Video: The dolphin “Delta” presses a button at exactly the same moment as his partner “Reese” at the other end of the test facility. © Current Biology/Sorensen et al.
When exposed to noise, we speak louder and clearer to convey information to a fellow human being. A study shows that dolphins behave in a similar way: In order to be able to work together successfully when there is a noise in the underwater world, they “scream” and stretch out their communication calls. Despite this compensatory behavior, the success of the cooperation suffers from the acoustic stress, the evaluations show. This illustrates the complex ways in which man-made underwater noise can harm marine mammals, the researchers say.
It has never been quiet in the oceans – there are many natural sources of noise in the sea and the water ensures that sound spreads particularly far. Some sea creatures take advantage of this: they produce sounds for various purposes and also have the corresponding hearing abilities. Marine mammals are particularly well known for this: Many species use their sounds as biosonar and for intraspecific communication. The representatives of the dolphins can convey complex information to each other, numerous studies show. For example, they can “agree” in a surprisingly sophisticated way to go hunting together.
However, as is well known, humans are increasingly interfering in the natural soundscape in the sea: ship traffic, construction projects and many other anthropogenic sources of noise cause a roar under water in many places, which can harm marine animals – including the dolphins. “For the same reasons that make it so beneficial for these animals to use sound, they are also vulnerable to disruption from environmental noise,” says Pernille Sørensen of the University of Bristol. Previous studies have already shown that dolphins are disturbed by underwater noise and also change their vocalizations. However, Sørensen and her colleagues have now investigated the extent to which underwater noise affects communication behavior and success when dolphins solve tasks together.
Amazing cooperation skills
The study involved bottlenose dolphins “Delta” and “Reese”, already known for their amazing cooperation skills: Researchers taught them to simultaneously press – within one second – two buttons, each attached to the bottleneck, for a reward both ends of a test facility are in the water. In the delayed-release experiments, the animals must swim to their button and then “coordinate” with their partner via communication calls to precisely coordinate the joint button press. They usually succeed in triggering with a delay of less than a second in about 85 percent of the cases.
But for the current study, the scientists brought noise into play: During the tests, they caused different levels of noise pollution in the water. In addition to loudspeakers, a high-pressure cleaner was used to sometimes clean the jetties of the facility, which is located in a bay. During the experiments, Delta and Reese wore a suction-cup-mounted device that recorded their communication sounds during the cooperation task. In addition, the movements of the animals were recorded by video recordings.
Louder and clearer – but with limited success
As the researchers report, the evaluations showed that as the noise level in the water increased, the dolphins increased the volume of their calls. When the loads were high, they literally “yelled” at each other. The animals also changed the characteristics of their sounds: the duration of the whistle became longer and longer. In addition to these adaptations, a change in the movement behavior apparently also served to improve the transmission of information in the disturbing background noise: the animals turned in such a way that their hearing system was optimally aligned to the partner, the scientists report.
“But as it turned out, despite these compensation mechanisms, communication was significantly impaired by the noise,” says Sørensen. Because the evaluations showed: From the lowest to the highest noise level, the success rate of the dolphins fell from 85 to 62.5 percent. “Our work shows that despite the fact that the two dolphins know this cooperative task so well, are highly motivated and show sophisticated compensatory behavior, the noise impairs their ability to coordinate successfully,” Sørensen summarizes the study’s results.
Although this research was conducted with dolphins living in captivity, the results may well extrapolate to wild populations, the researchers say. This makes it clear once again that man-made underwater noise can harm these marine mammals in a variety of ways. “For example, when groups of animals in the wild are less efficient at cooperative foraging, this can have a negative impact on the health of an entire population,” says senior author Stephanie King from the University of Bristol.
Source: Cell Press, Article: Current Biology, doi: 10.1016/j.cub.2022.12.063