If you thought keeping quiet in the water would help you avoid a shark, think again.
Scientists have discovered the predators can hear underwater sounds from nearly 250 feet (75 metres) away and even work out exactly where they’re coming from.
Using an underwater speaker and drone cameras, researchers monitored how blacktip sharks behaved when a range of sounds were played in the waters off southeast Florida.
The animals consistently responded to low–frequency noises played through the speaker, often veering away from the source even when it was hundreds of feet away.
‘The ocean is an acoustic environment, and sharks are clearly tuned into it in ways we are only beginning to understand,’ senior author Professor Stephen Kajiura, from Florida Atlantic University, said.
‘Being able to detect and respond to sounds from hundreds of feet away gives these predators an important source of information about their surroundings.
‘The next question is how their sensory system allows them to pick up and interpret these distant sounds.’
For the study, the researchers took advantage of a natural shark hotspot off southeast Florida where blacktip sharks gather in huge numbers each winter.
The seasonal influx allowed scientists to track the predators in crystal–clear, shallow waters without disturbing them.
‘Their abundance and accessibility made it possible for us to observe them from above without disturbing their natural behaviour, while also presenting controlled underwater sounds,’ Professor Kajiura said.
The team anchored a boat and deployed an underwater speaker which drifted with the current up to 62 feet (19m) away, to minimise the boat’s influence on the sharks.
They tested three ranges of low–frequency sounds – 100 to 200 Hertz, 200 to 400 Hertz and 400 to 800 Hertz – along with a 10–kiloHertz control sound outside the known hearing range of sharks.
They played the sounds at a high intensity to startle the sharks rather than try to attract them.
The study, published in the journal Integrative Organismal Biology, found the sharks reacted to all three low–frequency sounds played but ignored the high–frequency control sound.
The predators were able to detect noises from as far as 243 feet (74 metres) away, further than has previously been demonstrated in free–swimming sharks.
Research Video of Shark HearingA new Florida Atlantic University study provides the first quantified evidence that free-swimming sharks can detect and respond to underwater sounds from distances extending nearly 250 feet. Using controlled low-frequency sounds and aerial drone tracking, researchers found that blacktip sharks rapidly changed course when they detected a sound, providing evidence that they can also determine its direction.
YouTube • Florida Atlantic University
Remarkably, many swiftly changed direction after hearing the sounds, suggesting they could pinpoint where the noise was coming from.
The researchers also found the animals were particularly sensitive to lower–pitched noises, detecting them from greater distances and at quieter volumes.
‘What makes this finding particularly interesting is that the sharks were responding to sounds beyond the acoustic near field, where the sound behaves differently than it does close to the source,’ Professor Kajiura said.
‘This suggests that they are detecting the particle motion associated with sound even at considerable distances from the source – something we have not previously been able to demonstrate in free–swimming sharks.’
The discovery is particularly surprising because sharks lack the gas–filled swim bladder used by many fish to detect sound.
Instead, scientists believe they rely on highly sensitive inner–ear structures that allow them to pick up vibrations and movement as sound waves travel through the water.
‘Trying to do hearing experiments in a tank results in the sound bouncing off the walls which causes complex and confusing signals – it is like being in a house of mirrors,’ lead author Caroline Sullivan said.
‘This is why it is so important to do these types of experiments in the ocean with wild sharks to get a natural response.’
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