In the years since the 1859 publication of “On the Origin of Species,” Charles Darwin’s ideas about evolution have become foundational to the field of biology. Yet though his ideas were revolutionary, Darwin was not all-seeing — and recent scientific work has revealed a missing element in his theories. That missing piece has to do with sperm.

Indeed, beginning in 1970 and continuing full throttle with new research published earlier this year, biology experiments highlighting “sperm competition” among males are filling in some of the blanks left by the great 19th-century evolutionary scientist. As it turns out, there are evolutionary battles raging in microscopic, behavioral, and physical fields. These skirmishes are about the rights and abilities of rival males to impregnate females.

Darwin used the term “natural selection” to describe the ability of genetically lucky organisms to survive long enough to procreate under environmental conditions that kill others. Animals’ supreme and universal urge to send their DNA into subsequent generations drives evolution. Indeed, to them, it is the very meaning of life. Copulation is how animals satisfy their urgent evolutionary impulse. And for almost all species of multicellular animals, procreation requires that sperm meets egg.

In 1871, twelve years after the publication of “Origin,” Darwin’s “The Descent of Man and Selection in Relation to Sex” made clear that getting sperm to meet egg relies on “sexual selection.” By that term, he meant the preference in one sex for certain attributes in the other. For example, male peacocks with spectacularly feathered tails get sexually selected more often to copulate with fertile females than do less well-ornamented males. Stags with big antlers score more often than stags with small ones.

As world-enlightening as Darwin’s ideas of natural and sexual selection were, there’s a tiny whiff of failure about him as a scientist. Brilliant as he was, he never realized that natural selection and sexual selection aren’t quite enough to explain evolution.

The piece of the evolutionary puzzle Darwin missed

In many species, females take more than one mate during a breeding cycle. (The technical term is that they are “polyandrous.”) For males, this means that successfully copulating isn’t a guarantee that they’ve fertilized eggs. A rival might get to those eggs instead.

Some modern scholars consider Darwin’s failure to recognize polyandry as the wrench jamming up the works of evolution to be one of his biggest oversights.

Here are some of the disruptions to the simple “sperm + egg = baby(ies)” equation that polyandry creates. Say, for example, that multiple males have copulated with the same fertile female. Probably at least one of those males will impregnate her.

But which one?

If she’s carrying more than one egg (and some insects carry hundreds), many males might get to be biological fathers. Some males will probably end up fertilizing more eggs than others, and some may not get to fertilize any at all. Usually, a competition of sorts determines who fertilizes what and in what number. The battle can happen on a microscopic scale, with immediate physiological changes to semen and sperm quality. On the other hand, behavioral adaptations may give an animal his needed edge. (Sometimes, to the casual human eye, these behavioral adaptations can seem impossibly weird.) Even physiological adaptations can boost a male’s chances. (Physical adaptations don’t happen on the spur of the moment during sex like physiological changes to sperm do. They evolve over millions of years.)

 

As a group, the adaptations are called “sperm competition.” The first evidence of sperm competition was identified by Dr. Geoff Parker of the University of Liverpool. His 1970 research described mating behavior in polyandrous yellow dung flies (Scatophaga stercoraria, common in Northern Hemisphere pastures). Females copulate with many males, all while carrying the same egg sac. Dr. Parker noted that, usually, the last male to inseminate before the female drops her sac in dung wins. He becomes the biological father of around 80% of the eggs.

A male of any species can only make so much sperm in a day or maybe even a lifetime. Since 1970, Dr. Parker and others have found that males in polyandrous species spend sperm prudently. For the yellow dung fly, sperm is best spent on virgins. Some male insects boost their paternity chances by flooding only one female with sperm. Still others spread sperm promiscuously, and so on.

