Study confirms that monkeys with small genitalia act more aggressively

A joint study conducted by the University of Western Australia and the University of Zurich, which has been published in The Royal Society B journal, tells us something most women may already know. They studied 103 primate species with a focus on “the presence/absence of female monopolization”.

What they found was that male primates with smaller genitalia compensated for their lack of size with aggressive behaviors and “badges of status”. Phys.org gives details;

“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,” UWA researcher and study co-author Dr. Cyril Grueter told Phys.org. “This finding clearly shows that you can be well-adorned or well-endowed, but it’s hard to be both.”

Sounds familiar?

The Swiss National Science Foundation has concluded that having other flashy traits (think brightly colored Mandrill butts) compensated for their lack of large testes. For each species of monkeys, this could mean different flamboyant features. Long beards, large manes, and bright colors are used to attracted mates and discourage other males.

They have also concluded that most monkeys can either look great or have big ‘equipment’. But not both.

“Ornament elaboration comes at the expense of testicle size and sperm production,” says Lüpold, one of the researchers, “In a nutshell, the showiest males have the smallest testes.”

It is suggested that in communal groups – like in Bonobos communities – these ornaments or flashy physical traits are seen at a higher rate when a male becomes dominant. Because this dominant primate will not have to compete for female attention, his sperm does not have to be ‘vigorous’ or plentiful – thus allowing more testosterone to enhance other beautifying features.

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The only ‘ornamentation’ or ‘weapon’ that seems to be positively correlated with large testicles is teeth. It is unclear why. It is possible that the genes associated with teste growth are also associated with the genes that control canine growth. Or perhaps it takes less energy and upkeep to grow large canines in comparison to colorful butts and big furry manes.

They took into account different social structures as well.

Cosmos magazine gives us details into the sexual prowess of male monkeys in communities that share mates:

In species in which multiple mates are a feature of social life, however, extravagant ornamentation is less valuable because every male is presented with opportunities to fertilise, and exclusivity is not a factor.

In these cases, reproductive fitness is determined by sperm volume and motility, so large testes – in as much as these are regarded as proxies for fecundity – are an advantage.

In these cases, it’s a plus to look like this guy: