We’ve all heard the rumor: a shark can detect a drop of blood in an Olympic-sized swimming pool, so if you’ve got the tiniest of cuts on you, don’t step into the ocean!
Sharks indeed have exceptionally precise sensory apparatus in their brains and bodies to pick up scents, sounds, and even electrical impulses. But are they attracted to blood or urine?
Sharks are attracted to blood in the water. While they can smell human urine, they aren’t particularly interested in it. Any tasty, smelly odorant is likely to get a shark excited but their sense of smell is no better than that of a tuna, and tiny amounts will go unnoticed.
Let’s look at how sensitive sharks are to these bodily fluids, how they pick them up, and what effect it has on them.
Are Sharks Attracted to Blood?
Sharks are formidable hunters, but they’re also adept scavengers when they can get away with it. To capitalize on a recent kill or mop up an injured animal in an easy attack, sharks have developed a keen sense of smell or olfactory sense.
Paired nasal organs on the shark’s face give the shark a sense of direction by differentiating between minute differences in concentrations of odorous materials, and a significant proportion of some sharks’ brains is dedicated to processing smells.
However, this is more common in deep-sea and pelagic sharks than reef sharks.
While stories of their sensitivity to blood are overblown, sharks are capable of picking up the scent of blood in the water and have a keen attraction to it.
A shark’s sense of smell is, in general, about the same as other fish such as tuna, but there’s not much recent evidence of quite how sensitive that is and no conclusive evidence for their sensitivity to blood in, for example, parts per million (ppm) across many species.
In Hawaii, in 1961, researchers experimented with blacktip reef sharks and their reactions to blood. They found that the reef sharks were attracted to fresh human blood in concentrations as low as 0.01ppm.
Interestingly, they were repelled by aged blood. Similar studies with other species are hard to find.
Despite this, there are a lot of unsourced claims online that point to the fact that a shark can pick up blood from a kilometer away, detect one part per 250 million, or any other unfounded statistic you want to hear. As far as we can tell, all these claims are unsourced and not backed by any evidence.
Chum, a mixture of blood and dead fish, is often used to lure sharks, but it does take quite a bit to get them interested.
It’s not likely that the myths around a shark’s hypersensitivity to blood are true, since dilution happens at an exponential rate with distance from the source, to the point at which it’s safe to say a shark will not be able to pick up the scent of a drop of blood from a kilometer away.
Are Sharks Attracted to Urine?
Urine is another odorant that sharks can sense; however, they don’t seem particularly interested in it one way or another.
However, this information comes from trials with human urine and reef sharks, which are not a typical food pairing. The fact that the sharks could sense the urine may suggest that urine from their natural prey might be attractive to them.
Fish do urinate, so sensing this would be advantageous to their predators, but so far, not much information is available on whether sharks are attracted to fish urine.
One interesting digression here is that in the same experiment with reef sharks and human fluids, researchers found that blacktips and tiger sharks can sense human sweat in concentrations of 1 to 2 ppm, but they hate it!
In some cases, the smell was foul enough to elicit a stress response from the sharks, such as head shaking.
This suggests that in the ocean – much like on the bus – a strong, sweaty body odor is a great way to be left alone.
What can a Shark Sense?
While we’re on the topic of shark senses, it’s worth mentioning a handful of the other highly-adapted receptors a shark’s body is decked out with.
- Vision – Sight is a short-range sense in the ocean, but sharks do use it. Tests show that white sharks use their large, cone-rich eyes to identify prey at short ranges after using olfactory senses to find it.
- Hearing – sharks can hear and have well-developed ear structures, but there’s evidence that their hearing is not as good as in other fishes, though the ability to localize a sound source is strong in some sharks.
- Pressure – Sharks have a sensory array that is used to respond to pressure and depth. It’s not known whether this is also used to ‘hear’ the pressure wave of sound, but it is possible.
- Mechanosense – Detecting water movements is critical for a shark. Avoiding predators, sensing prey, and finding mates all rely on picking up changes in the water. However, not much is known about this sense in sharks yet!
- Electrosense – This is one of the most sensitive receivers a shark has. All sharks have this sense, and it is fine-tuned to detect weak electrical signals coming from the nervous systems of pretty items.
Sharks have incredible senses – but they’re not magic. In fact, their sense of smell is similar to that of a tuna. They can smell blood, but they’re not going to hunt you down if you enter the ocean with a paper cut.
The range of sensitivity of smell depends on the shark’s habitat; sharks who live in deep or murky expanses tend to have a better sense of smell than those who live in the crystal-clear reef waters. They can also rely on other senses most animals don’t have, like electroreception.
One thing they seem to hate, however, is the smell of human sweat. So, keep that in mind if you’re worried about shark attacks when swimming!