Sex and the Single Sperm

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Furthermore, she adds, the idea that rape is about domination and control isn't necessarily at odds with evolution. Describing a 1998 review she wrote about how birds resist copulation, Gowaty says, "What's going on is that the so-called 'forced copulation' is not about immediate fertilization but about conditioning female behavior for the advantage of males." In other words, despite Shackelford's skepticism, evolution might well have designed men to use rape to dominate and control women rather than to create more children.

Another biologist, Alan Dixon, who studies sexual selection in primates in New Zealand, is also a critic. "My own view is that the evidence for human sperm competition has been exaggerated, and some of the work, especially that of Baker and Bellis, is deeply flawed."

Even Randy Thornhill, who put rape-as-an-adaptation on the map, as it were, says that Shackelford and Goetz's approach to human sexuality is "cutting edge" but also a little extreme.

"I would say it's a bit fringe," he says.

Shackelford, meanwhile, finds it ridiculous that anybody would attack his work as biased or badly designed. "I'm attacked for being a sexist and for trying to construct the world so that women would have a difficult time. And here I am with sisters and a mother and daughters... but no, my goal is to make life difficult for women. It's just bizarre."

In his FAU office, Shackelford opens a slim black case to reveal one of the instruments he plans to use in future research: a pair of calipers, capped by a small digital screen, designed to measure the exact size of a man's testicles.

The instrument will be the heart of an experiment that will test another of sperm competition's predictions: Men with larger testicles are programmed by evolution to be promiscuous.

This theory, which Shackelford calls the "bad boys have big balls" hypothesis, seems to work across species. Scientists have long been puzzled by the fact that testicles come in a wide range of sizes among different kinds of primates. Gorillas, for example, which can weigh 500 pounds, have surprisingly small testicles in relation to their body size — smaller than a typical man's. At the other extreme, chimpanzees, which are much smaller than humans, have enormous balls, weighing in at more than double a man's sack.

But gorillas and chimpanzees choose mates in very different ways. A dominant male gorilla maintains a harem by aggressively fending off other males. He doesn't need to produce much sperm to overwhelm the competition, because there isn't any. He makes sure of that.

Chimpanzees, on the other hand, have sex like maniacs multiple times a day with many different partners. A male chimpanzee's sperm, therefore, is constantly in competition with the sperm of other males, and the more volume he can produce, the better chance he will succeed.

When the testicles of primates are averaged against body size, gorillas as a group come in at the low end and chimpanzees at the top. Humans, however, have middling sized testicles, which appears to tell us little about how much our genitals evolved to deal with sperm competition. Did we develop medium-sized jewels because ancestral men guarded a small harem or happily traded lots of female partners with other men or maintained monogamous relationships? There's just no way to know for sure.

But Shackelford, undaunted, wants to measure individual men while surveying them for how promiscuous they are.

"We'd say, 'The final part of this study is to please take these calipers to the restroom,'" he says. "We'd show them how to turn it on, and then we give them a diagram and show them 'Here's where you're going to put the width of your testes' and 'Here's where you put the length.' They'd measure, write it down, and then you would have them put it into an envelope and bring it back, so you don't have them worry if we're trying to see anything."

Shackelford expects that the men with larger balls would report more sleeping around than those with smaller balls. He'd also ask the men about their attitudes toward children and child-rearing in order to test another prediction: that big-balled men are more likely to invest their time and energy in impregnating as many women as possible, while the small-balled would focus on being good fathers. Shackelford calls it a "cads versus dads" scenario.

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Julia Reischel