proboscis monkey Credit: Getty
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