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What the Animal World Can Tell Us About Human Sexual Selection

You and me baby ain’t nothin’ but mammals, but that doesn’t mean you can act like an ape.
Male hartebeests competing. Image: ​Filip Lachowski/Wikimedia Commons

Bathing regularly, putting in time at the gym, trimming rogue hairs—we may tell ourselves we do these things for ourselves or our health, but they're really intended to make us more comely to potential sexual partners.

Humans, like all animals, compete for the most attractive mates in order to have the most successful offspring in a process called sexual selection. Like a peacock's plumage, humans have some specific characteristics that indicate what we find more attractive.

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What we consider attractive has changed over time—and, if we extrapolate from a recent study in birds, over our own lifetimes, too.

As individuals get older, their distinctly feminine and masculine traits become less obvious or important. In the  study published last week in The American Naturalist, researchers studied a lifetime of mating behavior in the black grouse. Male grouse are all black save for spots of white plumage as well as a bright red flap above its eyes. They have a distinctive mating behavior called lekking in which the males spread their feathers, squawk and peck at each other.

The researchers found that male grouse compete more fiercely when they are at their peak reproductive fitness, at about three to four years old, then declines with age. ​David Puts, an evolutionary anthropologist at Pennsylvania State University, does see a similar change in sexual fitness and competition over human life spans, starting in earnest in around puberty and declining over time.

Humans don't have distinct plumage, but both men and women have a number of traits that are a veritable battleground for evolutionary success. Both sexes are selecting for the most attractive traits in their mates, Puts said, but it's mostly the males that are competing for the females' attention.

"This is because men can have many offspring (in extreme cases, hundreds of offspring) if they have access to many mates, whereas women can generally only produce one offspring every couple of years regardless of how many mates they have," Puts said.

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Characteristics that we consider extremely masculine in men didn't come about because they're more attractive to women; they just make the individual better at competing against other men. "Beards and deep voices are far more effective at making men appear dominant than they are at attracting women, so a more apt comparison might be deer stags roaring to intimidate their rivals," Puts said.

Women also compete, though, because parenting takes a lot of time and investment, and they are vying for the males who are going to be more reliable to help out with that. "Some researchers have argued—and I agree—that women's body fat deposition on the breasts and hips has a display function, possibly signaling fertility to potential mates," Puts said.

Sometimes, species develop traits somewhat arbitrarily, because the choosy ones prefer them. But by looking at our closest evolutionary relatives, apes like chimpanzees and orangutans, we can get a sense of when and why our preferences changed as a matter of utility.

Our penchant for bearded men and wide-hipped women must have come about since our last relative in common with chimpanzees, Puts says, because chimps don't display these characteristics today. More predators or less food may have forced females to group together, which would have changed which males women wanted more, as is the case in gorillas. "If female [gorillas] form small groups, those groups might be defensible by a single male, leading to very strong sexual selection for male size and fighting ability," Puts said.

It's unclear exactly which events of warming or cooling, migration, or resource changes would have pushed humans into our various structures of sexual relationships. Evolution is a brutal game, and while generally the most attractive and best adapted are the ones with greater evolutionary success, that rule isn't foolproof. Understanding the other types of sexual relationships in the animal kingdom is useful for understanding ourselves, Puts said, but those comparisons can sometimes be problematic.

In other words, there's a limit to how much we can compare ourselves to other sexual relationships in the animal kingdom: it falls apart when we start justifying our behavior that way.

"The biggest issue arises when people think that explaining why a particular behavior evolves amounts to justifying it," he said. "For example, we could say that boys and men are more physically aggressive and homicidal across societies because some level of physical aggression tended to win mating opportunities for our male ancestors. I think that's probably an accurate statement, even if it may not be the whole story. But it's a logical error to say that an evolutionary explanation for a trait somehow morally justifies it."