A study of over 100 primate species, including humans, by three evolutionary biologists from the University of Western Australia and the University of Zurich, found that the more “showy” and “ornamental” the males were, the smaller their testicles were likely to be.
The study by Stefan Lüpold, Leigh W. Simmons and Cyril C. Grueter was supported by the Swiss National Science Foundation and published as “Sexual ornaments but not weapons trade off against testes size in primates” in the journal, Proceedings of the Royal Society B, on April 10, 2019.
“Sexual ornaments” of male primates refer to secondary sexual characteristics used by males to attract females and to signal social dominance within the group. Those sexual ornaments include red chest patches of geladas, cheek flanges of orangutans, hair capes of hamadryas baboons, enlarged noses of proboscis monkeys, and the beards of men.
“Weapons” refer to physical traits used by males in direct male–male combat, such as the size of their canine teeth.
Here is the study’s Abstract, written in typical academic jargon:
Males must partition their limited reproductive investments between traits that promote access to females (sexual ornaments and weapons) and traits that enhance fertilization success, such as testes and ejaculates. Recent studies show that if the most weaponized males can monopolize access to females through contest competition, thereby reducing the risk of sperm competition, they tend to invest less in sperm production. However, how males invest in sexual ornaments relative to sperm production remains less clear. If male ornaments serve as badges of status, with high-ranking males attaining near-exclusive access to females, similar to monopolizing females through combat, their expression should also covary negatively with investment in post-mating traits. In a comparative study across primates, which exhibit considerable diversification in sexual ornamentation, male weaponry and testes size, we found relative testes size to decrease with sexual ornaments but increase with canine size. These contrasting evolutionary trajectories might be driven by differential selection, functional constraints or temporal patterns of metabolic investment between the different types of sexual traits. Importantly, however, our results indicate that the theory of relative investments between weapons and testes in the context of monopolizing females can extend to male ornaments.
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.
As reported by the UK Evening Standard, the researchers say that the reason for males either being well-adorned (sexual ornaments) or well-endowed (large testicles) is that it takes too much energy to be “showy” and have larger testicles at the same time. In other words, males who invest their energy in sexual ornaments have less energy to invest in developing large testicles. As one of the study’s authors, University of Zurich evolutionary biologist Stefan Lüpold, 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.”
H/t Clash Daily
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