The most conspicuous evolutionary characteristic of the proboscis monkey is, of course, the enormous nose of adult males.
Why do proboscis monkeys have such big noses
The journal Science Advances suggests that the prominent adornment may have developed through the proboscis monkeys’ sexually competitive harem group social system, as researchers have found a clear link between nose size and the number of harem females. Evidence supports male-male competition and female choice as factors in the evolution of large male noses.
According to the 2018 study, “The relative size of an enlarged nose would act as an advertisement signal for conspecific male rivals and an informative visual signal for the recognition of male qualities, such as physical strength (body mass) and reproductive ability (testis size), by females.”
The species’ outsized organs also create an echo chamber to amplify the monkey’s call – useful for intimidating rivals and impressing females.
For this species, vocalisations transmit sexual signals more effectively than visual cues in a dense rainforest with poor visibility, and can be carried over 100– 200m, depending on habitat conditions. They may also be used to keep the harem close to the core male and protect offspring – the most vulnerable members of the group.
“Their strong communication through vocalisations could also be a factor explaining their peaceful social system,” explains Professor Benoît Goossens, director of the Danau Girang Field Centre, a collaborative research and training facility managed by Sabah Wildlife Department and Cardiff University. “Pacific challenges are made via their strong vocalisations.”
Flexible group organisation in the multilevel society (a distinct type of primate social system, the key features of which are single-male, multi-female core units within larger social bands) at their sleeping trees also demonstrates how harmony is achieved.
“Because the sleeping sites of harem groups are often closely located within 50m, with occasional occurrences of sharing the same tree (we have also observed bachelor groups located very close to harem groups), showing their badge of social status may lessen male-male physical antagonistic interactions, thereby avoiding seriously wounding each other,” Benoît says.
Among primates, larger noses with prominent sexual dimorphism are only seen in this species: females are about 60cm long, weigh 12kg and have smaller noses compared to males that can reach up to 75cm long and weigh about 24kg.