It is now well established that bats can develop a mental image of their environment using echolocation. But we’re still figuring out what that means: how bats take the echoes of their own vocalizations and use them to figure out the location of objects.
In a paper published today, researchers provide evidence that bats participate in echolocation in part because they are born with an innate sense of the speed of sound. How did the researchers study this phenomenon? By raising bats in a helium-rich atmosphere, where lower-density air causes an increase in the speed of sound.
Echolocation is pretty simple in principle. Bats produce sound, which bounces off objects in their environment and then returns to the bat’s ears. For more distant objects, the sound takes longer to return to the bat, providing a sense of relative distance.
But bats can use echolocation to identify their prey in mid-flight or choose a place to land. For that, they need to have a sense of absolute distance. It is not enough to know that the branch you want to land on is closer than the house behind it; you have to know when to start all the complex movements involved in holding onto the branch or you can collide with it or try to come to a complete stop in midair.
The easiest way to get an absolute distance is to get an idea of the speed of sound. With that, the delay between a vocalization and the return echo will provide an absolute distance. But how do you test whether bats have any sense of the speed of sound?
They were Amichai and Yossi Yovel from Tel Aviv University decided there was a simple method: change the speed of sound. One of the factors that influences the speed of sound is the density of the air. And there’s a simple way to alter the density of air: add lighter-than-air gases to it. 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.
(Sadly, it has not been tested whether bats raised in this environment thought they sounded funny or not.)
Distances measured by bats
A faster sound speed would mean that the reflected echoes would return to the bat more quickly. That, in turn, would mean that the object creating those echoes would be perceived closer than it actually is. So if we could somehow figure out how closely a bat perceives an object, we can get a measure of its understanding of the speed of sound.
Fortunately, the bat species used in these experiments changes its echolocation sounds as it approaches an object. So by tracking the noises bats make when approaching an object, we can get an idea of how close they think they are to it.
To do this experimentally, the researchers grew the bats in an enclosure with a feeding station at a set distance, with one group reared in normal air and another in helium-rich air. They then exchanged the atmospheres of the two groups. For bats that were raised on helium, the slower air velocity would make the echoes take longer to arrive and thus the feeding station seem more distant. The opposite would be true for bats that had been raised in normal air.
It turns out that both groups of bats behaved the same. They perceived that the platform was closer in helium-rich air and farther away in normal air. So it doesn’t matter what bats have learned from the environment in which they grew up; his perception of the speed of sound was identical. This suggests that perception is innate to bats.
That’s a bit surprising given that bats experience changes in climate and altitude that can alter the speed of sound as well, often by more than five percent. Therefore, it may seem advantageous to be able to adjust echolocation according to conditions. But Amichai and Yovel placed mature bats in the helium environment for a few weeks and found no indication that the bats could adjust their perceptions of where the feeding station was. This was true even in an atmosphere that contained 27 percent helium. Therefore, the bats’ knowledge of the speed of sound appears to be blocked.
Does it matter? It is difficult to say. The bats in the experiment often did not land properly, but that could be due to differences in aerodynamic lift produced by pressure changes. In contrast to echolocation, the bats actually seemed to make adjustments here, moving their wings at a larger angle to compensate for the lack of lift.
In any case, the problem of flying did not influence the bats’ perception of distance. Bats used to start echolocation before taking off; this provided an indication of how far the bats thought the feeding station was.
So while it might be advantageous to have a more accurate perception of distance in a variety of conditions, bats do not appear to have developed the ability to adjust their perception. That could be because the lead isn’t big enough to make a difference. Or it could be offset by competitive advantages, such as the ability to perceive distance relatively accurately without having to learn, which could make a big difference on the animals’ first flights.
PNAS, 2021. DOI: 10.1073 / pnas.2024352118 (About DOIs).