Dolphins Can Scream Underwater, But It’s Never Loud Enough

Mammals swim in the ocean through a sound world. But in recent decades humans have turned up the volume, blasting the waters with the noise of ships, oil and gas exploration, and military operations. New research suggests that this anthropogenic noise can make it difficult for dolphins to communicate and work together.

When dolphins cooperated on a task in a noisy environment, they were not so different from city dwellers trying to be heard over the noise of jackhammers and ambulance sirens.

They screamed, calling louder and louder, researchers reported Thursday in the journal Current Biology. “Even so, there is a dramatic increase in the frequency with which they fail to coordinate,” said Shane Gero, a whale biologist at Carleton University in Ottawa, Ontario (Canada), who was not involved in the work. The effect of the increased noise was “remarkably clear”.

Scientists worked with a pair of male dolphins named Delta and Reese in an experimental lagoon at the Dolphin Research Center in the Florida Keys, Florida. The pair were trained to swim to different spots in their enclosure and press a button within a second of each other.

“They were always the most motivated animals. They were really excited about the task,” said Pernille Sorensen, a biologist and doctoral student at the University of Bristol in England. The dolphins would talk to each other using whistles and would often whistle right before pressing the button, she said.

Sorensen’s team channeled sounds using underwater speakers. Tags attached behind the animals’ vents captured what the dolphins heard and “said” to each other, as well as their movements.

Through 200 trials with five different sound environments, the team observed that the dolphins changed their behavior to compensate for the loud noise. Cetaceans turned their bodies towards each other and paid more attention to their location. At times, they nearly doubled the length of their calls and amplified their whistles, in a sense screaming to be heard above background noise or a recording of a pressure washer.

The noisier it got, the less successful the dolphins were at the task. At the loudest condition, they were successful 62.5% of the time, pressing their buttons simultaneously about 20% less than at ambient sound levels.

“It was surprising to see how much the success rate dropped,” Sorensen said.

In the past, researchers have observed wild dolphins changing their behavior when boats are around. For example, scientists in Australian waters have observed fewer dolphins as the number of dolphin-watching tour boats has increased. But no one had yet investigated how anthropogenic sounds can disrupt the ability of animals to cooperate.

“It’s usually very difficult to do these kinds of studies in the wild,” said Mauricio Cantor, a behavioral ecologist at Oregon State University in Newport who was not part of the study. But the experimental setup used by Sorensen’s team provided “clear evidence of the effect of noise,” he said, because the researchers were able to control for almost anything that might interfere with their results.

Dolphins hunt together, using sound to communicate and find their way through echolocation. They also use sound to bond with their families and whistle to signal their presence in pairs, Gero said. In noisy environments, “animals cannot talk to each other”. In the long term, Cantor said, these conditions can affect food intake and reproductive ability.

There may already be parts of the oceans that are no longer usable for these animals, Gero said. Dolphins can migrate from places where they cannot communicate successfully. This phenomenon may already be taking place in large ports in places like Los Angeles or Boston, he said.

Ships are a major driver of noisy ocean soundscapes, and in some spots, ships are slowed down to protect marine life from noise pollution, Gero said. For example, along parts of the Pacific coast, from Washington state to Chile, boats are instructed to reduce speed or change course to reduce noise that could disturb marine mammals.

“We’re already impacting animals in this way,” Gero said. “The sad reality is that, in some ways, this story is 35 to 50 years too late.”