It’s 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 own vocalizations and use them to figure out the locations of objects.
In a paper released today, researchers provide evidence that bats engage in echolocation in part because they’re 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 the lower-density air produces an increase in the speed of sound.
Putting the location in echo
Echolocation is rather simple in principle. A bat produces 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 prey in mid-flight or pick out a location to land on. For that, they need to have a sense of absolute distance. It’s 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 latching onto the branch or you might either run into it or try to come to a complete stop in mid-air.
The simplest way of getting an absolute distance is to have a sense 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 some 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 influences the speed of sound is the density of the air. And there’s a simple way to alter the density of air: spike it with lighter-than-air gases. In this case, the authors chose helium and raised a group of bats in an atmosphere that had enough helium in it to increase the speed of sound by 15 percent.
(Whether or not the bats raised in this environment thought they sounded funny was sadly left untested.)
A faster speed of sound would mean that reflected echoes would return to the bat more quickly. That in turn would mean that the object that creates those echoes would be perceived as closer than it actually is. So if we could somehow figure out how close a bat perceived an object to be, we could get a measure of their understanding of the speed of sound.
Fortunately, the species of bat used in these experiments changes its echolocation sounds as it gets closer to an object. So by tracking the noises the bats make as they approach an object, we can get a sense 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 a set distance away, with one group being raised in normal air and another being raised in helium-rich air. They then swapped the atmospheres for the two groups. For the bats that were raised with helium, the slower speed of air would make the echoes take longer to arrive and thus make the feeding station seem farther away. The reverse would be true for bats that had been raised in normal air.
As it turns out, both groups of bats behaved the same. They perceived the platform as being closer in the helium-rich air and farther away in the normal air. So it doesn’t matter what the bats learned from the environment they grew up in; their perception of the speed of sound was identical. This suggests the perception is innate to the bats.
That’s a bit surprising given that bats experience changes in weather and altitude that can also alter the speed of sound, often by over five percent. So it might seem to be advantageous to be able to adjust the echolocation according to conditions. But Amichai and Yovel put mature bats into 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 was 27 percent helium. Thus, the bats’ knowledge of the speed of sound appears to be locked in place.
Does it matter? It’s hard to say. The bats in the experiment often failed to land properly, but that could be due to the differences in aerodynamic lift produced by the pressure changes. In contrast to echolocation, the bats actually did seem to make adjustments here, sweeping their wings across a larger angle to compensate for the lack of lift.
In any case, the flying trouble didn’t influence the bats’ perception of distance. The bats would often start echolocation before they took off; this provided an indication of how far away the bats thought the feeding station was.
So even though it might be advantageous to have a more exact perception of distance under a variety of conditions, bats don’t seem to have evolved the ability to adjust their perception. That could be because the advantage isn’t large enough to make a difference. Or it could be offset by competing advantages, such as the ability to perceive distance relatively accurately without having to learn—which could make a big difference in the animals’ first few flights.