Choosing a mate is a funny thing. While other animal species are probably less likely to make the poor alcohol-fueled choices most of us regret, albeit fondly—and less likely still to wake up in a hungover fog in a strange place the next morning, grabbing articles of clothing up off the floor and checking the waste bin to make sure the number of used condom wrappers matches up with our hazy memories—females of other species are subject to a lot of the same bravado and competitive posturing we endure from human males, and they act just like we do: sometimes accepting an offer, sometimes walking away.
This process of picking—whether you’re mating for lifelong partnership, to make babies, or just for a recreational quickie—is known as sexual selection. Just like us, animals mate for a staggering variety of reasons. And also like us, they frequently make questionable decisions.
Why we pick the mates we do has been the subject of countless research studies since Charles Darwin coined the term “sexual selection” 150+ years ago, but we still know way less than you’d expect.
One thing we do know: animals are show-offs, and will do just about anything to impress a lady.
Darwin was so impressed by animal courtship that he included a description of sexual selection in On the Origin of Species—a term he defined by contrasting it to his theory of natural selection. That is, while natural selection is shaped by “a struggle for existence,” sexual selection depends “on a struggle between the males for possession of the females,” in an effort to produce the most viable offspring.
Darwin’s examples of sexually selected traits that confer an advantage range from “special weapons confined to the male sex,” such as horns, spurs, or overall strength and dominance, to the “more peaceful character” of sexual selection seen in many bird species—in particular, demonstrations of singing, dazzling plumage, or entertaining antics to win a female’s affection. Today, any characteristic or trait that gives a reproductive advantage to one sex or the other is generally considered to be related to sexual selection (although there’s still some debate on the definition of sexual selection in many scientific circles)1.
The Bird of Paradise
mating dance from Planet Earth is one of my favorite examples of the influence of sexual selection. The male calls to attract a female audience, then performs a remarkable song-and-dance routine you have to see to believe. (Jump ahead to the 2-minute mark for the good stuff if you want to skip David Attenborough’s wind-up, but why would you want to do that?)
Like the Bird of Paradise, the male peacock also has a tail that is essential to his mating success. Peahens are pretty bland-looking but very picky about their mates, so their men really put on a good show to win their affections. Skip ahead to the 6-minute mark or so of this video to see what happens when peacocks suffer the ignominy of having their tails trimmed.
Elaborate vocalizations like birdsong, and even whale songs, are also widely thought to be the result of the sexual selection process. Attracting a mate is the reason for the orange spot patterns on guppies, for the variety of patterns and colors in Lake Malawi’s cichlid fish, and for many of the chemical, visual or acoustic signals that insects give off.
While easily visible differences between male and females—like the peacock’s plumage—make easy targets for researchers studying sexual selection, more subtle sexually selected traits can be harder to spot.
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