Nursery web spiders

Perhaps because they’re easy to catch and breed, much of the research about sperm competition has been done on spiders. February 2022 work from biologists at Ludwig Maximilian University in Munich and Aarhus University in Denmark shows the benefit to mating males of long copulations. When a male nursery web spider (species Pisaura mirabilis, found all over Europe) offers a female a “nuptial gift” of a silk-wrapped bug, she allows him to copulate. What’s more, she lets him continue to flood her receptacle with sperm for as long as the proffered meal lasts. In an email, co-investigator Dr. Cristine Tuni explained the logic of this adaptation. The spider’s ejaculate doesn’t arrive as a brief, happy burst and then stop. Rather: “In this species, sperm is transferred continuously over time from his copulatory organ into hers,” Tuni says. “So, the longer a male has his organ coupled to a female organ, the more sperm is transferred. The relationship is basically linear.”

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One egg sac can carry hundreds of eggs. Because of this, any male wanting a big bang for his f**k probably intuits that size (of the gift) matters. Pumping as much semen as possible can help send his DNA on its way.

Malabar spiders

The Malabar spider (Nephilengys malabarensis, found in Asian rain forests) wields a far more dramatic sperm competition adaptation. Each male has two genital appendages extending from behind the mouth. As semen pulsates out of one, the spider detaches it and leaves it inside the female’s receptacle. Even severed like that, the genital continues to ejaculate. Meanwhile, it also plugs the receptacle, making it difficult for another male to get a genital in. Ready to fend off anyone who tries, the mating male stays on the web near the female. Unfortunately for him, each female’s semen receptacle has two openings. He has only plugged one. This means that, if a rival approaches, the mating male will have to fight fiercely to keep him at bay. To that end, and while ejaculation from the abandoned genital continues, many males eat their only remaining genital.

Of course, that seems like a counter-intuitive strategy. Why get hungry at that very moment? Why hurt yourself right when you may need all the energy you can muster?

A team of biologists from several institutions in Europe and Asia seem to have an answer. They compared the battle survival rates of spiders who’d severed one genital to those of spiders who’d severed one and eaten the other. Additionally, they tested the battle survival rates of genitally intact males. The name of the team’s paper — “Eunuchs Are Better Fighters” — says a lot about why, under duress, a Malabar spider would eat its only remaining genital.

RELATED: How testosterone and oxytocin hormones influence male behavior

God knows why that’s true, but apparently it is. What’s more, turning himself into a eunuch is not where a Malabar spider’s sperm competition adaptation ends.

Immediately after mating, a eunuch male usually lets the female eat him. Why not? Doing his best to fulfill his biological imperative to guarantee the intergenerational survival of his DNA, he has left himself incapable of copulating. At least any eggs he has already fertilized will benefit from his calories and protein.

Sperm competition in mammals (including humans)

Coyotes and other canines have an anatomical sperm competition adaptation. A bulbous gland at the base of the penis inflates, locking it inside the vagina and giving ejaculated sperm time to reach the female’s eggs before another male mounts the female. This doesn’t necessarily mean that the first canine’s sperm will fertilize the entire littler. Even so, the arrival time of a canine’s sperm may make a critical difference in whether he can fertilize at least one egg.

In great apes, a primary adaptation is semen volume, which is reflected in testes size. For example, silverback gorillas are not particularly polyandrous. Typically, several females are sexually dominated by a single male. With no need to out-compete the inseminations of other males, gorillas have small testes relative to their enormous bodies.

Female chimps and bonobos, on the other hand, are decidedly polyandrous. Indeed, bonobos are notorious libertines; a fertile female chimp might copulate three or four times an hour with different males. Both males and females copulate with same- and different-sex partners freely and joyfully as a way to have fun, make friends, soothe tempers, and (yes) make babies.

Humans, too, are great apes. In general, women are less polyandrous than chimps and bonobos and more polyandrous than gorillas. Relative to their body sizes, men’s testes are not typically as large as those of chimps and bonobos and they’re not as small as those of gorillas.

Certainly, many modern humans have intercourse for reasons other than procreation. Providing sexually active people with ways to avoid pregnancy is a multi-billion-dollar industry. Even so, humans’ mating behaviors may be driven to some extent by the same “meaning of life” urge to ensure the survival of their DNA that motivates other animals.

In 1993, environmental biologists R. Robin Baker and Mark A. Bellis found one way in which men may have evolved to help sperm meet egg. Testing 35 men, they found that the ones whose mates had recently been out of their sight had more sperm in their ejaculate. The biologists reasoned from the data that a partner’s temporary absence might make a man uncertain about her fidelity. Niggling suspicion that his sperm has to compete with that of other men might create a physiological response increasing sperm count.

