It seems like a simple question: bees buzz when they flap their wings.
But the truth is more complicated. Indeed, the vibration from flying certainly produces sound.
However, sometimes, even when bees are not flying, they still buzz continuously.
"Buzzing" is Actually the Sound of Thoracic Muscles
If you've had the chance to observe bees collecting nectar up close, you'll notice that when they are on flowers, even though their wings are not flapping, they still make a buzzing sound. If you find this hard to believe, with proper protection, you can catch a bee, put it in a test tube, and place your ear against the tube to hear the buzzing from inside.
So where does this buzzing sound come from? The thorax. The mechanical vibrations of the thorax cause nearby air molecules to vibrate, creating high-frequency sound waves, which are the buzzing sound.
The first person to discover that bees use thoracic vibrations to buzz was Reginald J. P. Phillips. He conducted detailed studies in the 1950s and documented the mechanism by which bees use thoracic muscles to create vibrations.
He found that during flight, the indirect flight muscles in the thorax, namely the dorsal longitudinal and dorso-ventral muscles, contract and relax rapidly, causing the thoracic shell to vibrate, which in turn drives wing movement.
Even when not flying, such as during pollination and defense, the bee's wings remain still and folded, but the thoracic muscles still contract rapidly. These vibrations are transmitted through the bee's body to the flowers or the air, producing the buzzing sound.
Dual Purpose of Vibrations: Collecting Nectar and Defense
Why do bees vibrate their thorax while collecting nectar?
We all know that bees and many plants have a mutualistic relationship: bees help with pollination, and in return, they get nectar. The buzzing helps bees with pollination.
Many plants, especially those with tubular anthers like tomatoes, eggplants, and blueberries, require strong vibrations to release pollen. Bees achieve this through rapid thoracic vibrations. These high-frequency vibrations cause pollen to be ejected from the anther pores and stick to the bee's body.
After collecting pollen, the bee needs to release it to complete pollination, and buzzing helps a lot. When a bee lands on another flower, it grabs the anthers and uses the high-frequency vibrations produced by its thoracic muscles to transfer energy to the anthers. This efficient energy transfer effectively releases the pollen. Studies have shown that this vibration frequency is typically between 200-400 Hz, which maximizes pollen release.
Studying non-flight vibrations in bees in the lab is not easy. Replicating the scene of bees pollinating in the lab is challenging, with many uncontrollable factors such as getting plants to bloom and having bees perform specific pollination behaviors.
However, as mentioned earlier, non-flight vibrations are used not only during pollination but also in defense. Getting bees into a defensive posture is much simpler.
Although the ultimate goal is to understand the vibrations bees produce during pollination, using data from defensive vibrations is reasonable. Whether for defense or pollination, bees use the indirect flight muscles in their thorax to generate vibrations. The contraction and relaxation mechanisms of these muscles are basically the same for both types of vibrations.
Different Bees, Different Buzzing
Professor Mario Vallejo-Marin, a well-known scientist in the fields of ecology and plant evolution, and his research team are conducting an intriguing experiment:
They want to know if bigger bees buzz louder.
The first step of the experiment is to collect bees. Professor Mario and his team members captured bees in different regions of Australia, Scotland, and Mexico. They wore protective clothing and used insect nets to carefully capture busy bees in the flower beds.
They selected different types of bees, covering 70 species from six families to ensure diverse experimental data. The captured bees were carefully placed in plastic bottles, one bee per bottle, and then put into a box with ice packs to calm them down.
Back at the field lab, researchers first photographed and measured each bee's thorax width with calipers. The bees were briefly chilled in a refrigerator to induce a short period of cold anesthesia, preventing injury during the experiment and making them easier to handle.
A cold-anesthetized bee was fixed onto a small nylon ring, just 0.18 mm wide, securely holding the bee's thorax. The bee was then allowed to regain consciousness at room temperature. Once the bee recovered, the team gently but firmly pressed its thorax against a miniature piezoelectric accelerometer, connected to the experimental setup by a thin bamboo stick, with a resonance frequency of about 17 Hz.
