Evolution appears to have presented male primates with an unfortunate trade-off – they can either have a showy, more masculine appearance or large testicles.

But, rarely can they have both.

A study comparing the gonad sizes of more than 100 primates suggests the energy required for these features poses an evolutionary dilemma, allowing males to be either 'well-adorned or well-endowed.'

Teams from the University of Western Australia say animals can either invest energy attracting females with impressive manes and beards or spend their energy growing larger gonads.

While the implications for humans aren't yet clear, 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.

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The smallest testicles were found to be the size of peppercorns while the largest were comparative to that of a tennis ball in diameter (file photo)

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 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.

'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.

Researchers explain that poorly endowed males commit all their energy into attracting mates using beards and long hair and then have none left to invest in their genitalia (file photo)

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.'