It might seem like a pretty obvious question, but why do bees buzz inside the hive? It makes sense that they make a buzzing sound during flight, as the vibrations from their flight muscles create the noise, but why do they keep buzzing when they return home?
If you listen to sound from inside a beehive, you will hear a constant background buzz with as many as 50,000 bees contributing to the noise. The diagram below shows the constant buzzing as a waveform.
So the bees use their wing muscles for more than just flight, and it turns out the wing muscles play an important part in many tasks, including pollen collection, heating, cooling, honey making, defence, and communication. Studies have also shown that many of these activities generate sound at slightly different frequencies, which means we can use audio analysis to learn more about what the bees are up to.
So while the recording from inside the hive might sound like a loud buzzing noise, this noise consists of multiple frequencies overlaid on top of each other. Human ears aren’t very good at distinguishing frequencies which are so close together, and it’s a bit like trying to pick out an individual conversation in a crowded room. But if we split the audio into the component frequencies using fourier transforms, we get a much better view of what’s happening.
The image below shows a spectrogram of the same sound data shown above, but this time the data is split out by frequency from top to bottom, so the lower areas of the chart show lower frequencies, and the higher frequencies are near the top. The greener areas indicate louder volumes so that we can see some patterns within the audio data.
So what do these patterns mean?
Some of the volume changes will occur during the main daylight flying hours when thousands of bees leave the hive for foraging flights. This can be seen in the chart below, where the audio data has been overlaid with a red line showing the number of bees exiting the hive. We can see the peaks in the red line correspond roughly to the audio peaks, and these occur during daylight hours.
But there are also more subtle activities we can pick up.
For example, a natural part of the lifecycle of a healthy bee colony is swarming, where the queen leaves with up to half of the workforce and starts a new colony in a new location. But one of the first tasks required in order to swarm is to get the queen ready for flight. She spends most of her life laying around 1500 eggs a day, and any spare time is taken up with eating, so she isn’t usually in the best shape for a flight. So one of the first things the worker bees will do is to chase her round the hive in order to get her ready for departure. This is done by the nurse bees and the sound they make when they do this occurs at a very specific frequency, which is detectable in the audio. This means we can get an early warning of potential swarms just by listening in at the right frequency.
The chart below shows and example of audio filtering, where a potentially interesting ‘spike’ of sound has been identified.
The bees also use their wing muscles as part of communication, but it isn’t just the audible sound that is important. For example, when scout bees find a rich source of food, they communicate this information to their colleagues via the waggle dance, where the direction of the food is indicated by the direction of the dance, and the distance is indicated by the dance duration.
But inside the hive it’s very hard for other bees to see or hear what the dancers are doing, so the bees came up with a genius solution to this problem.
For a long time it was thought that bees were deaf, as they don’t have visible ears, but it turns out bees hearing is more sophisticated than scientists first realised, as bees use not only sound pressure vibrations, but also vibrations in surfaces. So instead of ears located on the side of their head, bees detect vibrations using organs located both in their antennae as well as in their legs.
So when a scout bee does a waggle dance, she does it while grabbing the honeycomb with her feet, and this causes vibrations to be passed through the honeycomb. Nearby bees feel these vibrations through the organs in their legs and this makes it much easier for the nearby bees who are waiting for directions. As while their antennae might pick up lots of competing noises inside the hive, their legs pick up just the vibrations from the nearest dancer. The hearing organs in the bees legs are called subgenual organs and they are located near the tibia-femur joint, so they are literally the bees knees.
Audio data is proving to be a valuable area of research, with each hive producing around 12 hours of sound every month. The audio data can then be combined with the other bee related data including flight counts, hive temperature, humidity, weather etc to provide a much more detailed picture of bee behaviours and activity.
Audio data (inner chart) versus bee foraging trips (outer chart) for 3 months.
