Increasing levels of human noise may impair dolphins’ ability to communicate and cooperate, making them “shout” over the din to get the job done, according to a new study.

“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,” first author of the study Pernille Sørensen, from the University of Bristol, said in a statement. “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,” co-author Stephanie King added.

Two male bottlenose dolphins of the species Tursiops truncatus, named Delta and Reese, were studied while they performed a cooperative task where they had to each push a button in different locations within one second of each other. The dolphins completed the task with low noise, medium noise, high noise, very high noise, or an ambient control, with 40 trials taking place for each condition. The noise was either broadband filtered noise or the noise of a pressure washer used to clean the lagoon.

The amplitude of whistles increased with noise levels, with Reese getting 0.08 decibels louder and Delta getting 0.14 decibels louder per 1 decibel increase in noise. The duration of whistles also increased, with Reese lengthening them by 2.8 milliseconds and Delta by 7.2 milliseconds per 1 decibel increase. Compared to the ambient noise control, Reese’s and Delta’s whistles were on average 1.85 and 1.66 times longer, respectively, in the very high noise trials.

Reese and Delta were also more likely to visit the opposite end of the lagoon and come closer to each other with increasing noise levels, although there was no evidence of them spending more time in sustained close physical proximity. The study authors also point out that “recent work has shown that bottlenose dolphin hearing sensitivity is directional, not only for echolocation clicks, but also for communication signals.” Indeed, the dolphins were more likely to orient toward each other as noise levels increased.

However, their success rate still dropped from 85 percent in the control trials to 62.5 percent in the very high noise trials. “This shows us that despite them using these compensatory mechanisms, their communication was impaired by noise,” Sørensen said. “Despite being highly motivated and the fact that they know this cooperative task so well, the noise still impaired their ability to successfully coordinate.”

Bottlenose dolphins aren’t the first species observed to shout over human noise – in 2015, male eastern bluebirds (Sialia sialis) were shown to increase the volume and pitch of their songs in noisier environments. A 2016 study showed that noise from ships impairs orca communication. And a 2019 study showed that human noise pollution could threaten the survival of a wide array of species, both terrestrial and aquatic.  

The new study is published in the journal Current Biology.