Men with long hair and beards may have smaller testicles than those who struggle to grow facial hair, a study claims.
It found that humans, and more than a hundred other primates, face an evolutionary dilemma when it comes to their genitals.
Teams from the University of Western Australia, who carried out the research, say a male 'can be well-adorned or well-endowed, but it's hard to be both'.
Animals can either invest energy attracting females with impressive manes and beards or spend their energy growing larger gonads.
This means hairy human men with bushy beard and long hair are likely to have smaller testes than their follicly challenged brethren.
For the rest of the world's primates, this trade-off comes in the shape of having either the bright buttocks of a baboon or the plain features and large testes of plainer primates.
Primatologist and study co-author Dr Cyril Grueter, from the University of Western Australia, said testicle size varies enormously and is linked to a male's appearance.
'Some [primate] species sport flamboyant accouterments such as beards, manes, capes and cheek flanges, and various shades of colour in their faces and fur,' he said.
'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.'
The finding comes from a study that looked at testicle size of more than 100 primate species and found gonads vary dramatically depending on the animals.
Animals with the smallest testicles had gonads the size of peppercorns while the largest were comparable to that of a tennis ball.
Researchers explain that poorly endowed males commit all their energy into attracting mates using beards and long hair and have none left to invest in their genitalia.
Researchers chose to study primate testicle size as it varies so widely in different populations.
One of the reasons, the researchers suspect, is that trying to do both just takes too much energy.
He explains that all the males are striving for the same thing – to father offspring.
'But not all of them can have what they want,' Dr Grueter said. 'So how do they succeed?
'Well, next to simply fighting, 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.'
Source: Daily Mail Online