Deaf: That’s how text books long summed up the hearing of honey bees. As it turns out, humans were using their own hearing to make the definition. Now it is being revealed that bees live in a world of sound that goes beyond our range – just as bees see ultraviolet light invisible to humans.
Bees, as social insects with divisions of labor, require cooperation. Cooperation requires communication, but most of the life of a colony takes place in a dark hive. Vision, important for orientation and navigation outside the nest, is useless inside. It was long thought that communication within the hive was based only on chemical signals: They do play a vital part, and understanding of the complexity of pheromones is growing. But, in addition, many sound signals have been identified, and science is beginning to decode what the bees are signaling there in the darkness.
For centuries it was known that bees somehow communicate about forage, but it was a mystery until the 1940’s, when Karl von Frisch followed foragers through the rubble of war-ruined Munich to understand the significance of bee dances. For the next decades, most scientists believed that messages were relayed by silent movement.
In the 1960s, two researchers independently discovered that dancers emit low-frequency sounds — Adrian Wenner of UC Santa Barbara, and Harald Esch at the University of Munich. Their assertion that the bees use sound to communicate fell, as it were, on deaf ears, as the pervasive belief was that bees could not hear.
Sound is vibration that travels through a medium — most often air, but for the bees, comb, too. Vibration causes molecules in the medium to pulse outward, colliding with nearby molecules, creating waves. Humans hear by detecting the resulting oscillations in pressure. Bees detect air particle movements. Because traveling sound waves have both components, either can be used in sound perception.
Sound waves are measured by their frequency in Hertz, or cycles per second. The frequency of sound waves is heard as pitch; a higher wave frequency creates a higher pitch. Musical notes correspond to particular frequencies of those waves: for example, an orchestra tunes to the A tone, which has the frequency of 440 Hz. Honey bees produce many frequencies of vibration and sound – from less than 10 to more than 1000 Hz. So far it has been shown that they can detect sound frequencies up to about 500 Hz. How much of this spectrum is used for communication is unknown. It is thought that bees produce vibrations and sounds they do not use or even detect, but some researchers are banking on the fact that whether or not the bees use them, some of these sounds may provide valuable information about the colony.
The most familiar bee sound is the buzz that comes from wing movement: the larger the bee, the slower the wingbeat and the lower the pitch of the resulting buzz. The often-quoted rate of honey bee wing beats is 11,400 times per minute, but beekeepers know very well that the sound varies with circumstance, individual colony and even subspecies; reports range from 190 Hz to 250 Hz. Howard Kerr at the Oak Ridge National Laboratory found the wing beat frequency of Africanized bees to be about 50 Hz higher in pitch than that of European bees. Perhaps they are that much smaller.
As insect wings beat, they create turbulence in the air. Like the wake of a ship or eddies behind a plane, this turbulence has enough stability to capture pheromones for chemical signaling, according to researcher Jurgen Tautz – as seen in fanning.
Experienced beekeepers recognize sounds as they work bees: “You hear and see personalities — agitated, angry, hungry, calm,” said Bonnie Morse. “When you open splits, you know who went with the queen and who stayed,” said John Jacobs.
When she works bees, Kaat Byrd, a nearly deaf beekeeper, prefers to remove her hearing devices to eliminate the confusion of sounds. She moves her hands in silence: “carefully cracking the lid of the hive creates a roaring-rumbling surge of vibration…”. Then, if all is well, the bees return to a gentle rhythm; otherwise Byrd can tangibly feel distress. “Placing a frame of eggs and young brood into a queenless colony almost instantly soothes the angry and anxious vibe…”
“The decibel level is above 65 if the bees are queenless,” said Diana Sammataro, now a retired USDA entomologist. “They are more agitated and more runny, but it can sound similar if they have been disturbed by an animal.”
In response to an intruder, guard bees rock forward and issue a short burst of sound, repeating these warning bursts. When the hive is jarred, their collective reaction is sharp, loud buzzing, followed by faint beeps from workers in the hive at about 500 Hz. Wenner observed that the bees appeared to be soothed by the audio signal, and found that disturbed bees were quieted when he played that recorded sound.
“We can’t hear a lot of the vibrations that the bees give off, but we can hear some of them” said Juliana Rangel, assistant professor of Entomology at Texas A&M University. It has long been known that bees respond to vibrations in the comb, also known as substrate-borne sound — for example, they respond to striking a hive by moving upward, even absconding. But it is relatively recently that it has been shown that bees can perceive airborne sound as well.
Hearing in honey bees was researched in the 1980s by William Towne and Wolfgang Kirchner, who showed that bees can detect …