Animal Senses #10: Bats and Dolphins

How bats navigate is well known – via sonar (sound waves) and their reflection. Correct, and it involves at least 10 challenges, lists Ed Yong in in Immense World: (1) distance. Sound loses energy quickly, so the bat can only detect objects at 6-9 yards; (2) volume. Bats “scream” at volumes of nearly 138 dB (that’s as loud as a jet engine). It must be deafening for the bat; (3) speed. It needs to send and process sound very quickly. After all, its prey will dodge and move; (4) overlapping. It would be easy to get lost in its own echoes. Which is the first echo? The third echo?; (5) multiple frequencies. Bats sound out at multiple frequencies – lower frequencies help identify large objects; higher frequencies with small objects. There is so much different info coming in via the echoes and it has to be made sense of; (6) adjustments. In the scan mode, a certain combo of frequencies and duration makes sense. Once a prey has been identified, a different combo is needed; (7) cluttered environments. Echoes from two branches at the same distance would reach at the same time. How to make out if the echoes are from the same branch or from two different branches? (8) background. Take a moth on a leaf. The echoes from both would reach at the same time. How to even identify that there’s a moth on a leaf? (9) groups. Bats fly in groups, which means they need to differentiate their screams and echoes from those of their neighbors; (10) backfiring of optimization. Solving all of the above problems is exhausting and uses up precious energy; so bats optimize by remembering stuff closer to home. But if things close to home change, the bat wouldn’t realize it since it is navigating by memory, not sonar. Often with disastrous consequences.

 

I won’t get into the bat’s solutions for all of the above – with so many problems to solve, it would be too long to describe them all here – but the list of challenges alone should make you appreciate the bat.

 

In the eternal cat and mouse game, what are some of the defenses of the bat’s prey? Some species have developed “acoustic armour”, scales on their body that absorb or muffle the bat’s sound. Others can hear the bat’s shriek – they then dodge and take other evasive action before the echo can reach the bat. Tiger moths can “talk back” and confuse the bat – what was that sound? An echo? Something else? Luna moths have long tails that they can live without. They flap the tail a lot such that it is the part of the moth that echoes the most; the bat attacks and tears off the tail and spits out the unappetizing bite by which time the luna moth has vanished.

 

The other animal that uses sound heavily is the dolphin. It too uses sound and echoes. Since dolphins are larger and friendly, they have been studied in great detail. Dolhpins can differentiate objects by shape, size and material. They can even distinguish between cylinders filled with water, alcohol, and glycerine. Such is their precision that they can even detect a difference of a few millimeters in thickness.

 

Dolphins can even echolocate a concealed object and then recognize it when it is shown on a TV screen. Read that last sentence again. It probably isn’t obvious what was so great in that, right? Which is why Yong explains.

“The animal isn’t just working out the object’s position but constructing a mental representation of that object (using sound) which can be translated to its other senses (e.g. vision). And it’s doing that with sound – a stimulus that doesn’t naturally carry rich, three-dimensional information.”

 

This was the only time Yong dedicated an entire chapter for specific creatures (bats and dolphins) that use a particular sense. Totally worth it.

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