Martin Wikelski sits alone in the cockpit of a small biplane, listening to the ping of a machine at his side. The sonic beacons help the ecologist to stalk skull butterflies (Acherontia atropos) fluttering through the dark sky over Konstanz, Germany – about 80 kilometers north of the Swiss Alps.
Nicknamed for the skull-and-cross pattern on their backs, the moths migrate thousands of miles between North Africa and the Alps in the spring and fall. Many migrating insects go where the wind takes them, says Ring Carde, an entomologist at the University of California, Riverside, who is not a member of Wikelski’s team. Skull butterflies seem anything but typical.
“When I follow them with an airplane, I use very little gas,” said Wikelski of the Max Planck Institute of Animal Behavior in Munich. “That shows me that they are supposedly choosing directions or areas that are likely to be supported by a little bit of updraft.”
A new analysis of data collected from 14 skull butterflies suggests that these insects do indeed self-steer, possibly relying in part on an internal compass tuned to Earth’s magnetic field. Not only do the moths fly along a straight path, they also stay on course even when the wind changes, Wikelski and colleagues report on Aug. 11. Science.
The findings could help predict how the moths’ flight paths might change as the Earth warms further, Wikelski says. Like many animals, skull butterflies are likely to migrate north in search of cooler temperatures, he suspects.
To keep an eye on the moths, Wikelski’s team taped radio transmitters to their backs, which is easier to do than you might expect. “Skull butterflies are totally cool,” Wikelski says. They are also huge. Weighing no less than three jellybeans, the moths are the largest in Europe. That makes attaching the little tags a cinch, although the moths don’t like it very much. “They talk to you, they yell at you a little bit,” he says.
After the researchers freed the newly tagged and slightly irritated moths, Wikelski went after them on a plane. As the insects flew south toward the Alps, an onboard device pings the transmitters at a frequency related to the moths’ distance from the plane.
While detailed tracking of eight of the moths allowed him to track the insects for an average of about 63 kilometers, he chased one for just under 90 kilometers. That’s the longest distance an insect has continuously tracked, he says. “It’s insanely crazy work,” he says of the low-altitude night flights. “It’s also a bit dangerous and it just goes to show that it’s possible.”