However, dolphins are severely disturbed in many areas of their lives – most notably by noise. But even so, they continue to communicate.
Dolphins are social animals that live together in so-called “pods”. They protect each other, raise young animals together, hunt prey together and play together. To do this, they produce, among other things, whistles and clicks for echolocation. The exchange of information with members of the group, for example, in hunting or mating, is essential for the survival of both the individual and the population.
However, this peaceful coexistence can be disturbed by excessive underwater noise. When there is noise in their environment, dolphins increase the volume and duration of the whistles they use to communicate. If the animals are in a particularly noisy environment, they even need to “scream” to communicate.
Despite these attempts to compensate, marine mammal communication can be significantly impaired by loud noises, reports a team led by Pernille Sørensen of the University of Bristol in Britain. The results, published in the journal Current Biology, show how underwater noise can affect dolphins’ lives.
Navigation and drilling threaten dolphins
Meanwhile, marine mammals’ underwater communication is increasingly being disrupted by humans. Underwater drilling or shipping by sea generates a lot of noise, which means that animals can no longer communicate normally and cannot coordinate complex behaviors properly. Orientation, foraging or recognition of conspecifics are significantly impaired. In the long run, this can lead to serious changes in behavior, damage to health and even an increased risk of death for the animals.
To compensate for the increased noise pollution, some animals use their specific calls louder, longer, or more frequently. Or leave the site to go to quieter areas.
So far, these behaviors have only been demonstrated in single individuals. The international team has now investigated how two bottlenose dolphins – a type of dolphin – reacted to increased noise as they were required to solve a task together.
Increased noise makes cooperation difficult
Male dolphins, Delta and Reese, were asked to simultaneously press two underwater buttons located at opposite ends of the experimental pond. In previous tests, the pair had already proven that they could solve such a task through precise communication. The new challenge was to complete the task in different background noises – from normal ambient noise to the huge noise of a pressure washer.
Result: The success of bottlenose dolphins decreased with increasing ambient noise. While the two managed to press both buttons at the same time in normal ambient noise in 85 percent of the attempts, their success rate dropped to 62.5 percent in the loudest noise annoyance.
Human noise affects foraging
The researchers also observed that the dolphins increased both the volume and duration of their whistles as the noise level increased. The animals literally had to “scream” to coordinate. At the noise of the pressure washer, they hissed almost twice as loudly as usual. The animals also changed their body language: as the noise increased, they more often reoriented themselves towards each other and swam to the opposite side of the pond in order to get closer and better understand their partner’s signals. “Our study shows that dolphin communication is significantly impaired by noise – despite their various attempts to compensate,” said first author Sørensen.
Although the study was only carried out on dolphins in human care, the researchers assume that human-made noise also affects wild dolphins. “For example, noise can make shared collection less efficient,” said co-author Stephanie King, also from the University of Bristol. “This not only harms the health of individuals, but ultimately the entire population.”
(APA)