Dolphins have to shout to make themselves heard over underwater background noise, scientists have found.
The marine mammals use a series of clicks and whistles to communicate with one another and are highly intelligent social beings that live in large social pods.
Having the ability to communicate over long distances helps them hunt and breed but this is getting harder as a result of increased human-made noise, scientists say.
Ships and drilling are key noise sources in the wild and the University of Bristol has found when background noise is present, dolphins up their own volume but still struggle to be heard.
Delta and Reese, two bottlenose dolphins living at the Dolphin Research Center in Florida, were tasked with working together to simultaneously push two buttons which were placed 21 inches underwater and were at opposite ends of a 60-foot long pool.
Researchers also varied when and where the animals were released, with one dolphin sometimes being delayed by up to ten seconds to ensure they were only using verbal communication to synchronise.
University of Bristol scientists monitored how loud the calls of the two dolphins were when there were no background sounds and under four conditions where they used a speaker to introduce noise of differing volumes.
The scientists conducted 40 trials in each condition and found “both dolphins significantly increased the amplitude of their whistles as noise levels increased”.
For every decibel the team increased the background noise, the dolphins upped their own volume by between 0.08 dB and 0.14 dB.
Noise impairs communication
The more noise there was the less effective the dolphins were, data show, with the quietest environment producing an 85 per cent success rate, while just 62.5 per cent of trials in the noisiest trial (which was the sound of a jet washer) were a success.
“This shows us that despite them using these compensatory mechanisms, their communication was impaired by noise,” study first author Pernille Sorensen, from the University of Bristol, said.
As well as changing their calls, the animals also altered their body language.
The study found that as noise levels increased, they were more likely to reorient themselves to face each other, and they were also more likely to swim to the other side of the lagoon to be closer.
Although the research was conducted with dolphins living in human care, researchers suggest human-generated noise could also have detrimental effects on wild dolphins.
Co-author Stephanie King, associate professor at the University of Bristol, said: “If groups of animals in the wild are, for example, less efficient at foraging co-operatively, then this will negatively impact individual health, which ultimately impacts population health.”
The study is published in the journal Current Biology.