Dolphins ‘shout’ to each other to ‘drown out manmade noise’, study reveals

Dolphins “shout” to each other to try and make themselves heard above increasing underwater noise levels, new research reveals.

But they don’t always succeed, which scientists say indicates that manmade noise could have a long-term negative impact on the health of the species.

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

However, that means that manmade noise generated by activities such as drilling and shipping has the potential to negatively impact the health of wild dolphins.

Now the new study, published in the journal Current Biology, shows that dolphins “shout” when trying to work together in response to increasing underwater noise levels.

Study first author Pernille Sørensen, a PhD candidate at Bristol University, said: “Those same reasons that make sound so advantageous for animals to use also make them susceptible to disturbance from noise in the environment.

“Within the last couple of decades, we’ve seen a dramatic increase in human-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 suction-cup sound recorders to document their vocalisations.

The dolphins had to work together to both press their own underwater button placed at either end of the lagoon within one second of each other.

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

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

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

From the lowest to highest levels of noise, the dolphins’ success rate dropped from 85 per cent to 62.5 per cent.

Researchers found that not only did the dolphins change their calls, but they also changed their body language.

As noise levels increased, the dolphins were more likely to re-orient themselves to face each other, and they were also more likely to swim to the other side of the lagoon to be closer.

Ms Sørensen said: “This shows us that despite them using these compensatory mechanisms, their communication was impaired by noise.

“Our work shows that despite their attempts to compensate, despite being highly motivated and the fact that they know this cooperative task so well, the noise still impaired their ability to successfully coordinate.”

The researchers said that while the study was conducted with dolphins living in human care, manmade noise can potentially have detrimental effects on wild dolphins, too.

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

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

She said that since dolphins rely on their communication skills to successfully hunt and reproduce, noise levels can affect their behaviour, which in turn affect population health.

Ms Sørensen said: “This collaboration with international colleagues at the Dolphin Research Centre provided us with a unique opportunity to investigate the impact of noise on animals working together in a controlled setting, something that is almost impossible to do in the wild.”

She said that to conduct similar research in the wild, the team would need further understanding of when animals are actively working together and how cooperative behaviour is coordinated.

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