A large eyespot on the forewing of a Bicyclus anynana butterfly is sometimes hidden by the hindwing. Researchers at the University of Singapore conducted an experiment that suggests the eyespot, which reflects ultraviolet light, plays a role in dry-season B. anynana females (such as the one shown here) attracting mates. (Photo credit: Antónia Monteiro, Ph.D.)

By Viviane Callier

Bicyclus butterflies have many eyespots, and it turns out that each eyespot serves a different signaling role. Although it’s often the case that females use male eyepots to pick the best mate, a new study shows that a little-understood eyespot on the ventral forewing may actually help males choose the most fertile females, researchers report in a new paper published this week in the Journal of Insect Science.

Viviane Callier

Bicyclus anynana butterflies come in wet and dry season forms. In the wet season, males are not picky in choosing females because resources are abundant. But in the dry season males become choosy because they give females a spermatophore—a package of sperm that also contains nutrients and resources, which is energetically expensive to produce. As their investment in reproduction increases, males become more picky.

As a rule, the eyespots on the dorsal side of the wing are used to attract mates, while the ones on the ventral surface are used to scare predators. But Antónia Monteiro, Ph.D., associate professor of evolutionary developmental biology at the University of Singapore, and her students became interested in a bright white, UV-reflecting eyespot on the ventral side of the forewing that is hidden from view when the butterfly is resting, because it’s covered by the hindwing. What could that eyespot be doing?

Initially, Monteiro and her students—Manizah Huq and Shivam Bhardwaj, Ph.D., lead and second authors on the study, respectively—thought that maybe the big beautiful eyespot in that hidden location was a signal that females used to attract males. To test what the eyespot was doing in females, they blocked that eyespot center with black paint, effectively erasing the white color from the wing. As predicted, the males didn’t like females with blocked out eyespots, and they found that, in the dry season, males use it to discriminate female mates.

“In the wet season, females care about the dorsal eyespots of males,” Monteiro explains. “In the dry season, males care about the dorsal eyespots, and the hidden ventral eyespot, of females.”

In Bicyclus anynana butterflies, eyespots in their wing patterns (ventral sides shown here) reflect ultraviolet light (bright areas). WS: wet season form; DS: dry season form. (Image originally published in Huq et al 2019, Journal of Insect Science)

Now, Monteiro and her student are testing which female eyespots in B. anynana are most important. If they black out the dorsal eyespots, or the ventral hidden eyespot, which females do the males prefer?

It’s possible that the size of the hidden eyespot correlates with the number of eggs a female has, Monteiro said, because both the development of the eyespot and the development of eggs are influenced by the pulse of ecdysone hormone that happens at the end of larval development. That developmental correlation could make the eyespot an accurate indicator of female fertility. So, the males, by choosing females with a larger eyespot, could get more offspring. That’s a hypothesis she’s now investigating.

“It’s possible the whole thing started in females,” Monteiro says. “This idea of the eyespot being correlated with fitness in females, and females using the ornament to attract males, and males zooming in on it because of its relationship to female fertility—that could be a very old thing,” she says. “Maybe it’s much older than the use of eyespots in males to attract females. We still don’t know yet.”

Viviane Callier is trained as an insect physiologist and is now a freelance science writer in Washington, DC. Twitter: @vcallier. Email: vcallier@gmail.com