The results were stark. The monogamous flies were much more likely to wander toward the dangerous smell than the polygamous ones. In other words, they had done a much worse job of learning.

“I think this is a compelling and interesting study,” said Emilie Snell-Rood of the University of Minnesota, who was not involved in the research. The experiment, she said, suggests that the struggle to find a mate favors the evolution of better learning.

The evolution of learning remains a puzzle for scientists. A smart animal can learn how to find more food or how to avoid predators. But if learning were such an unalloyed good, then one might expect all animals to be as smart as we are.