Click! Click-clickity-click-click. Unghhhh. Cliiiiiiiiick! A bottlenose dolphin tries to communicate with nearby friends, but they cannot hear the calls. There are too many ships in the water making noise. CLICK! To be heard over man-made din, whales and dolphins must effectively raise their voices, which they do by changing the frequency, amplitude or duration of their vocalizations or simply by repeating their calls over and over. Unfortunately, that acoustical alteration also affects the animals' health.

National Oceanic and Atmospheric Administration biologist Marla M. Holt and her colleagues turned to a pair of bottlenose dolphins at the Joseph M. Long Marine Laboratory at the University of California, Santa Cruz, to find out how. The dolphins were trained to produce a quiet, low-amplitude vocalization on command, as well as a high-amplitude call, 10 decibels louder. The researchers monitored the dolphins' oxygen intake during both types of calls and found that the louder the dolphins vocalized, the more oxygen they needed.

The team then combined its observations of oxygen use with data from wild dolphins to calculate how many extra calories the animals would have to eat to compensate for the energy they burn while making louder calls. Wild dolphins, the estimates show, would need to gobble two extra nutritional calories of fish for every two minutes they spend whistling, clicking and squawking to overcome boat noise. Although this metabolic cost is small, it adds up over time. “To survive and breed, you have to make sure you have enough calories every day to support those activities,” Holt says, and animals living in food-limited, noisy environments that rely on sound for communication, hunting or breeding may not be able to find enough food to make up the difference. The health risk is even more serious for juveniles and for nursing females, which already must perform additional foraging to obtain all the nutrition they need. The results were published this spring in the Journal of Experimental Biology.

Human-created noise underwater, whether from the spin of a ship's blades, the hum of an engine, the clanking of construction or the bangs of seismic exploration, does more than force odontocetes to speak up. Other research shows that whales and dolphins breach, spy-hop and tail slap the surface more often when vessels are nearby, all of which sap more energy. Military sonar also disrupts cetacean hearing and alters their diving behaviors, most likely leading to illness and stranding.

Next, Holt and her colleagues will investigate specific actions that could be taken to mitigate the effects of human-generated noise on dolphins and other sea creatures, such as requiring ships to slow their motors while coming into a harbor or keeping whale-watching boats a minimum distance away from the marine mammals they pursue. Besides, shouldn't humans know better? Interrupting a conversation is rude.