Male primates who show off more to attract a female mate tend to have smaller testicles, according to enlightening new research.
Researchers from the University of Western Australia (UWA) and University of Zurich have revealed that on average, male primates (including humans) who are great at attracting mates through displays of strength and status have smaller testicles.
Publishing their findings to the Proceedings of the Royal Society B, the researchers described how male primates typically live in highly competitive environments, all with the common goal of fathering offspring.
The ‘showy’ males typically produce ‘badges of status’ that include flashy ornaments, which allow them to control access to females and intimidate other males.
However, those who would be shyer typically have larger testicles. So, while they might not be able to show off as much as some of the flashier primates, they can win “by producing a lot of sperm to swamp those from their rivals”, said the study’s co-author, Dr Cyril Grueter from UWA.
The research focused on primates because of their tremendous variation in both testicle size and male ornamentation, with some having testicles as large as 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,” Greuter said. “Others are pretty drab and look more like your ‘Mr Average’.”
The data was compiled from more than 100 primate species, including humans, demonstrating for the first time that ornaments come at the expense of testicle size and sperm production.
“This finding clearly shows that you can be well adorned or well endowed, but it’s hard to be both,” Grueter said. Explaining further, the researchers said it was likely that both would take up too much energy.