One of the most widespread myths about sharks is that they are able to smell a tiny droplet of blood from kilometers away and rush to the scene of the crime to check if there is something to eat. In reality, their methods of detecting prey (and more generally other underwater objects) are more complex than this, and use a combination of sensory stimuli. One of which has so far been very little studied: let’s talk about hearing, put to the test for the first time in a study published in Integrative Organismal Biology which shows how sharks hear us very well, and even at great distances.
The shark hearing test
The study was conducted on a population of blacktip sharks living off the southeast coast of Florida. The choice fell on these selacians in particular because they are relatively easy to study: every winter they aggregate in large quantities along the Floridian coast to reproduce, close enough to the beaches to swim in perfectly transparent waters, and it is therefore possible to analyze their movements from above without disturbing them. The team from Florida Atlantic University that conducted the study followed the Floridian sharks for a few days, subjecting them to acoustic experiments to study their reactions to certain sounds.
The researchers anchored a boat off the coast of Palm Beach and attached a 20 meter long chain to the keel with an underwater speaker attached to the end: in this way they were able to make them listen to a series of sounds without the presence of the boat disturbing them too much. They tested with three different sounds, all at low frequencies (from 100 to 200 Hertz, from 200 to 400 and from 400 to 800), to which they added a “control” sound at 10,000 Hertz, outside the sharks’ acoustic range.
Up to what distance do they hear sounds?
They then filmed the blacktip sharks’ movements in response to different sounds, filming them from above with a drone flying 50 meters above the ground (another way to reduce disturbance). A frame-by-frame analysis of the videos allowed them to calculate exactly the distance at which the sounds had any effect on the sharks. Because they had it: the sounds were not designed to attract but to scare the Selacians, who demonstrated very clearly that they heard them and reacted, clearly changing direction.
What surprised the researchers is the distance at which the sharks begin to hear noises: 74 meters, much further than we had measured so far with other selachians.
According to the researchers, sharks are able to hear sound at this distance by sensing the movement of particles caused by the sound itself. Something that bony fish do using their swim bladder, which however is absent in sharks: the study explains that they use their inner ear to listen to underwater sounds. The next step, the authors say, will be to find out how these signals are interpreted by their brains.
