On a humid afternoon a few years ago in the Peruvian Amazon, a flicker of motion caught Phil Torres' eye. A cherry-spot metalmark butterfly (Adelotypa annulifera) was drinking nectar from the tips of bamboo shoots.
As he watched, he realized something strange was going on. Normally butterflies only sip at nectar for a few seconds, minutes at most—but these butterflies were feeding for hours, Torres later discovered.
Even more bizarre, ants that live on the bamboo and chase away other insects from their home ignored the butterflies.
Many animals steal food, but this is the first time that scientists have discovered adult butterflies swiping food from ants.
“It was really cool. I had never seen anything like it,” says Torres, co-author of a new study on the behavior in the Journal of the Lepidopterists' Society.
Biologists already knew that metalmark larvae produce a nutritious goo and feed it to ants in exchange for their protection from predators.
After the larvae made the transition to adults, however, they had to immediately fly away or these same ants would attack and try to eat the adults. What the adults did afterward—how they mated, what they ate—remained largely a mystery.
Now Torres' observations, in addition to more by study co-author Aaron Pomerantz, an entomology Ph.D. student at University of California, Berkeley, suggests the adults have figured out a way to pilfer food.
What's more, victims of these brazen nectar thefts appeared to ignore or be completely unaware of the butterflies, says Pomerantz, a National Geographic explorer.
Even bullet ants—the “most badass ants of the jungle” due to their pugnacious nature and painful sting—got swindled, notes Andy Warren, a lepidopterist at the University of Florida who wasn't involved in the study.
“Ants normally treat this bamboo like their castle, aggressively fighting off intruders,” Pomerantz says.
The scientists even documented a butterfly stealing a droplet of nectar straight from the mandibles of an ant, the first time this has been documented.
It's unknown how the butterflies get away with it. For example, they could use chemical signals to conceal themselves from the ants, or perhaps the ants get some unknown benefit from the butterflies in their midst.
Another element of the story piqued the interest of Susan Finkbeiner, an entomology postdoctoral researcher at Boston University and expert in butterfly wing patterns. When she first saw pictures of this behavior, she noticed A. annulifera wing spots look remarkably like ants.
“It definitely looks like mimicry to me,” says Finkbeiner, who wasn't part of the study.
Given that ants have poor eyesight, the wings likely don’t help camouflage the butterflies from ants. Instead, the markings may help to deter the many animals that would like to make a meal of the butterflies as they enjoy their nectar, the study authors say.
Finkbeiner and Warren say many more questions remain about this "unique" behavior, including whether other metalmark butterflies do this.
“In the tropics, anything is possible,” Finkbeiner laughs.