Showy male primates have smaller testicles

Credit: CC0 Public Domain

Male primates equipped with all the bells and whistles to attract a female mate tend to have smaller gonads, according to a study by researchers at The University of Western Australia and University of Zurich.

Primatologist and study co-author Dr Cyril Grueter, from UWA's School of Human Sciences, said male typically lived in highly competitive environments where they all wanted 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."

The study, published today in the Proceedings of the Royal Society B, investigated the relationships between different indicators of male virility.

The research team, which included Professor Leigh Simmons, director of UWA's Centre for Evolutionary Biology and Dr Stefan Lüpold, from the University of Zurich, focused on primates because of their tremendous variation in both testicle size and male ornamentation.

Dr 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 international team compiled data for more than 100 including humans and demonstrated for the first time that ornaments come at the expense of testicle size and sperm production–or simply put, at least in primates, the showiest males have the smallest testes. 

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

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More information: Stefan Lüpold et al. Sexual ornaments but not weapons trade off against testes size in primates, Proceedings of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rspb.2018.2542
Citation: Showy male primates have smaller testicles (2019, April 10) retrieved 18 September 2019 from
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Apr 10, 2019
Thus the big shiny pickup truck.

Apr 10, 2019
Unless you're plodding along neked, your hefty plums offers little advantage in this regard.

BTW. It's not the size of the ship, but the motion in the ocean.

Apr 10, 2019
No Lori. Trucks, planes, trains and heavy equipment have a utilitarian purpose, they aren't to attract you. Any attraction you feel to industrial/agricultural equipment is entirely independent of the purpose of the equipment.

Apr 10, 2019
In any event, it is not the balls that count for mating, it is the size of the dong.

Jeez, I thought everyone knew that. And these primates are not having sex because they want to procreate (at least not the males). They don't even know that results from gang-banging.

A big shiny pickup truck wouldn't hurt! A Lamborghini would work a lot better, but it might depend on where you are banging it out.

Apr 15, 2019
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