I was reading my handy checklist for avoiding shark attacks last night, and it said that sharks are attracted to blood. Makes sense, but it occurred to me - how does it attract sharks?
It makes sense that bears, for example, are attracted to the smell of blood, because the scent can carry very far, and they have a hyper-sensitive sense of smell. But in the water, how, physically, does the shark sense the blood? Are they sensitive to iron or some component of red blood? Would they be attracted to blood plasma, blood serum? What about dried blood?
My checklist went so far as to suggest women not swim in the ocean when they menstruate.
I don’t know what component of blood the sharks are sensitive to, but molecules diffuse through water just like they do through air, and sharks have a damn keen sense of smell. From here:
It is not just blood. They eat live fish too. I have a couple turtles in a large tank. If I feed one while the other is at the bottom across the tank sleeping with his back to me ,he will wake up when the meat hits the water.It is not splashing. The turts like to have the ham put right into the old food hole.
CookingWithGas’ quote leaves a little to be desired. Sharks can’t smell anything that isn’t right in the water that goes through their olfactory glands. So it can smell blood at one part per million, but it can’t do that if the one part is nowhere near them…
Well, presumably it starts as one million parts per million close up, and then starts dissolving.
to me, though ‘one teaspoon in a swimming pool’ isn’t really that impressive. How much does that need to be to get a shark’s attention half a mile away, in water eighty feet deep?
I’m picturing sharks “inhaling” water through their nose(?) and detecting the “smell” of blood via particulates of blood? Is is the same process humans use, except with water instead of air?
Okay, I’ve worked out my arbitrary example.
A cylinder 80 feet high, and 1/2 mile, (2640) feet in radius, would have a volume of approx 1.75 billion cubic feet. To get 1 ppm of blood in that whole volume, you would need 1750 cubic feet of blood.
By comparison, the 6 quarts of blood in a person are about one fifth of a cubic foot. :eek:
I guess maybe I was expecting a bit too much from the poor sharks
On preview, to Sateryn76:
Yes, it’s a similar process I think. Sharks, like all fish, take in water through their mouths I believe, and pass it over their gills to extract oxygen. Somewhere along the way this water encounters the olfactory epithelium, which has all kinds of chemical receptors to recognize particular molecules and report them to the brain via the olfactory nerve.
Cecil points out in his article that the spread of chemicals in the ocean depends mostly on current flow, which moves things much faster than diffusion does. Presumably, the distance at which a shark could detect a given quantity of blood depends on how far the current takes it before it diffuses too much to be detectable.
Well, I wasn’t commenting so much on the speed of diffusion, so much as how quickly you can get below the 1ppm threshold.
I would think that a current wouldn’t affect the total volume, but would get you more distance in the direction of the current, at the expense of almost no concentration in other directions.
So the implication is that sharks only eat wounded ,bleeding prey. Seals and surfers would disagree. It is not just blood that strikes a resonance . They are attracted to many things including, noise ,movement and maybe things we still do not know.
It really isn’t that impressive at all. Think about what a tiny amount of substance diffuses through the air. Put the equivalent of one teaspoon of frying bacon in a room the size of a swimming pool, and I guarantee I’ll be able to smell it. The amount of bacon that actually dissolves in the air will be much less than one teaspoon - in fact, I doubt the most sensitive scale could detect the loss of mass from the bacon.
A shark’s sense of smell is pretty good but it gets a bit exaggerated sometimes. It’s not so incredible that sharks can smell blood from “miles away”, but it is pretty good. Sharks can follow scent trails the same way bloodhounds follow scent trails on land, so a shark can come from miles away if the blood trail in the water happens to be a couple of miles long (which can easily happen).
Sharks are VERY attracted to fish blood. They aren’t so much attracted to human blood, but they will react to it. Mythbusters did a demonstration of this on one of their episodes. You can see the difference in this video clip:
Sharks are also attracted to other strong smells. It’s not just blood. There’s a common legend that urine will mask the smell of blood and save you from shark attacks. What actually happens is that the shark will smell the strong smell of urine and will usually come over to investigate, so peeing on yourself to mask the blood actually tends to attack sharks not repel them.
It’s even less impressive than that. The 1 teaspoon per swimming pool is so wrong its not even close- 1 gallon contains 768 teaspoons, and a *small *swimming pool has 10,000 gallons, the average pool has around 20,000 gallons. So instead of 1 teaspoon per swimming pool, it’s actually about 10 times that amount, 7-20 teaspoons.
On the other hand, Cecil claimed that “experts claim they’ve seen sharks go nuts over a single drop of blood in a 2,000-gallon tank.” Assuming a drop to be 0.05 mL, which is supposedly a common standard, 2000 U.S. gallons is over 150 million drops.
I think it’s a misnomer to say sharks can smell blood from miles away.
What they can do is smell trace amounts of blood (one part per million) that happen to make their way into their nostrils miles away from their source.
If I’m not mistaken, sharks have nares (“nostrils”) in the form of pits on the dorsal side of the upper jaw, lined with olfactory sensors but not connecting with the respiratory system.
