Shipping traffic sounds drain dolphins of energy

Trying to communicate over the sound of ship traffic and other noise sources, dolphins need to use extra energy to raise their voice and get themselves heard, says a study.

While dolphins expend only slightly more energy on louder whistles or other vocalisations, the metabolic cost may add up over time when the animals must compensate for chronic background noise, the study pointed out.

"If they are repeatedly exposed to a lot of noise, the repeated effort to call louder or longer or more often -- that's where the impacts could become more significant," said study lead author Marla Holt, research biologist at the Northwest Fisheries Science Center in Seattle.

The impacts could be pronounced for young, growing animals or nursing females already struggling to eat enough to maintain their energy balance, the researchers noted.

Some animals also react to nearby vessels and associated noise by slapping their tails on the water or breaching -- jumping clear out of the water. That could add to the extra effort required by louder calls to further drain their energy.

The research examined the energy expenditures of trained captive dolphins at the University of California Santa Cruz.

The dolphins were trained to whistle softly as they might in quiet conditions and more loudly as they would in situations with greater background noise.

Plastic hoods over the dolphins measured their oxygen consumption as a gauge of how much energy they expended in producing the whistles of different volumes.

The study found that the dolphins consumed about 80 percent more oxygen when whistling at the highest vocal energy levels than they did at rest.

The results appeared in the Journal of Experimental Biology.