Sexual Competition May Actually Boost Men's Semen Quality

Driving flashy cars and pumping iron are obvious ways that men try to outdo sexual rivals, but it turns out there's a far more subtle, biological method too. According to a new study, men can unconsciously boost their semen quality if they believe their partner is spending more time with other men.

The study was conducted on 34 heterosexual couples in committed, sexually active relationships consisting of individuals aged between 18 and 32. Participants were surveyed about their relationship, including questions that measured the men's perceived risk of sexual competition for their partner. Six samples of ejaculate were then gathered over a 45-day period and their quality analyzed.

Sure enough, when men believed they had a higher number of potential sexual rivals – meaning their partner had more male friends and coworkers – they were found to produce higher sperm concentrations per ejaculation.

The evolutionary logic is sound: if a man is primed to suspect there's a higher chance (real or not) that their partner could potentially have sex with another male, then a last-ditch effort to outcompete them would be to enter more of their own swimmers into the race.

It's worth clarifying that men can't do this willingly, of course: the whole process, if it does occur, is an unconscious one. But either way, it's an intriguing quirk of biology.

The research originally started as a reproduction and update of a 1993 study by University of Manchester biologists Robin Baker and Mark Bellis. That study also investigated how men might unknowingly adjust their sperm concentration in response to a higher sexual competition risk.

However, Baker and Bellis only focused on one measure of this risk: how much time the couple had spent together since they'd last had sex. The idea is that more time apart would, at least in the eyes of the man, give his partner more opportunity to sleep with someone else. Indeed, males did seem to deliver more sperm after spending less time with his partner.

The new study, led by Oakland University psychologist Tara DeLecce, expanded the idea. Along with time spent together since last copulation, the researchers also measured the males' perception of their partner's fidelity, and how many potential sexual rivals they thought they had.

Over 45 days, six samples of each man's semen were collected – three from intercourse and three from masturbation. These were then analyzed for quality, specifically sperm count, motility, and concentration.

Intriguingly, the new study disagreed with the main finding of the older one: time spent together since last copulation wasn't associated with any of the indicators of semen quality. But other evidence did support the underlying sperm competition theory, particularly that the men produced ejaculates with higher sperm concentration when they perceived that their partner had more male friends and coworkers.

There are plenty of reasons that the two studies might reach different conclusions. The team behind the new work says that, for example, it could be a product of changing times: more time apart could matter less to partners in this age of constant contact via mobile phones.

Other interesting findings of the new study include that sperm concentrations were higher in ejaculations from sex than those from masturbation, which also makes sense from an evolutionary standpoint. Less easily explained though is that sperm released during a solo session tended to be more motile, as did those when the male perceived his partner as more faithful.

"The principles of sperm competition would predict that perceptions of lower faithfulness or suspicions of infidelity would result in more rapid progressive motile sperm to adjust for heightened sperm competition risk," DeLecce told Eric W. Dolan at PsyPost. "This might suggest that ejaculate adjustment in humans may be more complex than in other primate species, and additional research is warranted."

Other questions remain to be answered too. For instance, it's currently unknown whether ejaculate adjustment affects the production or just release of sperm. The sperm production process can take up to 64 days, so that's quite a long lag between a perceived threat and boosted swimmer numbers. Releasing more sperm per ejaculation could be a quicker way to respond.

Either way, the researchers say that future work could investigate the biology behind this question.

The research was published in the journal Archives of Sexual Behavior.