![]() That’s been established in other studies, and is reconfirmed here. Social structure also explains why more monogamous primates are less ornamented and have smaller testes than non-monogamous primates. They exist within a relatively promiscuous multi-male to multi-female mating system, so their best chance at reproductive success is having more sperm. "This finding clearly shows that you can be well-adorned or well-endowed, but it’s hard to be both."Īgain, we can consider the chimpanzee, whose testes weigh more than a third as much as its brain. This generates a competition between the sperm of male rivals, so males are better off investing their resources in their testicles rather than their “badges of status” because there’s no way they can become an alpha male anyway. But it’s also apparent that, in other species, the social structure can lead to multiple males mating with the same females. The genes underlying “badges of status” like ornamentation are then passed on and spread in the population over time. “In some species, socially dominant males may be able to monopolize access to females, and whatever trait allows males to rise to the top will be under strong selection, as only those who eventually get to mate can pass on their genes,” Lüpold says. Ornamented monkeys, like this gelada, are more likely to have smaller testes. And the reproductive strategy that works for the males who get to reproduce will be passed down as a selected trait to their progeny. Whether or not a male ends up investing in big testicles or a big beard or nose depends on the primate society it exists in. An evaluation of the primate data set showed that in primate societies where male competition is the most intense, sexual traits become the most pronounced.īut energy is costly, and males can only make limited reproductive investments - it’s hard, Lüpold explains, to have it all. “Shaving won’t make bigger.”įor primates whose social mating system is centered around female monopolization - in which all males compete for non-monogamous females - being chosen over other males is the big prize. “Our study only focused on between-species relationships, so we can’t jump to the conclusion, as done by a few news outlets, that bearded men have small testes,” Lüpold says. Importantly, this study evaluates species of primates to other primates, not primates within the same species group. ![]() The redder the face, the more likely a rhesus macaque is to mate. Chimpanzee penis skin#He explains to Inverse that after evaluating a data set including 103 primate species, a pattern of an evolutionary “trade-off between ornaments and testes emerged.” The primates with the most pronounced ornamentation - manes, beards, and colorful skin patches - had the smallest testes, while primates with the biggest testicles were the plainest. So the team, led by Stefan Lüpold, Ph.D., an evolutionary biologist at the University of Zurich, decided to look into this apparent mismatch. These primates have something the showy primates do not have: large testicles. On the other side of the primate spectrum are the plainer-looking bonobos and chimpanzees. In the new paper, researchers explain that showy males - for example, orangutans with the largest framing cheek pads or rhesus macaques with the darkest, reddest faces - have more mating partners than their thin-faced or paler peers. Showiness in the primate world can mean many things. "…we can’t jump to the conclusion, as done by a few news outlets, that bearded men have small testes." A strange repercussion of this process is that “showy” primates actually have the smallest testicles. ![]() Scientists reported Wednesday in the journal Proceedings of the Royal Society B primates have evolved different traits to achieve this same sexual goal. The large-nosed proboscis monkey, the large-testicled chimpanzee, and all primates in between want to have as much sex as possible so they can increase the chances of spreading their genes. ![]() Primates belong to a diverse taxonomic order, all united by one sexy pursuit. ![]()
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