Showing off takes stones for male primates. Small ones, it turns out.

Pppppffff, show-off!
Image via Pixabay.

Gird your loins, ladies and gentlemen, because today we have a rather peculiar (but still interesting, and quite amusing) study to talk about. Hailing from the The University of Western Australia and the University of Zurich, the paper reports that flashy male primates tend to have smaller gonads, while their more average-looking counterparts sport larger ones. It all seems to be a product of how male primates handle social hierarchies and reproductive strategies.

Two kinds of primates

Dr. Cyril Grueter, a primatologist from UWA’s School of Human Sciences and a co-author of the present paper, says that male primates tend to live in relatively high-competition environments. They also pretty much all want the same thing: to impress (and impregnate) the females.

“But not all of them can have what they want,” he adds. “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 focused on primates because of their “tremendous variation in both testicle size and male ornamentation,” a press release accompanying the paper explains. Dr. Grueter seconds this view, saying that some primates they looked at in the study had testicles no larger than a peppercorn, while others’ could easily pass for tennis balls. Pun intended.

Pretty surprisingly, however, the team found that there’s a consistent link between different indicators of male virility throughout primate species. The team compiled data from over 100 species of primate (including humans) to show that ornamentation seems to come at the expense of testicle size and sperm production ability.

“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.”

In blunter terms, showy primate males have smaller testes. Dr. Grueter jokes that the “finding clearly shows that you can be well-adorned or well-endowed, but it’s hard to be both”. This either-or approach likely comes down to economics, the team suspects: it simply takes up too much energy to invest in both reproductive strategies.

The findings also tie in with some of Grueter’s past research. In a study published in the journal  Evolution and Human Behavior back in 2015, he and his team reported that male primates have developed more ostentatious ‘ornaments’ or ‘badges’ (such as beards in humans, cheek flanges in orangutans and so on) to help them navigate big, multilevel societies.

Such elements could help males attract the attention — and affections — of females while also helping them deal with male competition in a more direct way. Male rhesus macaques (Macaca mulatta) with darker red faces receive more ‘come-ons’ from females during the mating season, the team reported in that study, while men with beards could be seen as more aggressive and dominant than those without beards — helping them intimidate competitors and attract women drawn to seemingly powerful men. That paper also suggested these badges are particularly useful for males in large, complex groups, as proxies to show dominance or rank.

“When you live in a small group where everyone knows everyone because of repeated interactions, there is no need to signal quality and competitiveness via ornaments,” Greuter explained at the time.

“In large groups where individuals are surrounded by strangers, we need a quick reliable tool to evaluate someone’s strength and quality, and that’s where these elaborate ornaments come in. In the case of humans, this may also include phenotypic extensions such as body decoration, jewellery and prestige items.”

So, primate ladies in the audience, take heed: flashy males may catch your eye… but there’s really only enough energy to pursue a single reproductive strategy at a time.

The paper “Sexual ornaments but not weapons trade off against testes size in primates” has been published in the journal Proceedings of the Royal Society B: Biological Sciences.