- It was Shark Week at the end of July.
- To celebrate, YouTuber Mark Rober decided to test out one of the most widely quoted theories about the underwater predators – that they can sniff out a drop of human blood from a mile away.
- In their first experiment, 41 sharks visited a site that pumped out 2 litres of cow blood, but there was little interest in human blood in the second experiment.
- This was probably because there was a lot less of the blood, as the experiment relied on human donors already aboard the boat.
- “I think it’s safe to qualitatively say that if no sharks came to check out 15 drops of human blood a minute in the middle of shark-infested waters, you’re probably going to be OK with a small scrape,” Rober said. “There certainly won’t be some kind of feeding frenzy with a single drop of blood from all sharks within a mile.”
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In the spirit of Shark Week at the end on July, the YouTuber Mark Rober decided to test out one of the most widely quoted theories about sharks – that they can smell a drop of human blood from a mile away and therefore hunt those with open wounds.
Rober, who is a former NASA engineer and calls himself a “friend of science,” conducted an experiment on his YouTube channel, which he said would be more rigorous than previous tests.
He travelled to the Bahamas and consulted with a marine biologist named Luke Tipple. The experiment consisted of four surfboards, all 50 meters away from the boat they would be watching from, and all with 2 litres of a different liquid to be pumped out in the vicinity – fish oil, cow blood, seawater as a control, and then urine.
“I’ve heard from surfers that a lot of them won’t pee in their wetsuits, because they feel like it’s going to attract a shark,” Rober said.
Then, over the course of an hour, they would count the number of sharks that visited that surfboard.
“OK Mark, I can confirm that all four boards are working great,” Tipple said from the depths of the ocean, where he spotted the blood coming out of the board, and several sharks. “I’m going to head back to the boat just so I’m not a variable in this experiment.”
There wasn’t much action after 10 minutes, though a couple of sharks took notice after about 20 minutes.
Then, about 15 minutes before the end, a train of sharks were following the blood trail that was spewing out in a line.
In total, 41 sharks visited the board with the cow’s blood, four visited the fish oil, and none fancied the urine or the control.
“The real question was just how much blood is interesting to them,” Rober said. “And there was one part of the experimental design that was really nagging at me.”
The researchers then decided to try human blood, for which they needed to ship in a phlebotomist (someone trained to draw blood) from the shore.
They placed the control in the middle this time, flanked by two boards that would pump out blood – one slowly at one drop a minute and the other quickly at one drop every four seconds.
After one hour, the footage showed zero sharks had visited any of the boards.
“I think it’s safe to qualitatively say that if no sharks came to check out 15 drops of human blood a minute in the middle of shark-infested waters, you’re probably going to be OK with a small scrape,” Rober concluded, noting that this wasn’t an exhaustive experiment.
“There certainly won’t be some kind of feeding frenzy with a single drop of blood from all sharks within a mile,” he said.
You can watch the full video, which has over 30 million views, below.