Wednesday, June 29, 2016

A shark can smell blood from a mile away....Just how does that work??

We've all heard that a shark's sense of smell is so acute that it can smell a drop of blood from a mile away in the ocean (or a quarter-mile, or whatever--some vast distance, the specific number hardly matters). But how does that work? How can a drop of blood, physically present at point X[1] be detected by a shark nostril at point Y, one mile away?

The answer is--it can't.

It's not like smell is transmitted as a fast-moving wave. It is based on parts per million. The key bit--parts. Molecules have to make their way to the shark nostrils. A molecule a mile away is, by definition, not being detected by a shark's nostrils.

So where does this myth come from? And is it a myth, or just a crude misconstruing of the actual facts?

I strongly suspect it's the latter (with the "a mile" part being a convenient exaggeration...I get the say "a kilometer" in the rest of the world). I think a correct illustration of the sensitivity of shark-smell would to say that a drop of blood, as it diffuses through the ocean, generates enough ppm that a shark can detect blood, at a much later time, after diffusion has spread it out a quarter-mile[2] distant from its origin.

That's not the same thing as saying that when you cut your foot on a rock, you risk summoning all the sharks within a quarter-mile radius.

NOTES
[1] More technically, diffusing slowly out from point X.
[2] Without researching it too heavily, it sounds like 1/4 mile, not a full mile, is the correct distance.

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