Field of Science

How can hammerheads smell blood a full kilometer away?

Here's a report about research into the mechanism of the hammerhead shark's incredible sense of smell.

It is said that hammerheads can smell a drop of blood up to one kilometer away. How the hell is that possible? I'm not surprised if the sharks can detect really, really small amounts of certain molecules. Rather, I am surprised that any blood molecules appear up to a kilometer away. Obviously, I understand that can happen, but the way it is told, it sounds like you drop the blood in the water, and next thing you know the sharks can smell it, and that seems impossible. Can two hammerheads located two kilometers apart both smell blood right in between them? Does the blood really diffuse that far, and that fast? How fast? Is this diffusion isotropic?

Very strange.


  1. I've had similar thoughts about that little "factoid". Your post inspired me to do some calculations.

    Let's generously assume that the drop of blood has been perfectly evenly dispersed among the volume of water between the bleeding organism and the shark. Obviously it's not going to go directly at the shark, so we have to assume it disperses in a roughly circular pattern -- well, spherical actually, but for the sake of optimistic calculation, I am going to assume fairly shallow water, so the spherical effect can be ignore.

    As I said, I'm assuming shallow water. Let's say 5m deep. So if the volume of water in which the blood has to be detectable for the shark to smell it a kilometer away would be pi*r^2*h where r=1km and h=5m, so about 1.6 * 10^6 cubic meters.

    Assuming human blood, there is something on the order of 5 * 10^9 blood cells per milliliter. A "drop" is ambiguous, but for optimism's sake, let's take the Imperial drop, which is about 99 uL. So our drop of blood gives us about 5 * 10^14 blood cells to work with.

    This leave us with 3.1 * 10^8 blood cells per cubic meter, or about one blood cell every 0.003 mL.

    So if the blood disperses very well, it seems quite plausible the shark could detect it. But of course, that number was assuming a perfect dispersal, and obviously that's not going to happen.

    I wonder if the "factoid" is based on a calculation somewhat like this? Like, somebody observed that a shark could detect a ridiculously diluted blood solution, but since audiences are not impressed by parts-per-trillion numbers, the number was twisted and exploited in to the "can smell it a mile away!" factoid...?

  2. My concern was not so much whether the shark could detect very small traces of blood, but rather if any blood molecule would travel a kilometer. And how fast.

    As you, I wonder how this evidence was obtained, that a shark can detect blood that far away. You drop the blood, and at the same time observe a shark that's 1 km away and it reacts right away? Or in ten minutes? What?

  3. Yeah, I was just wondering if it was even plausible after sufficient time for dispersal. If my calculations had come up with one blood cell per cubic meter, for instance, I would reject the entire factoid as "impossible no matter how you interpret it".

  4. In fact, the claim seems to be a factoid:

    "a questionable or spurious—unverified, incorrect, or fabricated—statement presented as a fact, but with no veracity."

  5. You know the average person is subject to 23 factoids each week.


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