Bats can Beat Physics, but not a Swing Set

By Lilly Lewison

Although active sonar was initially developed in the early 20th century as a means to detect enemy submarines underwater, various animals have been implementing this technique long before humans possessed this ability. Bats are a commonly cited example, paired with their technique known as echolocation. Initially believed to be more primitive in nature, the biosonar capabilities of various bat species are incredibly precise and demonstrate deeply complex capabilities well beyond the “blind” nature often assigned to them.

Firstly, we should probably understand what a wave even is. Waves can be classified into two main groups: mechanical, and electromagnetic. Since bats don’t tend to produce electricity or induce magnetic polarization as far as I’m aware, we’ll stick to working with mechanical waves. Mechanical waves occur as a result of some initial disturbance which propagates throughout some medium in the form of energy. An example of this is the noise generated by applause—the kinetic energy of our muscular movement which leads to the collision of our palms. Once our palms collide, the energy still needs a place to go, thus it travels through the medium that is the air around us. Our vocal cords function in a similar way, the reverberation generated by them is pretty analogous to clapping our hands.

Now that the energy from within us has been transferred into the air, this is where we can observe the common qualities of a wave. Waves oscillate; meaning they have a consistent peak and dip gathered around a central point. This is what we can call amplitude.

The amount of times a peak (or trough) passes an arbitrary point over a given amount of time is known as the frequency of the wave, and is usually measured in a unit known as Hertz. Many animals have the ability to control this frequency (commonly known as “pitch”), however bats take the cake in their ability to discern incredibly specific frequencies.

Bats contain a specialized notch within their ear which allows them to specifically discern a pitch of their choosing thanks to the shape, location, and size causing the notch to resonate at a specific frequency. During echolocation, a bat will propagate out an initial signal the frequency they are incredibly adapted to hearing, and upon detecting the echo (the “bounce back” of the initial wave), this will allow them to determine the position and relative distance of said object. “Great!” I hear you say, “what could possibly be a downside or challenge with this process?” you continue to say, being an astute reader.

Unfortunately, there is a slight complication that occurs when using this approach while you are moving throughout the air: the Doppler effect. As the source of the sound moves, the frequency heard by the observing party also changes. This is more easily visualized by the demonstration in which as the red dot (source of the sound) in the following image moves along the x-axis, the sound waves get bunched up in the direction of movement, leading to a higher frequency sound.

“But if the bat is flying around and trying to detect a target, then how will they utilize the special notch in their ear if the returning frequency is always different!” Scientists wondered this as well, and the answer to this solution is actually quite novel. Bats have the ability to alter their initial pitch based upon the speed they are moving! This is known as Doppler-shift compensation. Now this is a pretty remarkable technique, and how do you think we discovered this about these fascinating creatures?

“DSC was tested for by swinging the bats on a pendulum… a pendulum was constructed of heavy-duty PVC irrigation pipe and placed at a distance of roughly 10 m from the entrance to the cave.”

WE LITERALLY PUT BATS ON A SWING AND MEASURED THEIR PITCH AS WE SWUNG THEM. As the bats swung forward, they intentionally lowered their pitch such that the higher pitched echo that would normally be produced was toned down to their preferred detection pitch.

As if all of these findings weren’t beautiful enough: the science behind how a wave works in relation to how animals generate sound with our vocal cords, the specialized notch to detect these pitches, and how bats combat the laws of physics in order to eat insects while flying around at night, there is a pièce de résistance to the above graph. Notice on the “forward swing” how the red dots shift perfectly that if you were to average the frequency emitted with the induced doppler effect, it would arrive back at the resting frequency (the so-called preferred pitch). However, there seems to be no shift with the “backward” swing. The answer to why this happens is so incredibly simple: bats don’t fly backwards!