2023-05-23 13:03:58
A different ideal of beauty may apply to humans, but to the proboscis monkey (Nasalis larvatus) of Borneo, it’s a fait accompli: the bigger the nose, the smarter the monkey. Males of this species with a large nose have the best chance of going through life as a casanova.
Large nose (monkey), large scrotum
The large nose is not only an irresistible external characteristic, but has also caused evolutionary differences: male proboscis monkeys with a large nose are more robustly built than small-nosed congeners. Also, their testes tend to be larger, making them more likely to reproduce.
A larger nose has also resulted in a different sounding courtship call. The larger the nose of a proboscis monkey, the more likely it is to have a large harem. Males with a more refined appearance, on the other hand, are more likely to live in bachelor groups. The results, which were published in the magazine Science Advancespartly explain what the proboscis monkey owes to its striking appearance.
Lead researcher Ikki Matsuda, a primatologist at Japan’s Chubu University, says the study is based on a wide variety of data, including body measurements, audio recordings of courtship calls and videos of social behaviors. The findings offer an extraordinary glimpse into the life of a primate whose population has been down since the 1970s halvedmainly due to habitat loss.
The function of eye-catching appearance
In many animal species, the appearance of males and females differs, and primates are no exception. So do male mandrill a bright red or bright blue face, while male gelada’s can show off a bright red chest. These spectacular features are a status symbol that alerts other males or attracts females – or a combination of both.
For a long time, researchers thought that the nose of a proboscis monkey played a similar role, but the evidence for this assumption was lacking. From 2000 to 2005, Matsuda and his colleagues observed proboscis monkeys in the wild along Borneo’s Kinabatangan River. Whenever Malaysian game wardens caught a male proboscis monkey and fitted it with a collar with a GPS transmitter, the team was given the opportunity to weigh the animal and measure the size of its testes.
Why the big nose?
In 2014, Matsuda and his colleagues started a follow-up study. They studied the courtship calls of male proboscis monkeys in captivity. This showed that the frequency of the courtship call changed due to a larger nose. The consequence? Females could judge a male’s appearance without seeing him.
However, not all questions have been answered yet. Because how did that big nose come about? Alan Dixson, a biologist at the Victoria University of Wellington in New Zealand and a peer reviewer associated with the follow-up study, says that it is not yet clear how evolution led to a large nose in proboscis monkeys. The nose may have served as a status symbol, or as a billboard to impress females.
Proboscis monkeys as late bloomers
According to Dixson, it’s also possible that the noses of young proboscis monkeys don’t develop until later in life, which makes it more difficult to draw conclusions. Mandrill males that turn blue at an early age, for example, develop their striking facial color later, as a result of lower testosterone levels. A similar mechanism may also apply to proboscis monkeys.
‘It is possible that the growth of the nose of proboscis monkey males is inhibited in cliques of bachelors. And they may only adjust their call when they have a better chance of committing to a female,’ he says. Although scientists are still guessing at answers, Dixson praises the research. The first results are fascinating, and that’s something no one should turn their noses at.
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