Study reveals dolphins ‘shout’ to each other to ‘suppress man-made noise’

New research reveals that dolphins “yell” at each other to make their voices heard above increasing underwater noise levels.

But they don’t always succeed, scientists say, as man-made noise can have a long-term negative impact on the health of the species.

Previous studies have shown that dolphins are social, intelligent mammals that rely on whistles and echolocation to hunt and reproduce.

However, this means that man-made noise from activities such as drilling and shipping has the potential to adversely affect the health of wild dolphins.

The new study, published in the journal Current Biology, shows that dolphins “scream” in response to increased underwater noise levels as they try to work together.

First author of the study, Pernille Sørensen, PhD candidate at the University of Bristol, said: “The same reasons that make sound so advantageous for animals also make them susceptible to disturbance from ambient noise.

“Over the past few decades, we’ve seen a dramatic increase in man-made noise, and noise pollution in the oceans is no exception.”

The two dolphins observed in the study, Delta and Reese, were placed in an experimental lagoon and equipped with sound recorders with suction cups to document their sounds.

The dolphins had to work together to press their underwater buttons, one second apart, placed at either end of the lagoon.

They were released from a starting point in each trial, and in some trials one of the dolphins was held back for 5 to 10 seconds while the other was released immediately.

In the delayed release trials, the dolphins had to rely solely on voice communication to coordinate the button press.

The research team found that when playing increasing levels of noise from an underwater speaker, both dolphins compensate by changing the volume and length of their calls to coordinate the button press.

From the lowest to the highest noise level, the dolphins’ success rate dropped from 85 percent to 62.5 percent.

The researchers discovered that dolphins not only change their voices, they also change their body language.

As noise levels increased, dolphins were more likely to change direction to face each other and swim to the other side of the lagoon to get closer.

Ms Sørensen said: “This shows us that their communication is affected by noise despite using these compensatory mechanisms.

“Our study shows that, despite their attempts to compensate, their high motivation, and their great knowledge of this collaborative task, noise hinders their ability to coordinate successfully.”

While the study was conducted with dolphins living in human care, human-induced noise could also have potentially harmful effects on wild dolphins, the researchers said.

Co-author Dr Stephanie King, Associate Professor at the University of Bristol, said: “For example, if groups of animals in the wild are less efficient at foraging collaboratively, this will negatively impact individual health, which will ultimately affect population health.

“Our study shows that these adjustments are not necessarily sufficient to overcome the negative effects of noise on communication between individuals.”

Because dolphins rely on their communication skills to hunt and reproduce successfully, noise levels can affect their behavior, which in turn could affect population health, he said.

Ms Sørensen said: “This collaboration with our international colleagues at the Dolphin Research Center has provided us with a unique opportunity to investigate the impact of noise on animals working together in a controlled environment, which is nearly impossible to do in the wild.”

To conduct similar research in the wild, the team needs to better understand when animals are actively working together and how cooperative behaviors are coordinated, he said.

Ms. Sørensen added: “Our results clearly demonstrate the need to take into account how noise affects group tasks in wild animals.”

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