When it comes to butterflies, males are usually the pretty ones. They have to be, since they're also usually the ones that do the courting. But male and female squinting bush brown butterflies (Bicyclus anynana) that live in central Africa look alike, at least to us. Both are equally beautiful in the warm, wet season, when their ventral wings have multiple large eyespots. In the cooler dry season, though, both males and females are plainer and more cryptic. But it's not that side of their wings that the butterflies use to attract a mate—they care only about the couple of eyespots that appear on the dorsal forewing, specifically about the white center of those spots, called the pupil, which reflects ultraviolet light. And though those spots looks similar in males and females in the wet and dry seasons, the love lives of these butterflies are a bit more complicated than their appearance would imply, as scientists outline in a study in Science.
In the wet season, males actively court the females and the females tend to choose males with large dorsal eyespots with intact reflective pupils. In the dry season, the females are the ones doing the courting and the males the choosing, preferring mates with intact pupils. This is true in the lab, too, where the scientists raised butterflies from caterpillars that developed in cold and warm environments. When the caterpillars grew up in the warm, males did the courting. And when it was cold, the females took over.
The scientists also discovered that those dorsal eyespots aren't as similar as they appear to the naked eye, at least in males. In the wet season, the males' eyespot pupils reflected more ultraviolet light than did males' in the dry season, and the pupils were larger in the wet season, too, when the males had to attract the females' attention.
Why the difference in courtship strategies? Like many butterflies, males of this species give a "nuptial gift" to the female when they mate. That gift helps her survive and lay eggs. During the dry season, the females have greater need of that nuptial gift and so they have more incentive to actively seek mating opportunities. And it pays off—females that mate during the dry season live longer than unmated females. In the wet season, it's not as big of an issue, so the females can be choosier and wait for the males to court them.