Men with bushy beards and long hair may have smaller testicles than their counterparts who struggle to grow facial hair, suggests a new study.
This bold claim was made by a team from the University of Western Australia and the University of Zurich, who have studied the gonads of more than 100 different male primates as part of their research.
According to the scientists, male primates all fall victim to an evolutionary problem with their genitals.
Essentially the study found that males can either be "well endowed" and have large balls, or are instead "well-adorned" with hair.
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Researchers concluded that it's "hard to be both" of these things as it takes too much energy to do.
And while they may have been focusing on animals, many are saying the results can also be applied to humans, given that we too are primates.
Primatologist and study co-author Dr Cyril Grueter explained that they chose to study primates as their testicle size can vary enormously - with the smallest being the size of peppercorns and the largest comparable to a tennis ball.
The size, he says, is a factor linked to male appearance and the primate's desire to father offspring.
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He said: "But not all of them can have what they want. So how do they succeed? Well, they can produce so-called 'badges of status', showy ornaments that help their bearers control access to females by intimidating other males.
"And if males cannot keep others off their females, they can win by producing a lot of sperm to swamp those from their rivals.
He continued: "We found the same thing with ornamentation - some species sport flamboyant accoutrements such as beards, manes, capes and cheek flanges, and various shades of colour in their faces and fur.
"Others are pretty drab and look more like your Mr Average.
"This finding clearly shows that you can be well-adorned or well-endowed, but it's hard to be both."
