By Ed Yong. A honeybee returns to its hive after a productive visit to a nearby field of flowers, rich in pollen and nectar. It starts to dance. By waggling its bodyand strutting in a figure-of-eight, it conveys the duration and direction of the food source to its hive-mates. It was Karl von Frisch, an Austrian scientist, who first deciphered the waggle dance back in 1923. Now, 90 years after his pioneering work, we’re still learning amazing things about the messages that are exchanged within the hive.
When bees fly through the air outside the hive, they collide with charged particles, from dust to small molecules. These impacts tear electrons away from their cuticle—their outer shell—and the bee ends up with an positive charge. When they return to the hive and walk or dance about, they give off electric fields. And Uwe Greggers from the Free University of Berlin has shown that they can detect these fields with the tips of their antennae. Despite our long history with the honeybee, there could still be a secret world of electric communication within the hive that we know nothing about.
We’ve known that insect cuticle builds up electric charge since 1929, almost as long as we’ve known about the waggle dance code. “Many colleagues thought that the bees have a charge but it doesn’t matter. It’s too small,” says Greggers. But when he actually took measurements of living bees, he found that they can produce voltages of up to 450 volts! The insects’ waxy cuticles are responsible—they’re so electrically resistant that a substantial charge can build up and stay there.
Since the 1960s, scientists have speculated that these charges could be useful during pollination. Flowers, after all, tend to have a negative charge on clear days. When bees approach, pollen can actually fly through the air to their bodies. And just last month, Daniel Robert from the University of Bristol showed that bumblebees can detect the electric fields of flowers, and use them to tell the difference between recently visited blooms and fresh ones.
But what about social communication? Can the bees themselves detect each other’s electric fields? Can they extract useful information from them?
To find out, Greggers created Pavlov’s bees. He exposed them to artificial electric fields that mimicking those found in the hive, before giving them a rewarding sip of nectar. Soon, he found that the field alone was enough to make them extend their tongues in anticipation of a tasty treat, just like Pavlov’s dogs salivating at a the sound of a bell.
Greggers found that the bees detect these fields with their flagella—the very tips of their antennae. Picture a bee, dancing away in a tightly packed hive with many neighbours in close proximity. As it waggles, it also vibrates its wings. As the dancer’s positively-charged wing get closer to a neighbour’s positively-charged antenna, it produces a force that physically repels the antenna. As the dancer’s wing swings back to its original position, the neighbour’s antenna bounces back too. With their electric fields, the bees can move each other’s body parts without ever making contact. (Sure, the beating wing also pushes air past a neighbour’s antenna, but Greggers found that the force produced by the incoming electric field is ten times stronger.)
The bee detects these forces with small touch-sensitive fibres in the joints of their antennae, which send electrical signals towards the insect’s brain. If Greggers immobilised the joints by covering the antennal joints with wax, the bees couldn’t learn to associate electric fields with nectar rewards.
These signals from the fibres are intercepted and processed by a structure called Johnston’s organ within the antennae. By recording the activity of neurons in this organ, Greggers showed that it does indeed fire when an electrically charged object—like a Styrofoam ball—is brought close to the flagellum.
“This is a remarkable discovery,” says Robert. “After all these years of studies on bees, one comes to realise yet another secret aspect to their language. The exact function of such electric sense is not entirely clear but the evidence is strong that electric communication can take place between bees in the hive.
Indeed, now that Greggers has shown that honeybees can detect each others’ electric fields, the big question is: Do they? Is their electric sense an actual part of their everyday lives? To find out, Greggers now wants to study the electric fields of waggle-dancing bees. If he can interfere with the audience’s ability to detect those fields, will that disrupt their ability to interpret the dance?
PS: When I wrote about Roberts’ discovery about bees sensing the electric fields of flowers, the most common comment was something like: “Aren’t our own man-made electromagnetic fields are screwing the bees over? The short answer is: No. The fields produced by our technology are actually much lower in energy than those produced by the bees themselves. “They should be naturally protected,” says Greggers. “Unless a bee-keeper puts their hive directly under a high-voltage electric wire outside, the effects should be limited.”
Reference: Greggers, Koch, Schmidt, Durr, Floriou-Servou, Piepenbrock, Gopfert & Menzel. 2013. Reception and learning of electric fields in bees. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.0528