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Researchers from the University of Western Australia and the University of Zurich have found that male primates can either be ‘well adorned or well-endowed – but not both’.

The study, published in the Proceedings of the Royal Society B by evolutionary biologists, looked at primates and found that the more ‘showy’ they were, the smaller their testicles were likely to be.

A press release from the University of Zurich said: “To maximize their chances of passing on their genes, males of many primate species invest heavily in various sexual traits, such as a large body size, or long canines that can serve as weapons in direct contests over mates. What's more, showy sexual ornaments such as manes, beards, fleshy swellings, and colourful skin patches can help them intimidate rivals and woo females.

“And if males can't keep other males off their females, they will try to outcompete them at the level of sperm. By swamping the sperm of others, they can increase their chances of fertilization. But producing a lot of sperm requires large testicles.”

These findings were determined by comparing the sexual traits of over 100 primate species, including humans.

Stefan Lüpold, an evolutionary biologist at the University of Zurich, said in a statement: “Ornament elaboration comes at the expense of testicle size and sperm production. In a nutshell, the showiest males have the smallest testes."

This is the first study to examine all male primate sexual traits simultaneously and the researchers say that the reason for males either being well-adorned or well-endowed is that it takes too much energy to be ‘showy’ and have larger testicles at the same time.

As Lüpold stated: "It's hard to have it all.”