When choosing more attractive guys, girl guppies with larger brains have an advantage over their smaller-brained counterparts. But there’s a cost to such brainpower, and that might help explain one of the persistent mysteries of sex appeal, researchers report March 22 in Science Advances.

One sex often shows a strong preference for some trait in the other, whether it’s a longer fish fin or a more elaborate song and dance. Yet after millions of years, there’s still variety in many animals’ color, size, shape or song, says study coauthor Alberto Corral-López, an evolutionary biologist at Stockholm University. Somehow generations of mate choice have failed to make the opposite sex entirely fabulous.

Mate choice could require a certain amount of brainpower, with animals weighing the appeal of suitors and choosing among them. Previous research suggests a smaller brain dims guppies’ mental abilities, and the researchers wondered how brain size might affect the fish’s choice of mate.

To test the idea, researchers used female guppies bred for either a larger or smaller brain. Guppy brains are tiny to begin with, but after five generations of breeding the brain sizes in the study differed by about 13 percent, within the range of what biologists find in the wild.

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Each female was offered a choice between a colorful male with orange spots and a bigger tail versus a drab male of about the same weight but without much glory behind. The male fish were installed in compartments at either end of a tank, and females swam back and forth, forced to remember and mentally compare one suitor with his rival.

Females with larger brains showed a preference overall for the more colorful male. Smaller-brained females showed no preference. (The difference did not come from differences in color vision, Corral-López says. The researchers checked the eye genes of the fish and also tested their ability to distinguish colors.)

Interest in flashy-looking males may not be just a fashion choice for females. Orange colors come from pigments in food, suggesting that brighter males may be better fed and healthier, which could lead to healthier offspring. And more colorful males are typically better at finding food. Corral-López also tested females that had not been specially bred for brain size, and these fish preferred the colorful males, too.

But bigger-brained females did not beat their small-brained compatriots in all tests. The smaller-brained guppies tended to grow faster when they were young and to have better immune systems and more offspring.

Thus, circumstances might tip the balance toward or against braininess, the researchers say. Having more babies might be more useful than a discriminating brain, for instance, when food is plentiful and most males manage a decent orange. Such changes in fortune might help explain how variety in appearance persists despite strong mating preferences, Corral-López and colleagues argue. Sometimes flashier males win females, but sometimes drab is just fine.

“Exciting work,” says Molly Cummings of the University of Texas at Austin, who studies fish brains and sexual selection. Checking the fish’s vision was especially important, she says. The results show that females were not “simple slaves to their sensory system.”

The new paper, of course, tracked lab animals, and there’s little data on what differences in brain size mean for mate choice in the wild, says evolutionary biologist Kimberly Hughes of Florida State University in Tallahassee. The new guppy study suggests it’s certainly worth looking at what girl guppies do naturally, she says.