The research carried out by teams from the University of Western Australia say a male “can be well-adorned or well-endowed, but it’s hard to be both”.
This means that men who have bushy beard and long hair are likely to have smaller testicles that those that are clean-shaven.
The study also say that animals can either invest energy attracting females by growing out their hair and beards or spend their energy growing larger testis.
Primatologist and study co-author Dr Cyril Grueter, from the University of Western Australia, said testicle size is linked to a male’s appearance and varies enormously.
“Some [primate] 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.”