It is now well established that bats can develop a mental picture of their environment using echolocation. But we still understand what that means – how bats perceive the echoes of their own vocalizations and use them to understand the location of objects.
In an article published today, researchers provide evidence that bats are involved in echolocation in part because they were born with an innate sense of speed of sound. How have researchers studied this phenomenon? By raising bats in a helium-rich atmosphere, where lower-density air increases the speed of sound.
Echoing the location
In general, echolocation is quite simple. The bat produces a sound that bounces off objects in its surroundings and then returns to the bat̵
But bats can use echolocation to identify mid-flight prey or choose a place to land. For this, they must have a sense of absolute distance. It is not enough to know that the branch you want to land on is closer to the house behind it; you need to know when to start all the complex movements associated with fixing on the branch, or you may encounter it or try to stop completely in the air.
The simplest way to get absolute distance is to feel the speed of sound. Thus, the delay between vocalization and the return echo will provide absolute distance. But how to check if bats have any sense of the speed of sound?
Eran Amichai and Yossi Yovel of Tel Aviv University decided 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. And there is a simple way to change the density of the air: add it with gases lighter 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 percent.
(Whether or not bats raised in this environment sound ridiculous have not been tested.)
Measured distances with a bat
A faster speed of sound would mean that the reflected echo would return to the bat faster. This in turn would mean that the object that creates these echoes will be perceived more closely than it actually is. So, if we can somehow understand how close the bat perceives the object, we could get a measure of their understanding of the speed of sound.
Fortunately, the species of bats used in these experiments change their echolocation sounds as they approach an object. So, by tracking the noises that bats make when they approach an object, we can see how close they think they are.
To do this experimentally, the researchers kept the bats in an enclosure with a feeding station at a certain distance, with one group being raised in normal air and another being kept in helium-rich air. Then they exchanged atmospheres for the two groups. For helium-bred bats, the lower air velocity will make the echo take longer, making the feeding station look farther away. The opposite would be true for bats raised in normal air.
As it turned out, both groups of bats behaved equally. They perceived the platform as closer to helium-rich air and away from normal air. So it doesn’t matter what the bats have learned from the environment in which they grew up; their perception of the speed of sound was identical. This suggests that the perception is innate in bats.
Not set up
This is a little surprising, given that bats experience changes in weather and altitude, which can also change the speed of sound, often by more than five percent. So it may seem advantageous to be able to adjust the echolocation according to the conditions. But Amichai and Yovel put mature bats in helium for a few weeks and found no indication that bats could adjust their perceptions of where to feed. This was true even in an atmosphere that was 27 percent helium. In this way, bats’ knowledge of the speed of sound seems to be locked in place.
Does it matter? It’s hard to say. Bats in the experiment often fail to land properly, but this may be due to differences in the aerodynamic lift caused by changes in pressure. Unlike echolocation, bats actually seem to have made adjustments here, swinging their wings at a greater angle to compensate for the lack of lift.
In any case, the flying problems did not affect the bats’ perception of the distance. Bats often begin echolocation before taking off; this gave an indication of how far the bats thought it was a place to feed.
So, although it may be beneficial to have a more accurate perception of distance under different conditions, bats do not seem to have developed the ability to correct their perception. This may be due to the fact that the advantage is not large enough to make a difference. Or it can be offset by competitive advantages, such as the ability to perceive distance relatively accurately without having to learn – which can make a big difference in the first few fields of animals.
PNAS, 2021. DOI: 10.1073 / pnas.2024352118 (For DOI).