For many species, love is truly a battlefield. While the animal kingdom is full of diverse social structures, a pretty common one involves males tussling for access to females. For these males, being—or at least looking—physically powerful tends to be a perk: Rippling muscles can be an advertisement for future security, and a promise of robust, sexually successful offspring.
Bulking up is no easy task for any male. If there’s a lovestruck lady waiting at the end of this equation, the effort could be well worth the trouble—but attracting a mate is only half the battle. If a female mates with multiple males, for instance, individual sperm can actually compete with each other in a high-stakes race to the egg—and the road is often littered with casualties. Each male can only mount so much moxie, and allocating resources to one pursuit means sapping energy away from another. As a result, the males most competitive before sex don’t always come out ahead in the moments after. It’s a dark lesson in not judging a virile-looking book by its cover.
Scientists have been aware of such tradeoffs in other animals for years. Certain species of beetles grow weighty horns to clash with male competitors and guard female companions—but when horn growth is stunted, their testes will swell in compensation (a common indicator of prolific sperm production). On the other hand, howler monkeys that make the loudest hoots tend to tote the tiniest testes (conversely, other howler monkeys speak softly and carry a big… well, never mind).
The counterintuitive relationship makes sense from an evolutionary perspective. If a male with physical disadvantages will have a tougher time in the fierce competition for female attention, he better make every last bit count in the few encounters he has. This gives pint-sized suitors a fighting chance of still passing on their genes.
“This can be a real advantage, depending on the environment,” says Jennifer Kotler, an evolutionary biologist who studies sexual conflict at Harvard University and did not participate in the research. “Sometimes, it’s better to be a bit smaller or weaker, but mate really well only once.”
At an extreme, some insect-munching species of marsupials—like this sweet little brown antechinus—give new meaning to the phrase “going out with a bang.” Male antechinuses expend so much energy on the buildup to ejaculation that they can suffer immune system collapse and die in the days following their first breeding season. Because they get only one shot at leaving behind a genetic legacy, these males pour their entire selves into producing the most superb of swimmers, despite the horrifying costs.
Thankfully, though, humans aren’t antechinuses—or beetles or howler monkeys, for that matter. In humans, all sexes and genders put on mating displays, and they tend to be far less ritualized: Some swoon at serenades; others are content swiping through selfies on Tinder. Do the same rules about sperm tradeoffs apply to human men?
So far, the answer is… well, sometimes. Men who speak in deep voices or sport traditionally masculine facial features—both of which tend to be favored by women—have lower quality sperm. For the most part, however, the jury is still out on how sensitive human semen is to this type of resource allocation, especially when it comes to big investments like spending months at the gym getting swole.
A team of scientists led by Yong Zhi Foo at the University of Western Australia decided to investigate the relationship between machismo and sperm, publishing their results last month in Animal Behavior. First, 118 men posed for both headshots and full-body photos that showed off their physique. A sample of ejaculate was then collected from each individual and assessed for sperm count, motility and morphology (a basic assessment of how wonky-looking their sperm was).
Two groups of 12 heterosexual women then rated the men’s photos on attractiveness and masculinity. Finally, another group of 91 people rated the photos on how “strong” their subjects appeared to be. This last group included both men and women, acknowledging the idea that looking strong has an effect on both sexes—perhaps by intimidating other males they hope to oust, as well as charming the females they court.
Perhaps unsurprisingly, the strongest-looking men were also the ones rated as most attractive and masculine. In other animals, these guys are the ones that are best equipped to protect their mates and offspring, and pass on high-quality genes. But when the researchers inspected their subjects’ sperm, they found that the same men who boasted the most brawn—and were wooing the most women through their photos—also had, on average, lower sperm counts. Though there wasn’t a big change in sperm motility or morphology, a numerical decrease seems to denote lower quality sperm.
It would seem, then, that humans are not exempt from evolutionary sexual tradeoffs. Because the men involved in this study weren’t actually assessed for a change in the quality of their sperm, the researchers’ work is just the first step in establishing this relationship. But it fits with what scientists know about energy expenditure: After all, hitting the gym requires energy that could otherwise be spent on reproductive viability. In fact, a previous study from another group showed that men doubling down on a physically intensive regimen suffered reduced ejaculate quality. However, that work challenged men to regular endurance exercise on a treadmill, which doesn’t build brawn in the same way that, say, weightlifting would.
It’s not yet clear how resources are diverted from sperm production into building muscle mass, or vice versa—but Foo has theories. One possibility involves hormonal regulation. Testosterone fuels gains when muscles are put to work, but too much of this hormone can actually stymie the flow of ejaculate.
Moving forward, Foo hopes to follow up on this work by repeating his results in other study populations, noting whether sperm motility and morphology can also suffer from physical exertion. Additionally, to really pin down this particular sexual tradeoff, he and his colleagues hope to establish a more direct relationship between the strains of strength training and a less-than-stellar ejaculate—and which limited resources are important to both pursuits.
“Semen quality is very sensitive to many things,” Foo explains. “And [these changes are] happening both in the long and short term.”
Notably, the effects the researchers see may not be entirely due to training. Genetics can also play a big role in physique—and semen quality.
Given these additional considerations, Mollie Manier, a biologist who studies sperm and sexual competition at George Washington University and did not participate in the research, cautions against blowing the study’s findings out of proportion. “[The process of making sperm] can be ramped up or down—it’s flexible,” she explains. “It’s not that if you’re born big, you’re going to have a poor ejaculate.”
Similarly, clocking in an extra 15 minutes at the gym isn’t going to permanently compromise the family jewels. Sperm health is sensitive to environmental factors like diet and exercise, but human men are constantly producing new sperm—around 1,000 each second. So even if things take a testicular turn for the worse, it’s not all doom and gloom for those downstairs pairs.
What’s more, the relationship isn’t always so cut and dried: Sperm produced by hunky gym rats won’t always be found lacking. “There are going to be people who are good at both [physicality and sperm production], and people who are bad at both,” explains Manier. “Pragmatically, no one needs to be alarmed about their sperm quality.”
Finally, there may be a more heartwarming moral to this story. While sexual tradeoffs may cast a cynical light on romance, it’s worth keeping in mind that there are benefits on both sides of the equation, and most males—no matter their looks—likely have something to offer. This may even help explain why species haven’t just gotten uncontrollably burly over the course of millennia, which one might expect if there were no cost to a strapping stature.
“There’s natural variation in these traits,” says Kotler. “Getting stronger isn’t always going to result in mating success. There’s not one strategy that’s inherently better, and there isn’t one way to ‘win’ at evolution.”