Bats ‘know’ the speed of sound
Have you ever seen a bat fly? Even if you missed it, there’s a really good chance it saw you. But not necessarily with its eyes; bats can also ‘see’ with their hearing.
The listening ear
Most bats traverse the night sky making use of echolocation. They emit (broadcast) soundwaves—not audible to the human ear. These soundwaves are vibrations in the air that bounce off solid objects, such as cave walls, trees, or mosquitoes. The bat knows the location of its surroundings from the echoes that return to itself.
How does that work? For one thing, the time it takes for a wave to return gives the bat information about how far away something is. The calculation that takes place in the bat’s brain for this obviously involves the speed at which sound travels. The returning sound will also have a lower intensity than the emitted sound. It may have a different frequency too, a different pitch from what was sent out. This happens when the bat is on the move, and thus its own location is changing. It is the same phenomenon, called the Doppler effect, as when the sound of an approaching car changes pitch as it passes you and moves away. By doing this process repeatedly, bats can even assess whether an object is also moving, and in which direction.
Sound consists of waves of pressure travelling through a medium. Air is the medium through which bats—and other air-breathing creatures—normally hear sound. Interestingly, we can also hear sound in other media—for instance under water. But the speed of the sound waves changes depending on the medium. Scientists wanting to learn more about echolocation wondered: What if one were to change the medium in which bats fly, thus changing the speed of sound? Would they adjust their calculations accordingly, so they could still detect objects accurately?
Speed of sound not plucked out of thin air
Normal air consists largely of two gases: around 78% nitrogen and 20.9% oxygen. The scientists decided to experiment with bats flying in a chamber filled with a different mixture of gases (but of course still suitable for breathing and flying properly). They did this by lowering the nitrogen content of the air and replacing the nitrogen removed with an equal amount of helium. This meant the speed of sound increased by 15%. They tried this on bats that had been trained to land on a fixed perch in the flight chamber under normal circumstances, and then had them manoeuvre around in the new atmospheric conditions:
The helium interfered with the bats’ echolocation timing and caused them to aim short of the perch. At first, this was expected, but the adult bats never learned to adjust.1
This means that the bats not only failed initially, they did not learn to compensate for this new environment. The scientists then tried the same experiment with bats raised from birth in this different atmosphere of increased helium content, comparing them with a control group raised in normal air, and found no difference:
The researchers found that those flying in helium-enhanced air tended to underestimate the distance to the target—and it did not matter if they were raised in a helium-rich environment or not.2
The conclusion of the scientists was that the “bats’ reference to the speed of sound is innate and that it is not flexible during adulthood.”3 In other words, not only do “bats seem to be born knowing how to echolocate”1 but they are also “born knowing the speed of sound”—the speed of sound in air is programmed into the bat’s brain.1
Findings surprise evolutionists
“We were surprised by the results. Honestly, we didn’t trust them at first”, said the scientists after this discovery of bats ‘knowing’ the speed of sound.1 Of course, they then gave credit to “the evolution of innate and flexible sensory perception”.3 Such findings are interesting, but not a particular surprise. We can marvel at the wisdom of our sovereign Creator, who bestowed bats with the tools they need for successful living (and flying).
References and notes
- Cameron, D., Bats don’t have to learn the speed of sound—they’re born knowing it, newscientist.com, 3 May 2021. Return to text.
- Yirka, B., Bats found to have innate sense of speed of sound, phys.org, 4 May 2021. Return to text.
- Amichai, E. and Yovel, Y., Echolocating bats rely on an innate speed-of-sound reference, PNAS 118(19):e2024352118, 2021. Return to text.
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