Dolphins hunt with sound, but their prey may be listening back

Underwater recordings reveal how dolphins use different sounds to socialize and hunt, while their fish prey may be listening for danger too.

Dolphins live inside a world of sound. Whistles carry the social chatter, while bursts of rapid clicks work like sonar, sweeping the water for obstacles and prey. 

It’s the click part that just got more interesting. New research suggests the prey might be listening right back.

The findings come from Florida Atlantic University’s Harbor Branch Oceanographic Institute. 

For two months, the team eavesdropped on the waters off Florida’s Atlantic coast, and what they overheard changes a few assumptions about how dolphins actually spend their days.

An ear that never gets tired

Underwater is not the peaceful place people imagine. Both fish and boats make noise. 

Sorting out a dolphin’s voice from all of it usually means chartering a boat and hoping you show up at the right moment.

This time, the researchers skipped the boat. They used a wave glider, an autonomous, wave-powered vehicle towing an acoustic recorder, and let it drift the East Florida Shelf for two straight months, listening the whole way.

“The ocean is filled with sounds that humans rarely hear, and autonomous platforms give us a way to enter that hidden acoustic world,” said senior author Laurent Chérubin, a research professor at FAU Harbor Branch. 

“A wave glider can travel through the ocean and listen continuously, allowing us to detect patterns that would be almost impossible to capture with occasional boat surveys.”

“It is like giving researchers an ear that can remain in the ocean for weeks at a time, revealing a world of animal behavior that is otherwise largely unknown to us.”

Nearly 62 hours of sound came back. Dolphins turned up in more than 1,600 of the recordings.

Two very different soundtracks

Once the team lined the recordings up against water depth, temperature, salinity, and other conditions, a split appeared. Dolphins seem to switch acoustic modes depending on what they’re actually doing out there.

Whistles, the social sounds, clustered closer to shore. St. Augustine and Ponce Inlet stood out as hotspots, with smaller pockets near Melbourne Beach and just north of Ponce Inlet. 

These may simply be the places dolphins go to hang out.

Echolocation clicks told a completely different story. They clustered from Melbourne down to Ponce Inlet and out toward Long John Reef, right where sound-producing fish were most abundant.

“What we’re seeing is a fascinating separation between the acoustic behaviors dolphins use for social life and those they use for finding food,” said co-author Greg O’Corry-Crowe, a research professor at FAU Harbor Branch. 

“The fact that echolocation was associated with sound-producing fish, while whistles were linked to a different set of environmental conditions, suggests dolphins may be responding to their surroundings in very different ways depending on what they are doing.” 

“In essence, the same ocean can become a different habitat to a dolphin depending on whether it is looking for a meal or communicating with other dolphins.”

The prey that goes silent

The clicks lining up with noisy fish hints that dolphins may be tracking prey by ear, essentially following the fish’s own racket straight to dinner.

But the researchers also noticed the opposite happening. Sometimes the fish chatter and the dolphin activity didn’t match up at all, as if the fish had simply gone quiet. 

One possibility is that the fish had sensed a dolphin nearby and gone quiet, although the researchers don’t yet know whether that’s what happened.

“Everybody is snooping on everybody else,” O’Corry-Crowe said. “This raises a fascinating question for our future research: are dolphins listening for fish, and are fish listening for dolphins?”

Limitations of the study

Dolphin detections weren’t steady across the study either.

March brought noticeably more activity than April, likely a sign of different dolphin populations passing through the region at different points in the year.

Several of the nearshore spots richest in dolphin whistles also happen to be busy with vessel traffic. 

Boat noise can bury the very calls dolphins rely on, making life harder for the dolphins and the job harder for anyone trying to study them.

Building a map from sound alone

None of this required spotting a single dolphin. Sound alone can sketch out which patches of ocean matter most, and why.

Study lead author Jessica Carvalho is a research scientist at FAU Harbor Branch. 

“By listening to the sounds dolphins produce, we can begin to understand what they may be doing in different areas of the ocean and identify places that may be especially important for social activity or feeding,” said Carvalho.

“That information can ultimately help us better protect critical dolphin habitat, particularly in coastal waters increasingly affected by human activity.”

Bit by bit, the recordings are adding up to something bigger than a set of coordinates on a map. They’re starting to sketch out what the ocean actually sounds like from inside a dolphin’s world.

“Every sound gives us another piece of the puzzle,” O’Corry-Crowe said. 

“When we combine dolphin vocalizations with the sounds of their prey and the physical characteristics of the ocean, we begin to see the underwater environment from the dolphins’ perspective.”

“That can fundamentally change how we think about their habitat.”

The study is published in the journal PeerJ.

—-

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

——