In more recent years, Dr. Leigh Simmons of the Centre for Evolutionary Biology at the University of Western Australia has run a series of experiments with carefully controlled parameters. He and Dr. Sarah J. Kilgallon demonstrated that the very idea of sexual rivals can trigger sperm changes. They showed one group of men pornography involving one woman and two men. A second group saw pornography involving only women. The two researchers found that the sperm produced by men who viewed pornography involving one woman and two men swam faster.

At least one possible sperm competition adaptation in humans is anatomical. Simmons and collaborator Dr. Samantha Leivers have proposed that the piston shape of men’s penises may have evolved to help men rid their mates’ vaginas of a rival’s previously deposited sperm. With a flat-bottomed head and a shaft that is narrower at top than bottom, the human penis can scoop out semen each time it momentarily withdraws from a vagina in preparation for the next thrust.

 

Meanwhile, Simmons and many others have acknowledged mate guarding behaviors like sexual jealousy as a primary sperm competition adaptation. Surprisingly, work in the Simmons lab has also demonstrated that mate guarding behaviors like jealousy work against production of higher quality sperm. In 2014 in the peer-reviewed journal PLOS ONE, Simmons and Dr. Gillian Rhodes reported:

We found that men who performed fewer mate guarding behaviors produced higher quality ejaculates, having a greater concentration of sperm, a higher percentage of motile sperm and sperm that swam faster and less erratically.

Score one, perhaps, for the idea that the ideal mate is more courtly than boorish.

Sperm competition adaptations and human infertility

The Urology Care Foundation reports that up to 50% of infertility issues among human couples are linked to problems with male semen.

Males in polyandrous non-human species can ramp up sperm volume, count, and vitality in response to the presence of rivals. For humans, even the thought of polyandry can jack up a man’s sperm quality. Considering all of that, might turning the knob way up on polyandry as a real or imagined stimulus rocket launch the get-up-and-go of any given man’s sperm? Might overt and rampant promiscuity on the part of a woman help her man with his fertility problem?

That’s a bit of a sci-fi scenario, but by email and on Zoom I asked Dr. Simmons what he thought about it. By way of an answer, he told me about a body of work by Drs. Mariana Wolfner (Cornell University), Tracy Chapman (University of East Anglia), and Stuart Wigby (University of Liverpool). It shows that a protein called “sex peptide” in the ejaculate of male fruit flies (Drosophila melanogaster, originally an African species) diminishes a female’s sex drive. This reduces the likelihood that, once inseminated, a female will let additional males try to fertilize her enormous sac of eggs.

Unfortunately for the female of that species, sex peptide shortens her lifespan.

The research to which Dr. Simmons referred raises the specter of unpredictable results for humans of a modern reproductive strategy reaching to ludicrous lengths to boost sperm quality.

That said, there’s probably a second problem with my idea of operatic promiscuity, or at least there’s a problem with asking a well-respected scientist to weigh in on it. Generations of anthropologists have made clear that, across cultures, women sometimes take more than one mate. Even so, most women wanting to conceive take pains to seem to do so as part of a couple, harem, or intentional family. Potential risks to women who use exaggerated promiscuity to boost a male partner’s fertility have not been well-studied. Maybe, given the outré arrangement that would be needed to conduct such an experiment, the risks never will be studied at all.

With no easy sperm competition fixes to the infertility problem that 10 to 15% of American couples face, many couples are unable to revel in the meaning of life as Darwin’s natural selection theory defined it. They might take comfort in the realization that philosophers have wondered, without resolution, about “why life?” for millennia. They might also find comfort — or at least comic relief — in the 1983 Monty Python film, “The Meaning of Life.”

In particular, one of the movie’s songs might help boost moods. Granted, its lyrics aren’t about sperm competition per se. They bash narrow-minded ideas about fertility and masturbation. Even so, “Every Sperm is Sacred” might work as a fertility anthem. It could be sung in biology labs and bedrooms as well as in jungles, on spider webs, and near bowls of fruit everywhere.

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