Blacktip sharks have far more sensitive hearing than previously thought and can determine the direction the sound is coming from, according to a new study published in Integrative Organismal Biology.
Scientists at Florida Atlantic University found that free-swimming blacktip sharks dramatically changed direction to move away from sounds played from an underwater speaker up to 74m (243 ft) away.
This is the first time the responses of free-swimming sharks to particle motion in the acoustic far field (the region far enough away from a sound source where the sound spreads out in a smooth, predictable pattern) have been measured.
Sharks are most sensitive to irregular, low-frequency sounds that might indicate an injured or struggling fish. What we know of their hearing comes from laboratory trials, anatomical studies of the inner ear, and field work on bottom-dwelling species such as nurse sharks. Very little is known about the hearing of more pelagic species, such as blacktip sharks.
Each winter, blacktip sharks migrate to the clear, shallow waters off south-east Florida, where they gather in large numbers. Here, the study researchers were waiting with a boat, an underwater speaker, and an aerial drone.
- Orcas filmed eating great white sharks in the Gulf of California
- Sperm whales are pooping on orcas. Here's why
While filming from above, the team tested three low-frequency sounds – 100–200 Hz, 200–400 Hz and 400–800 Hz – along with a control sound outside the known hearing range of sharks. The sounds were played at high intensity to startle the sharks rather than attract them.
“We didn't know how it would work. We didn't know if the sharks would respond,” says Caroline Sullivan, lead author of the study.
The sharks responded to all three low-frequency sounds, detecting the lower frequencies from further away and at lower sound levels.
“Getting the data and having so much confidence in our experimental protocol was just so exciting,” says Sullivan.
More than 70% of responses occurred in the acoustic far field, something that had not previously been demonstrated in free-swimming sharks.
- Meet the weirdest sharks on the planet – including one that can walk:
- These are the rarest sharks in the world
The mechanism by which sharks – which lack the gas-filled swim bladder thought to help bony fish detect sound pressure – discern particle motion at such a distance is not certain.
“We think that's where the macula neglecta comes in,” says Professor Stephen Kajiura, senior author of the study, explaining how this sensory structure in the inner ear could transduce and amplify vibrations from particle motion in the far field. How might he prove such a thing? “I still need to figure that out,” he says.
More wildlife stories from around the world
- Chimps can carry their deceased infants for months. Scientists are now closer to finding out why
- This sloth has moths living in its fur. The reason why is remarkable
- New crocodile species discovered in rocks used to build Egypt’s Great Sphinx
- Marine scientists discover 57 new species in a canyon nearly twice as deep as the Grand Canyon
Top image: blacktip shark hearing study. Credit: Florida Atlantic University
