The oceans are not as quiet below the surface as you might think. In fact, natural processes and human activity generate significant amounts of underwater noise which can make it more difficult for marine animals, such as dolphins, to communicate.
Now a team of researchers from the University of Maryland Center for Environmental Science and Cornell University found that dolphins in the Western North Atlantic modified their whistle calls in response to high noise levels caused by ships, possibly hindering their ability to talk to each other.
Previous research has indicated that increased ambient noise levels may hinder the perception of acoustic signals (known as masking) in marine animals and have been associated with negative impacts on health and reproduction. In many species, vocal communication plays a critical role in bonding, parent-offspring interactions, warning calls, mating signals and territorial defense.
However, the effects of increasing noise levels on many marine species that primarily rely on sound for communication, are not well understood.
For the latest study, published in the Royal Society journal Biology Letters, the team investigated the effects of underwater noise on the social whistle calls produced by bottlenose dolphins off the coast of Maryland—a region of the ocean that experiences high levels of vessel traffic. These dolphins use whistles for conveying individual identity—almost like a name—and a host of other important information.
Between July and September 2016, the team made a series of recordings using underwater microphones about 30 kilometers off the Maryland shore. They calculated levels of ambient noise and the recordings were examined for indications of bottlenose dolphin whistles. These whistles were then measured for 11 characteristics, including duration and frequency.
The team found that the dolphins changed their vocalization characteristics during increased ambient noise. Specifically, the calls were higher in frequency and became more simplified in order to overcome the masking effect. The authors speculate that this could reduce the information content in the whistles and thus may be detrimental to the social cohesion of the group, although more research is needed to better understand this process.