When the bee began to recover, an amazing thing happened. Its thorax started to tremble slightly, producing a low buzz as it entered a defensive state. This vibration was precisely what Professor Vallejo-Marin wanted to study. The bee's thoracic muscles contracted and relaxed rapidly, generating high-frequency vibrations recorded by the accelerometer.
The experimental equipment recorded vibration data at a sampling rate of 10,240 times per second, transferring it to a connected portable computer. The computer screen in the lab began to display a series of vibration waveforms and spectra, revealing the frequency, amplitude, and duration of the bee's vibrations.
Each bee underwent multiple vibration measurements to ensure data reliability. The researchers recorded a total of 15,000 individual buzzes, averaging 49 buzzes per bee. After the experiment, Professor Vallejo-Marin and his team began an in-depth analysis of the data. They first applied a high-pass filter to remove background noise and then used an automatic algorithm to detect and mark each individual buzz.
By calculating the fundamental frequency and vibration amplitude of each buzz, the team found a strong positive correlation between thorax size and vibration amplitude.
Of course, not all bee species use vibrations for pollination. Among the 70 species studied, about 68.57% engaged in buzz pollination, while about 37.14% did not.
In the experiment, larger bees produced stronger vibrations, and bees that engage in buzz pollination generated significantly higher vibration amplitudes than those that do not.
In simple terms, the bigger the bee, the stronger the vibration it produces. Especially those skilled in buzz pollination, their vibrations are stronger than those not specialized in this technique.
However, vibration frequency is not related to size.
These findings have significant implications for botany and ecology. Professor Vallejo-Marin explained, "Bee vibrations are not only for flight; they also release pollen during pollination. This mechanism is crucial for the reproduction of many plants."
As the analysis deepened, scientists discovered that even within the same bee species, there are significant differences in vibration amplitude and frequency among individuals, indicating a high degree of flexibility and adaptability in their use of vibrations.
Why Can't Human Muscles "Buzz"?
Why can bee muscles vibrate at such high frequencies, while human muscles cannot?
The key lies in the unique structure and physiological mechanisms of their flight muscles, which are significantly different from human muscles in both structure and function.
Bee flight muscles are primarily divided into two types: direct and indirect flight muscles. The indirect flight muscles play a crucial role in high-frequency vibrations. These muscles include the dorsal longitudinal and dorso-ventral muscles, which do not directly connect to the wings but drive wing movement by changing the shape of the thoracic shell.
The indirect flight muscles are asynchronous muscles, meaning they can oscillate independently. This means they can undergo multiple contraction and relaxation cycles after a single neural impulse, unlike synchronous muscles (such as most human muscles), which require a neural impulse for each contraction.
This autonomous oscillation mechanism allows bee flight muscles to contract and relax at very high frequencies, typically hundreds of times per second. This enables bees to produce high-frequency vibrations, both for buzzing flight and buzz pollination.
Asynchronous muscle design also makes them more energy-efficient during high-frequency contractions. Since each neural impulse can trigger multiple contractions, bees can maintain high-frequency vibrations at a lower energy cost. Many other insects that rely on wing flapping for flight share this feature.
In contrast, human skeletal muscles are synchronous muscles, requiring a neural impulse for each contraction. The maximum contraction frequency of skeletal muscles is usually only a few times per second (in the Hertz range), far below the high-frequency capabilities of bee flight muscles.
In terms of energy consumption, synchronous muscles consume more energy during high-frequency contractions compared to asynchronous muscles, as each contraction requires an additional neural signal and energy input.
So, while bees can buzz for flight and pollination, humans cannot. Human muscles are designed for greater force and lower frequency actions, such as running, jumping, and lifting. Each has its own advantages!
References:
- Vallejo-Marin M, Field DL, Fornoni J, et al. Biomechanical properties of non-flight vibrations produced by bees. Journal of Experimental Biology. 2024;227(12).
- Iwamoto H, Yagi N. The Molecular Trigger for High-Speed Wing Beats in a Bee. Science. 2013;341(6151):1243-1246. doi:10.1126/science.1237266.
- Josephson RK, Malamud JG, Stokes DR. Asynchronous muscle: a primer. Journal of Experimental Biology. 2000;203(18):2713-2722. doi:10.1242/jeb.203.18.2713.
- New Phytologist. 2020;224(3):1068-1074. doi:10.1111/nph.15666.