Headbutts come as a surprise
By Sam Wong
Whoop whoop! A vibrational pulse produced by honeybees, long thought to be a signal to other bees to stop what they are doing, might actually be an expression of surprise.
Bees produce vibrations with their wing muscles that are inaudible to humans but can be detected by accelerometers embedded in the honeycomb.
In the 1950s, researchers noticed that this signal was often followed by bees exchanging food, and hypothesised that it was a request for food. Later, it was shown that the signal was produced when one bee tried to inhibit another from performing a waggle dance – a behaviour that tells other bees where to forage. It was interpreted as a “stop” signal that warns colleagues against foraging in a location where there might be problems, such as a predator or a researcher bothering the bees for an experiment.
To find out more, Martin Bencsik and colleagues at Nottingham Trent University in the UK used accelerometers to record vibrations inside hives over the course of a year. Then they used software to scan the recordings and identify the signal. Some of these signals have been collected and converted into the sound clip below.
They found that the signal happens much more commonly than we thought, with the accelerometer picking up around six or seven a minute from just a small area of the honeycomb. “There’s no way a bee was trying to inhibit another one that frequently, and there’s no way a bee would request food that frequently” says Bencsik.
They also found that the signal takes place mostly at night – in contrast to waggle dances, which happen in the day when bees are foraging. What’s more, the signal is easy to elicit from hundreds of bees en masse just by knocking gently on the wooden wall of the hive.
By placing cameras inside the hive, the researchers discovered that the signal often happens when a bee bumps into another bee near the accelerometer, and not when bees are waggle dancing or exchanging food.
“We suggest that, in the majority of instances, it is bees being startled that produce the signal,” says Bencsik. The team propose that instead of the “stop” signal, it should be called the “whooping” signal.
Earlier studies might have misjudged which bee was producing the vibration. When a bee is inhibiting others from foraging, it headbutts the bee that’s doing the waggle dance. Bencsik thinks the vibration might be a startled response by the bee that’s been headbutted and not the inhibitor, since it’s impossible to tell exactly where it’s coming from. Exchange of food between bees is also preceded by headbutting, so that could also be explained by this new interpretation.
It could also offer a way to monitor the status of the colony, by delivering a standardised stimulus and measuring the response. “I would imagine an unstressed colony would have less of a response and a colony that’s very stressed would be very reactive to a small stimulus,” says Bencsik.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0171162