Australasian Science: Australia's authority on science since 1938

Showy Males Have Smaller Testicles

If you’ve got it, flaunt it, but a study published in Proceedings of the Royal Society B ( has found that male primates that have all the attributes they need to be attractive to females are compensating for diminished assets elsewhere.

Study co-author Dr Cyril Grueter of The University of Western Australia said male primates typically live in highly competitive environments where they all want to father offspring. “But not all of them can have what they want,” 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.”

The study investigated the relationships between different indicators of male virility, focusing on primates because of their tremendous variation in both testicular size and male ornamentation.

Grueter said some primates had testicles almost the size of tennis balls, while others were barely larger than a peppercorn. “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,” he said. “Others are pretty drab and look more like your Mr Average.”

The research group compiled data for more than 100 primate species, including humans, and demonstrated for the first time that ornaments come at the expense of testicle size and sperm production. Simply put, the showiest males have the smallest testes – at least in primates.

“This finding clearly shows that you can be well-adorned or well-endowed, but it’s hard to be both,” Grueter said. One of the reasons, the researchers suspect, is that trying to do both takes too much energy.