Sex and the Single Sperm

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Baker and Bellis' work contained surprising ideas, such as their theory that some human sperm are "kamikaze" cells designed to sacrifice themselves to sabotage the sperm of competitors. They suggested that men adjust the number and quality of sperm they ejaculate depending on how likely it is their partner has had sex with someone else and that individual human sperm are designed to wrap around the sperm of rival males and strangle them. Some of their ideas — especially kamikaze sperm — have been shown to be thinly supported.

"Their work was pioneering but certainly not conclusive," Thornhill says. "They were reasonable hypotheses. Then Todd began to get interested in this stuff and has taken it very seriously. I would put Todd there as the person who has done most of the most important empirical work to look at human sperm competition."

Shackelford was inspired by Baker and Bellis' work, despite its methodological flaws. But he added his own approach — along with looking for physical signs of sperm competition, such as "attack" sperm, Shackelford and Goetz looked for psychological effects as well: behaviors, like jealousy and rape, that may have evolved in response to the pressures of sperm competition.

When it comes to procreating, men and women face very different outcomes. A woman never has to worry whether the baby she's putting all her time and energy into is passing on her genes: She knows it is, because she saw it come out of her body. But a man can never be sure: All he knows is that his sperm went into a woman and a baby eventually came out.

Because of this, our ancient male ancestor faced a quandary: If, while he was out hunting and gathering, the woman he thought he had impregnated stole away for sex in a nearby cave, he might end up putting all his time and energy into raising a child who didn't carry his genes.

In 1988, Shackelford's mentor, David Buss, predicted that an elaborate system of "mate retention tactics" would have evolved in response to this. Behaviors that range from giving lavish gifts to guilt trips, threats of violence, and outright rape. In 2003, Gordon Gallup, an evolutionary psychologist at SUNY Albany, succeeded in using dildos, artificial vaginas, and fake sperm made from one of Shackelford's recipes to show what Shackelford could not at FAU: that a human penis did indeed appear designed for "sperm displacement" — to shovel out any rival sperm hanging around the vagina. The displacement only worked, Gallup found, during deep-thrusting sex: when the penis was inserted at least 75 percent of its length into a vagina.

To find whether these sperm competition behaviors show up in modern humans, Shackelford and Goetz have surveyed men and women in monogamous relationships, asking very specific and often invasive questions about how they behave and feel in their relationships.

One study, published in 2004, quizzed 305 men about two different sides of sperm competition: whether they practiced any of the mate retention tactics (giving gifts, etc.) and how long and how deeply they thrust during sex. Men catalogued how many times they refused to introduce their partner to a male friend, peeked at their partner's personal mail, or bought a small gift. They were also asked to rate their thrust depth on a scale of 1 to 9. Meanwhile, Shackelford and Goetz correlated this information with the men's description of the attractiveness and personality traits of their girlfriends or wives and how likely they thought their partners were to cheat.

After crunching the numbers, Shackelford and Goetz got what they expected: The hotter and more outgoing the girlfriend, the deeper men thrusted and displayed "mate retention tactics."

A man, they concluded, would go out of his way to prevent a good-looking mate from looking for other men but, to cover his bets, would thrust deeply during sex in case the woman had been unfaithful.

In 2006, the two surveyed 246 men and 276 women who were in at least a year-long relationship (but not with each other), this time with a new set of questions about how often they had performed or experienced "sexually coercive acts" in the past month.

Shackelford and Goetz again found what they expected: Men in relationships with women they suspected were unfaithful forced their partners to have sex more often. (The fact that acts of coercive sex reported in this study may have constituted actual sex crimes, however, didn't seem to prompt reports to law enforcement authorities.)

Of course, subjects sometimes lie to survey takers, which is the motivation behind Shackelford's largest project. In 1999, he received a grant from the National Institute of Mental Health, the nation's preeminent underwriter of scientific research, to launch the most comprehensive study of human sperm competition ever conducted. Already two years past its original due date, the study's data is so voluminous that Shackelford has three of his graduate students processing it full-time.

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Julia Reischel