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It is now well established that bats can develop a mental picture of their environment using echolocation. But we’re still figuring out what that means: how bats take the echoes of their vocalizations and how they use them to represent the locations of objects.
In one paper released on Monday, the researchers show that bats are partly ecolocalized because they were born with an inherent sense of the speed of sound. How have researchers studied this phenomenon? Raising bats in a multi-helium atmosphere, where lower-density air increases the speed of sound.
Ecolocation is relatively straightforward in principle. A bat creates sound, bounces off objects in the environment, and then returns to the bat’s ears. In the case of more distant objects, the sound needs more time to return, providing a feeling of relative distance.
But bats can also use echolocation to identify prey in the middle of a flight or to select a landing location. To do this, they must have a sense of absolute distance. It is not enough to know that the branch you want to land is closer than the house behind it; you need to know when to start the complex movements involved in tying on the branch, or you may encounter or stop completely in the air.
The easiest way to achieve absolute distance is to have a sense of the speed of sound. Along with this, the delay between vocalization and return echo will provide a complete distance. But how do you test whether bats have a certain speed of sound?
Eran Amichai and Yossi Yovel of Tel Aviv University decided that there was a simple method: changing the speed of sound. One of the factors that affects the speed of sound is the density of the air. There is an easy way to change the density of air: the vertex with lighter gases than air. In this case, the authors chose helium and raised a group of bats in an atmosphere that had enough helium to increase the speed of sound by 15%.
(Whether the bats raised in this environment looked fun or not was left untested.)
It means that the echoes reflected by the faster speed of sound would return faster. This means that the object that creates these echoes will be perceived closer than it really is. So if we could somehow sense near how a bat perceives an object, we could get a measure that understands the speed of sound.
Fortunately, the bat species used in these experiments changes its echolocation sounds as they approach an object. So following the noise that bats make when they approach an object, we think they are close to it.
To do this experimentally, the researchers grew bats with a feeding station at a certain distance, one group growing normally in the air and another in helium-rich. They then exchanged atmospheres with the two groups. In the case of bats grown with helium, the slower speed of normal air would reach the echoes for a longer time, thus making the feeding stations farther away. The opposite would happen with bats that normally grow in the air.
As a result, the two bat teams played equally well. The platform was perceived to be closer to helium-rich air and normally farther away from the air. So it doesn’t matter what you learn from the environment in which the bats grew; the perception of the speed of sound was the same. This suggests that the perception of bats is innate.
This is surprising because bats experience changes in weather and pitch, which can also change the speed of sound, often by more than 5 percent. Therefore, it may be advantageous to adapt the ecolocation to the conditions. But Amichai and Yovel introduced adult bats into the helium environment for a few weeks and found no indication of where the feeding station was. That was true even in an atmosphere that had 27 percent helium. So the knowledge that bats have about the speed of sound seems to be blocked in itself.
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