Whales produce upcalls, sometimes called contact calls, when they are alone or in the process of joining with other whales. An upcall begins low and rises in pitch. It is the most frequent call produced by right whales.

Parks and her colleagues, Mark Johnson and Peter L. Tyack, Woods Hole Oceanographic Institution and Douglas Nowacek, Duke University, looked at short-term modifications of calling behaviour of individual North Atlantic right whales in varying environmental noise situations. They report their results in Biology Letters.

It appears that right whales increase the amplitude, or the energy in their calls, directly as background noise levels increase without changing the frequency. This suggests that right whales can maintain the signal to noise ratio of their calls in moderate levels of ocean noise.

"To our knowledge, this is the first evidence for noise-dependent amplitude modification of calls produced by a baleen whale," said Parks.

Increased risks

Changing calling patterns can, however, incur costs including increased energy expenditure, alteration of the signal and the information it contains and increased predatory risks. With increased noise, the effective communication range for feeding or mating will shrink and stress levels on individual animals may rise.

"Whether they can maintain their communication range in noisier environments still needs to be tested," said Parks. "Ocean sound levels will probably continue to increase due to human activities and there is a physical limit to the maximum source level that an animal can produce."