How must bats use their sound to get most help in catching insects ?
As we learned earlier,
A bat wants to be able to do two things with the help of his calls.
1. He wants to know about the latest location of the insect at small intervals.
2. He wants to be able to sense far away insects.
Can he do both these things at one time?
How should he produce sound to be able to do both of these tasks?
First let us find out how he can do the first of these tasks:
Getting the latest location of the insect at a fast rate:
What happens when a bat starts clicking at a very fast rate, which means he produces a large number of clicks in a second?
Faster clicks means echoes also return at a fast rate, bringing the new information about the location of the insect. The bat gets to know the location of the prey more and more accurately till it finally captures it.
So, if a bat wants the location of the insect at small intervals, it should emit calls at a high rate.
If there are too many calls in a second, what can you say about the gaps / intervals between the calls?
The gaps will get lesser and lesser as you pack more and more calls in a second.
For example :
Our bat is producing 10 clicks in a second and 1 clicks last 0.05 seconds.
1 click takes 0.05 seconds , so 10 clicks take 0.5 second.
But he produces this many clicks in the whole 1 second. So, how much time does he spend not clicking in a second?
1 second – 0.5 second = 0.5 second.
Suppose after each call , they remain silent for an equal time. This means if a call is 0.05 second long , the silence time or gap / interval between them is also 0.05 seconds.
If each interval is 0.05 seconds long, how many such intervals will be there in remaining 0.5 seconds during which our bats are not clicking?
0.5 divided by 0.05 gives you 10.
This means they emit a call 0.05 sec long, wait in silence for the echo for 0.05 sec, then emit a call 0.05 sec long, wait in silence again for 0.05 sec and so on. For how may times? 10.
So, the calls they emitted in a second looks like this :
This looks like the sound wave trains we studied earlier. The peaks tell you about the time in which the bat is making a call and the fall between the two nearby peaks tells you the time for which he is silent and waiting for the echo of a previous call to return.
Now let us understand what does he have to do to sense far away insects, the second thing he wants his calls to do for him.
We know that bats can only wait and listen for the echo of only one call at a time.
They emit a call, wait for its echo, when they get it send out another call and so on.
If our bat waits for 0.05 seconds, how far will the sound travel?
Can you recall the speed of sound we learned about before? 340 meters per second.
Also we learned that distance = speed x time
So, distance traveled by sound = 340 m/s x 0.05 sec = 17 meters.
But this sound first goes towards the insect and then comes back as an echo. So it has to travel the same distance twice.
Going both ways, he travels 17 m. So, the insect is half as far which tells us that it is 17 / 2 = 8.5 meters far.
This tells us that if a bat waits for 0.05 seconds, it can sense insects only as far as 8.5 m. No farther.
We did a similar calculation when we learned about how a bat can tell from the echo, how far is the insect.
What if it approaches the insect? We know that then it begins to click 100 times in 1 second.
As before we let the silence time be equal to the duration of the call.
Then there will be 100 call times + 100 interval times = 200 equal durations in a second.
So, one duration is 1 divided by 200 which equals 0.005 seconds long.
This is the total of the call time and interval which are equal.
So, we get these times as 0.005 divided by 2 = 0.0025 seconds.
So, bat calls for 0.0025 seconds , then waits for the same time for the echo and so on.
How far will the sound go in this 0.0025 second?
340 m / s x 0.0025 s = 0.85 m which is 85 cms.
Again , sound covers the same distance twice , so we divide this by 2 to get how far is the insect.
So, the bat detect insects as far as 85cms / 2 = 42.5 cms!
So, if a bat produces clicks with small intervals, he is able to detect only preys that are very near.
To be able to detect insects from afar, our bat should produce clicks at a large interval.
But small intervals mean many clicks in a second which help bats get newer locations of their prey very quickly.
So, we summarize what we have learned in the following way :
A bat has to make this choice constantly, If it wants the new successive locations quickly, it should produce clicks with small gap/interval. If it wants to be able to detect prey at a far distance, it should produce clicks with large gap/interval. A bat is always making this choice while flying : “ Do I want to know the new location of the prey or do I want to be able to detect prey at a far distance?” It produces clicks at a rate which it finds best.
Photographs below show them in action.
photo 8.3 photo8.1
Tiger Moths are one of their insect preys which can sense their echo location clicks, make a click of their own and trick them!