Wild Blacktip Sharks Can Hear a Sound Nearly 250 Feet Away and Swim Away From It

Wild blacktip sharks can detect low-frequency sounds from nearly 250 feet away and work out which direction they came from, according to a new field study from Florida Atlantic University.

The experiments took place in shallow, clear water off Palm Beach County, where blacktip sharks gather seasonally, which gave the team a rare chance to watch free-swimming animals instead of captive ones. The findings were published in the journal Integrative Organismal Biology.

Researchers lowered an underwater speaker into the water and played controlled sounds while tracking the sharks from above with drones flying 40 to 50 meters up. They tested three frequency bands, 100 to 200 hertz, 200 to 400 hertz and 400 to 800 hertz, and used a 10-kilohertz tone outside the sharks' hearing range as a control. The speaker was placed up to 19 meters from the animals. Video was then analyzed frame by frame to measure how the sharks reacted.

The sharks rapidly changed course when they detected a sound, and they turned away from its source. That reaction is the evidence that they can judge direction, not just notice that something is making noise. The control tone produced no such response.

The team found that sharks were more sensitive to lower frequencies, picking them up from greater distances and at lower sound levels. More than 70 percent of the shark responses happened in what scientists call the acoustic far field, where sound is traveling as a pressure wave rather than as the local water movement seen right next to a source. The distance at which the animals reacted came to about 74 meters, or nearly 250 feet.

"Being able to detect and respond to sounds from hundreds of feet away gives these predators an important source of information," said Stephen Kajiura, the lead researcher. Caroline Sullivan led the work as part of her master's degree, and Edmund Gerstein is a co-author.

Sharks lack the gas-filled swim bladder that many bony fish use to sense sound pressure. Scientists believe they rely instead on inner ear structures, including one called the macula neglecta, to pick up vibrations and movement in the water. The new results suggest that system works over a longer range than many had assumed in wild animals.

The study adds to the picture of how sharks combine senses, including smell, vision and electrical detection, to find food and avoid trouble.

The team says the field approach gives a more realistic measure of shark hearing than tests on captive animals, which are usually limited to small tanks.