Often people vacationing near shark habitats do not enter the water if there is even a small scratch on the body. This is because of the common belief that even one drop of blood will attract a predator swimming a kilometer away. We checked whether these concerns are justified.
Although from shark attacks per year die on average there are only five or six people, stereotypes that have been established over decades and the frightening appearance of predators influence people’s consciousness much more powerfully than not so frightening statistics. The fear of sharks and their olfactory abilities is also reflected in popular culture - for example, in the cartoon “Finding Nemo”, scented blood returned the animal’s instincts and interfered the shark adheres to the rule “Fish is a friend, not food.” In English there is even an idiom to smell blood (literally “to smell blood”), meaning “to detect the opponent’s vulnerability” and appeared, among other things, thanks to sharks.
Sharks do have an extremely developed sense of smell. Unlike people, their breathing and smell processes are not connected: these animals receive oxygen through their gills, and information about smells through the nostrils on their faces. The body of other fish is structured in a similar way, but the nasal cavity of sharks is filled with a large number of receptors, thanks to which a huge palette of odors can be distinguished.
Based on this premise, researchers from Florida Atlantic University decided to check, is sharks’ sense of smell developed enough to smell a drop of blood at a great distance. Biologists chose for the experiment diamondback and stingrays, as well as lemon and hammerhead sharks - all of these species are included in the subclass of elasmobranchs. Each animal was placed in a container with water, and a dispenser that released different amino acids and an electrode that measured activity in the cavity with olfactory receptors were brought to the nose.
As researchers have found, sharks do not differ significantly from their counterparts in the development of their sense of smell. The maximum concentration at which animals were able to distinguish an amino acid in water was about 1 in a billion. As biologists explain, this is several orders of magnitude higher than in the case when a drop of blood enters the ocean at a distance of a kilometer from the shark. The results were not surprising to scientists, because higher sensitivity would simply drive predators crazy - then they would smell literally all the smells and would not be able to distinguish among them the really important ones.
Similar experiment spent science popularizer and blogger Mark Robert. He took a boat out into the open sea, where many sharks live, and placed four surfboards at an equal distance from it. On each of these boards, the experimenter placed a container with one of four liquids: cow blood, human urine, fish oil and sea water (as a control). Using special devices, the containers were emptied evenly, and Robert calculated how many sharks would be attracted to different liquids. Over the course of an hour, the fish oil board attracted four sharks, the blood board attracted 41, and the other two were ignored.
Having made sure that the animals were really interested in blood, Robert wondered: how much is enough for this. This time, the experimenter filled two containers on two boards with human blood, which he supplied into the water at different rates: from one board - one drop every minute, from the other - one drop every four seconds. Another container, as in the first experiment, was filled with sea water for control. This time, none of the boards attracted the attention of the sharks. “No one was interested in a device that released 15 drops of human blood per minute in the middle of shark-infested waters. Perhaps everything will be fine with you in case of a small scratch,” Robert concluded.
Not true
- https://jeb.biologists.org/content/jexbio/213/20/3449.full.pdf
- https://www.popmech.ru/science/11003-zhazhda-krovi-razvenchanie-akul/
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