Sharks have long been feared and one of the main reasons is because we often think of them as being dangerous and bloodthirsty killers – something that isn’t helped by films such as Jaws. Besides, we’ve all heard the myth about sharks and blood – that chilling rumor that sharks can smell blood from several miles away and can detect even the smallest drop in the ocean. But is that really true? Join us as we discover exactly how far sharks can smell blood and learn how else they detect their prey.
How do Sharks Detect Prey?
Scent is only one of several methods that sharks use to detect their prey. The other methods that they use are sight, hearing, lateral line, and electroreception. They also use taste and touch, but we’ll get to those two later. One of the first senses that sharks use to detect their prey is sound and they hear sounds from long ranges – well before their prey comes into sight. Sharks are particularly attracted to low-frequency sounds, especially those made by wounded prey.
In addition to sound, sharks use the lateral line system to detect their prey. This system is a series of fluid-filled channels that are situated underneath the skin along the sides of the body and the head. There are tiny pores in the skin which allow water to flow into the channels where sensory cells are located. These sensory cells allow the shark to detect water movements – such as currents and movements or vibrations caused by struggling prey.
The other unusual way that sharks detect their prey is electroreception. Electroreception is when sharks detect the electrical impulses that are given off by every living thing. Sharks have the ability to detect these impulses because they have lots of small pores in their skin around their snout – known as the ampullae of Lorenzi. These pores help sharks to detect their prey at close range, even if they are hidden underneath the sand.
How do Sharks Detect Smells?
Before we explore how far sharks can smell blood we need to understand exactly how they can detect smells underwater. Sharks have two nostril-like openings which are known as “nares” and are located on the underside of their snout. However, unlike mammals, they don’t use them for breathing. Their nostrils aren’t even connected to their throat or lungs as they breathe through their gills. Instead, sharks use their nostrils solely for detecting scents.
Smells are carried in the water by the currents which disperse and carry molecules from things such as blood. Sharks nostrils are lined with sensory cells which are called “olfactory epithelium”. These cells can detect the tiny scent particles that are carried in the water. The water enters the sharks nostrils and flows over the sensory cells, allowing for the particles to be detected. Once a scent is detected a signal is sent to the sharks brain where the scent is interpreted. More than half of a sharks brain is made up of olfactory lobes. These olfactory lobes are what interpret the scent and help the shark to detect whether it is a predator, prey, or a potential mate that they can smell. After that it is up to the shark to decide what action it is going to take.
Amazingly, sharks nostrils work independently. There is sometimes a delay between the two nostrils detecting a scent, especially if the scent particles are coming towards the shark at an angle. Due to this, sharks always turn their head in the direction of the nostril that has detected the scent first – even if the time delay is only a fraction of a second. This means that sharks always turn towards the direction of the scent.
How far can Sharks really Smell Blood?
As we’ve just learnt, sharks rely on tiny particles entering their nostrils to detect scents. Sharks have extremely sensitive nostrils and can detect scents from far away, but how far they can detect these scents depends on a few things. Particles from different scents disperse in the water differently. However, depending on what it is, they can detect things in the water at between 1 part in 25 million and 1 part in 10 billion. To put the latter into perspective, that’s around one drop in an Olympic sized swimming pool.
However, it’s not just the number of particles that determine how far sharks can smell blood, it’s the water currents as well. This is because the scent particles get diffused (spread around) by the water. The only way a shark can pick up the scent is by the water carrying those tiny particles to them (or the shark swims into them). So because these particles are carried by the currents, the major factor that determines how far a shark can detect blood is the direction and speed of the water. As a rule, it takes a long time for particles to travel in the water once they are dispersed. However, faster currents mean that the scent will travel quicker and further.
Under optimum conditions sharks can smell blood from a quarter of a mile away, but it takes time for the smell to travel to them. However, that distance depends on the species of the shark (some can smell much better than others), and on the direction and speed of the water. So, while it’s still a pretty good distance, it’s probably much less than we expected.
Do Sharks go Crazy when they Smell Blood?
Along with the myth that sharks can smell blood from miles away, there’s also the myth that sharks go crazy if they smell even a drop of blood. This myth has led to many people being terrified of getting even a tiny cut while in the water. However, the truth is that sharks don’t go crazy as soon as they smell blood. We’ve already determined that sharks can only smell blood from a quarter of a mile away, and the length of time it takes for them to detect it depends on the direction and speed of the water currants. Therefore, even if they smell blood they’re not going to descend on it instantly.
But what if the blood is right in their vicinity? There have been a few experiments done by brave individuals to prove that the myth is wrong. Former NASA engineer Mark Rober and marine biologist Luke Tipple conducted an experiment in 2019 as part of Shark Week to find out. They used devices attached to surfboards to pump out cows blood, sea water, urine, and fish oil over one hour to see which attracted the most sharks in an area that was already shark infested. Four went to the fish oil, none to the sea water or the urine. However, after 45 minutes around 40 lemon and tiger sharks investigated the cows blood. The result of the experiment showed that even when it’s virtually on top of them sharks don’t go crazy just for a single drop.
However, they went even further and donated blood so they could see how interested sharks were in human blood. This experiment involved a control, one that pumped out blood quickly, and one that pumped it out slowly. The result – no sharks were interested in human blood.
Why do Sharks Attack?
So, if sharks can’t smell blood from miles away, and don’t go crazy when they do smell it, why do they still attack people? Well, as we mentioned above, taste and touch are both methods that sharks use to detect their prey. Many scientists believe that a lot of shark attacks are actually the result of the “bump and bite” action. When sharks are curious about something and want to find out what it is they touch it. However, as they don’t have hands they have to touch it with their noses. This is known as the bump. The bite is when the shark then takes a bite to see if something (or someone) is edible. So, some shark attacks are simply a result of the shark being curious and taking a “taste test”. But, unfortunately for us, that taste test is often fatal.
So, what all this tells us is that sharks do have a really good sense of smell, especially when it comes to blood, but anything more than a quarter of a mile away is unlikely to reach them. Also, a little bit of blood is unlikely to attract a feeding frenzy of sharks to you. However, if one comes up to you and bumps you with it’s nose then you might be in trouble.