An Eye for Survival

Understanding butterfly mating behavior begins with looking the insect deep in the eye.

Butterflies are beautiful in our eyes. We don’t feel inclined to squash or swat them—our instinctive reaction to other bugs and creepy crawlies. Many of us actually plant butterfly-attracting plants in our gardens. Somewhere in its evolutionary past, the butterfly began to make an aesthetic argument for its own existence. Humans bought that argument. On the other hand, could it be we just got suckered by a coincidence?

We know what butterflies look like to us. This gives us little in the way of understanding how they recognize and seduce one another. Ron Rutowski says that vision is still key, just not our own. Rutowski is an entomologist and behavioral ecologist at Arizona State University. He says that understanding butterfly mating behavior begins with looking the insect deep in the eye.

The ASU biology professor know. Supported by the National Science Foundation and its Swedish counterpart, Rutowski has studied butterfly vision for more than 25 years. His international collaborators include Eric Warrant of Lund University in Sweden. Together, the scientists have uncovered much of the structure behind butterfly vision. More important to Rutowski, however, is that the work has helped him to understand how that structure might affect the animal’s behavior in the wild.

How do butterflies see each other? The answer may at first seem incredibly ironic. The intricate designs collectors use to identify one butterfly species from another are useless when one butterfly is trying to locate another.

Butterflies are extremely nearsighted. They can spot color and conspecifics, but may never recognize much else about what pattern their companions are sporting. The world, including other butterflies, can be a blur if it’s not placed right in front of the butterfly’s eye.

Compared to humans, butterflies have about 100 times less visual acuity at a distance. Also, there is virtually no overlap—this means their vision is monocular, not binocular like ours. The butterfly cannot assess depth or distance in the same way we can. The butterfly eye, in terms of what is apparent to us, is not ideal for locating or selecting a mate.

Rutowski’s group found that this nearsightedness is in fact the side effect of an evolutionary trade-off with a very useful adaptation.

“We began by asking whether or not the evolution of eyes was shaped by mating tactics, whether or not the eyes were structured in such a way that would enhance mate locating,” said Rutowski. “The eyes are involved in so many activities during the life of the animal that the selection pressures that arise from mating just don’t size up well against all these other selection pressures. The eye reflects the demands of detecting predators, and the demands of very rapid, highly controlled flight through the environment.”

In butterflies like the Empress Leilia, Asterocampa leilia, males have a visual field of about 344 degrees on the horizontal plane. That’s only 16 degrees short of seeing all the way around its body. Vertically, it is virtually a full 360. Compare this to the average human, who has a visual field of a mere 190 degrees.

Most butterfly species have visual fields that are equally impressive. If you’ve ever tried to catch one, you know—butterflies are nearly impossible to sneak up on. This, when working properly, keeps them safe from their number one predators—hungry birds.

Assuming you live long enough, how do you find a mate when you’ve got extra hindsight but can’t see very far ahead of yourself? Once he knew the structure of the eye, Rutowski could make detailed observations and predictions of butterflies’ lives in the wild.

While intricate details may be lost on butterflies, some still rely on color, particularly colors not visible to us, to find and judge a mate. To see how this works, Rutowski studied the Orange Sulfur butterfly, Colias eurytheme. These small butterflies flutter above the Arizona alfalfa fields in summer.

In the wild, Rutowski’s group found that virgin females of the species are able to visually assess the relative age and fitness of competing males. They do this thanks to ultraviolet scales on the males’ wings, and receptors in their eyes allowing them to see UV wavelengths.

Missing scales are signs of aging and reduce a male’s ability to seduce a mate. Behavioral adaptations on the part of males help them to compete. Males should do their best to get in good position to display their wings to passing females. Behaviors that take care not to damage UV scales when a young male help too.

One tactic Rutowski predicted butterflies might use to make the most of their limited vision is to develop a highly systematic approach to finding a mate. The chances of successful mating are greatly improved if a male is in the right place at the right time. To see an example of this, just watch the Empress Leilia male in action.

Asterocampa females only mate once in their short lifetimes. Males go right to the source. They start the morning by sunning themselves on the ground underneath desert hackberry trees, where caterpillars of their species feed and pupate. All the while, they keep an eye out for young virgins just emerging from their cocoons.

According to Rutowski, the butterfly eye seems well adapted for looking up, where the background is bright and uniformly lit. On the ground is a good place for a male to be. On the ground near a plant where females are emerging from pupae is even better. This observation led Rutowski to predict that males should be doing whatever they can to stay on the ground.

In the field, Rutowski found that in fact a male only leaves the ground when it becomes too warm. He then moves up into the trees, to the level of the average flight plane of his female counterparts.

“As temperatures rise over the course of the morning, males will switch from being on the ground to being up off the ground, but one thing we often see them do is fly off, come back, then sort of swoop down near the ground maybe once or twice to see if it is too hot. We never see them investigating the plant first and then coming down the other way. There is a polarity to their investigative behavior that supports this idea that they are trying to stay on the ground for as long as possible.”

No matter what their body position or where they move to, once off the ground the male butterflies at all times point their heads so the eyes look horizontally into the flight path. According to Rutowski, this fits with predictions made from his laboratory observations. Holding their heads in such a way insures that the area of greatest acuity in their visual field is always looking in the right direction.

These are just a couple of the impressive vision-associated behavioral adaptations Rutowski and his team plan to continue studying. They have much ground to cover. About 12,000 species of butterfly are on record. The behaviors preceding butterfly sex have been described in only a few dozen.—Matthew Shindell