Nature
PUBLISHED
Ever been out in the water splashing around, shouting with glee, and then thought to yourself: gee whizz, I wonder if a shark can hear me? Turns out they could, provided they were already within around 75 meters (250 feet).
That’s according to a new study from Florida Atlantic University that used controlled low-frequency sounds and aerial drone tracking to establish the eavesdropping skills of blacktip sharks. Not only could they hear sounds from that far away, but they could determine where the source was.
Yeah, suddenly all very quiet now, aren’t we?
Just to establish the basics real quick: yes, sharks have ears. Though regrettably not as externally visible as humans’. Instead of big auditory flaps, they have modest holes on either side of their heads that lead into the inner ear inside their cranium.
To a human, the idea of sound underwater might seem a little superfluous, but it’s actually a very efficient medium. Sounds carry faster and farther in water than in the air, so sharks will be very familiar with the ocean soundscape in their local area.
They can listen out for signs of nearby prey, predators, or other things to avoid like boats. We know they’re listening, but scientists wanted to figure out just how far.
They focused on blacktip sharks because they keep to a schedule of aggregating in the waters of Southeast Florida that are conveniently clear and shallow, aka, fieldwork conditions. This meant the researchers could remotely deploy sounds and observe the sharks’ behavior without disturbing their natural behavior.
Sharks, it seems, can be incredibly shy. As co-author of the new study, Dr Stephen Kajiura, told IFLScience in a previous podcast about the hammer-shaped heads of hammerhead sharks, a lot of his work involves pretending to be a rock just so the sharks will come close enough.
The team used a boat to drag sounds of varying frequencies, using hydrophones to measure how loud the sound was wherever a shark was swimming. A drone recorded the sharks from above, and then the researchers reviewed the imagery to see how the sharks responded.
The results showed that the sharks were reacting to all of the sound ranges except the control. They could detect sound at a distance of up to 74 meters (243 feet), which is about the length of the Bayeux Tapestry.
And while our definitely-not-photoshopped header image added a little thrill to the concept of a shark hearing you scream – we’ve got great news. The sharks quickly changed direction in response to the sound, swimming in the opposite direction.
As we said, sharks really are rather shy. What’s even cooler, however, is that the study has taught us something we never knew about shark hearing before.
“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,” Kajiura said in a statement.
“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.”
Now that we know sharks are able to detect and respond to sounds from hundreds of feet away, the next step is to try and figure out what it is about their sensory system that enables them to do that. As the ocean soundscape becomes increasingly polluted by human activity – boats and oil rigs make quite a din – it’s these sorts of questions that can help us to better protect the oceans’ charmingly sensitive predators.
The study is published in the journal Integrative Organismal Biology.
