Butterfly wings can be defensive — or sexy.
A new study from Yale biologists suggests that the eyespots on the underside of butterflies’ wings are used to ward off predators while those on the upper side attract mates. The finding is the first to confirm an offhand observation evolutionary biologist Charles Darwin once made in his book, “The Descent of Man,” that most scientists consider common sense: butterflies use the same signal — eyespot patterns — for opposing purposes. The study, which was published last week, can be found online in the Proceedings of the Royal Society B: Biological Sciences.
“I think that for a lot of people it was a matter of conventional wisdom that [butterflies] were using the upper side [of their wings] for attracting mates and the underside for protection from predators,” said Jeffrey Oliver, the lead author of the study and an ecology and evolutionary biology postdoctoral associate. “But there was no testing or study on it.”
Working alongside Yale Professor Antonia Monteiro and State University of New York at Buffalo’s Kendra Robinson, Oliver found that the eyespots on the upper side evolved more quickly than those on the underside. Sexually selected attributes usually evolve more rapidly than other attributes, giving butterflies a distinct survival advantage when encountering potential mates and predators. Additionally, eyespots on the upper side were found to show different rates of evolution between the sexes — another expected pattern if upper side eyespots are evolving under sexual selection, Oliver said.
“When the butterflies are sitting on a leaf, the wings are held high and are brightly colored to attract a mate,” said Oswald Schmitz, professor of population and community ecology and director of the Center for Biodiversity and Conservation Science. “But in the event that a predator comes, they’ll fold their wings down and then there’s a modeled eyespot discoloration that makes it look like it has huge eyes or is some sort of beast.”
The team used DNA sequences from specimens of Bicyclus butterflie, a brown species with relatively simple wing patterns, to reconstruct their evolutionary tree. The study builds on past research, including Monteiro’s, which shows certain coloration patterns are more likely to attract mates.
Moving forward, the work — done entirely in a lab — is now ready for study in the field, Schmitz said.
“This is sort of a study that we use from specimens collected from the lab, but really we need to go out — and it’s hard work — into the field and see what these insects are doing,” Oliver said, echoing Schmitz’s suggestion with plans to follow through. “[But] they are in tropical Africa, which is obviously hard to get to sometimes.”
Oliver, who will be working on this study in Monteiro’s lab, said the next step is to dig deeper into the evolutionary history of butterfly eyespots to figure out how they first evolved.
This pattern of using a single signal to display different things, Schmitz said, is likely to apply to other insects as well, particularly other species of butterflies.
In fact, it may provide insight into other species as well.
“I do think this can be used for other organisms,” Oliver said. “Birds can very well be using this [strategy] in their feather patterns, especially if parts of their feathers are shown to potential mates and other parts to potential predators.”
The research was partially funded by the National Science Foundation.