Dolphins must "shout" over underwater noise to communicate

Dolphins are highly intelligent, social mammals that tend to form alliances that rely on communication between group members. Their underwater calls consist of a series of whistles, that are used to communicate with conspecifics, and clicks that are used in echolocation to identify prey items and objects in their surroundings. The animals are able to carry out cooperative tasks, such as rounding up a school of fish, that depend on successful underwater communication with others in the group.

Given the importance of underwater sound communication to dolphins and other cetaceans, it is natural to be concerned that the increasing noise levels in oceans worldwide could have significant negative impacts on the successful functioning of these animal. Ocean bed drilling, the establishment of offshore wind turbines, and increasing shipping traffic all produce underwater noise that is potentially disruptive. 

Previous studies have found that animals from several different taxa, including mammals, birds, fish and invertebrates, respond to increased noise levels in their environments and may adopt mitigation strategies such as modifying the frequency, amplitude and duration of their calls, orientating themselves to minimize the interference from the noise source, or even moving to an entirely different area. 

However, little is known about how successful these compensatory mechanisms are at overcoming the impacts of noise disturbance when individuals are actively working together. 

A new experimental study by scientists from the University of Bristol, U.K., Syracuse University in New York, The Dolphin Research Center in Florida, and the Woods Hole Oceanographic Institution in Massachusetts has now tested whether the ability of two dolphins (living in human care) to solve a cooperative task is impacted by the presence of anthropogenic noise at different intensities. The results are published in the journal Current Biology.

The two bottlenose dolphins (Tursiop struncatus.), known as Delta and Reece, had previously been trained to carry out a cooperative task that required them to understand the role their partner must play in the task, and use the sound of whistles to coordinate their behavior with extreme precision. The task required both dolphins to swim to their own stations at opposite ends of a pool. There, they had to press their own underwater button simultaneously with the other dolphin, or within a one-second time window. 

The two dolphins were equipped with suction-cup sound recorders to document their vocalizations as they carried out the task. During each of the 200 trials, the dolphins were released from a starting point but, for certain trials, one of the dolphins was held back for five to ten seconds while the other was released immediately to swim to its underwater button. In the delayed-release trials, the dolphins had to rely solely on vocal communication to coordinate the button press with precision.

At the same time, the researchers chose five different noise-level categories. These were ambient noise level, and low, medium, high and very high noise treatments.  A total of 20 trials was carried out at each noise level. The noise levels corresponded to 115 dB, 119 dB, 126 dB, 135 dB and 150 dB respectively, as recorded adjacent to each dolphin’s underwater button.

The researchers found that Delta and Reece were able to coordinate their button-pushing to within one second of each other in 85 percent of trials at the ambient (lowest) noise level. As the levels of noise played from an underwater speaker increased, both dolphins compensated by increasing the volume and duration of their whistles. Reese’s whistles were on average 1.85 times longer in the highest noise exposure trials than in the ambient noise trials, and Delta’s whistles were on average 1.66 times longer in the very high noise trials than in the ambient noise trials.

In addition to “shouting” louder and longer, the dolphins also changed their body positions to face each other more often, and swam to the other side of the pool to be closer to one another. Despite these strategies to help them cope with the loud noise, they were only able to achieve a success rate of 62.5 percent in trials at the highest noise level. 

“This shows us that, despite them using these compensatory mechanisms, their communication was impaired by noise,” explained study first author Pernille Sørensen of the University of Bristol. “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.”

“Those same reasons that make sound so advantageous for animals to use also make them susceptible to disturbance from noise in the environment,” said Sørensen. “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.”

While this research was conducted with dolphins living in human care, human-generated noise can potentially have detrimental effects on wild dolphins, too. “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,” said study co-author Stephanie King, associate professor at the University of Bristol.

“Our work shows that these adjustments are not necessarily enough to overcome the negative effects of noise on communication between individuals,” said Professor King. Since dolphins rely on their communication skills to successfully hunt and reproduce, noise levels can affect their behaviors, which in turn affect population health.

“This collaboration with international colleagues at the Dolphin Research Center 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,” said Sørensen. To study this in the wild, the researchers would need further understanding of when animals are actively working together and how cooperative behavior is coordinated. “Our results clearly show the need to account for how noise affects group tasks in wild animals.”

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By Alison Bosman, Earth.com Staff Writer

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