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If you’re reading this then you must be a primate, and that means there’s only one thing on your mind. But whether you’re a capuchin looking for copulation or a man in need of a mate, you’re going to have to be pretty sexy if you want to get noticed – and that comes at a price: according to a new study published in the Proceedings of the Royal Society B, primates with more attractive features tend to have smaller testicles.

While it might sound counter-intuitive, this moderation of masculinity actually provides an advantage in the reproductive stakes, as it allows for more genetic and energetic investment in ornamental features. In other words, there has to be a trade-off between beauty and bulge, so if you want to look good then the goolies have to go.

High sexual competition has led to the development of an array of “sexual ornaments” among male primates. These include the facial flanges of orangutans, the exaggerated noses of proboscis monkeys, and even the beards of men – all of which are designed to attract females.

This male orangutan has traded big balls for a handsome face. Svetlana Orusova/Shutterstock

 

The study authors searched for a relationship between these seductive embellishments and the size of the testes across 103 different primate species – including humans – and discovered that specimens with more impressive ornaments tended to have contracted cojones.

By way of explanation, the researchers suggest that individuals who are able to out-compete their rivals due to their physical attractiveness don’t need to invest as heavily in the production of sperm. On the other hand, ugliaer primates are likely to get fewer opportunities to mate, which means they have to be ready to release a large of number of sperm in order to increase the chances of fertilization. As a result, they have to invest more in the development of large testicles.

Yet good looks are not the only requisite for securing mating rights, and brawn is often equally necessary when it comes to competing for females. Individuals with larger weaponry, such as canine teeth, have a better chance of sex, so logic dictates that a similar trade-off should be present between canines and testes.

However, the researchers discovered the opposite to be true, as specimens with the largest canines also tended to have the biggest balls. While they have no definitive explanation for this, the authors suggest that the development of large weaponry requires much less genetic or energetic investment than the maintenance of attractive features, which is why big teeth needn’t cost a testicle.

In a statement, study author Stefan Lüpold from the University of Zurich explained that “ornament elaboration comes at the expense of testicle size and sperm production. In a nutshell, the showiest males have the smallest testes.”

Similarly, human males with the most expensive cars also tend to be the least well-endowed.

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